How Good are Chip Resistors as Low Power 50 Ohm Terminations?

I purchased some 49.9 Ohm 1206 resistors to use as low cost, low power terminations or dummy loads in experiments. It turns out that when I measured them with the nanoVNA they were very good up to 150MHz and usable to at least 70cm. If it was non-critical then I'd use them at 23cm also.

 

Two samples of 49R9 1206 Chip Resistors

Dishal Filter Alignment with the NanoVNA

When aligning bandpass filters it only takes me a few minutes of twiddling before frustration sets in. I had previously used the method attributed to Dishal as set out in Solid State Design. It is a very effective method although there is some set up needed. 

Over the years I had seen some very vague, if not useless, explanations of how to do this with a VNA. So I set out to discover for myself how this was done.  I threw together a 2 section narrow band pass made with coaxial stubs, tuned with high quality beehive capacitors. In the process I learned not only how to tune these filters but also how to design them with coaxial stubs.

 

Here then is how I use the NanoVNA to align multistage filters. (apologies to keen eyed readers who might noticed the dates/times of these images are mixed up. I lost a few so I had to mix two filter tuning records I had)

1. I used OneofEleven's PC software to drive the nanoVNA but this guide should be near universal.
2. OSL calibration as required
3. Connect the filter to be aligned to the VNA. I'm going to use Channel 0 as the Input, Channel 1 as the output in this guide. 
4. Use a suitable frequency span and add a marker for the desired center frequency eg 146.5MHz
   
5.  Select a polar view of S11 and place a short to ground at Point A
   
   
   
   
   
   
6. Adjust the marker using the "e-delay (ns)" field so it lies on the X axis. This is for convenience, the Y axis would also work. 

   
   

   

   
   

   
   

   
   

   7. Move the short to Point B

   

   
    

   8. Adjust the tuning capacitor on the first resonator until the marker rotates to be on the X axis but opposite to the rotated starting point.

   

   
   9. Move the Short to point C and tune the second resonator
   

   

   
   Remove the short. That's it. The picture in point 9 makes an important point. The whole trace is now on one side of the plot. That's irrelevant. What is important is the marker is on the right hand side of the two points which intersect the X axis. It started on the left hand side in point 8. Just try this, it is easy once you have had a go. Your traces will look very different to mine but the procedure is the same.

   
   So how did it work? As expected:

   

   
   I then decided to rebuild this for a narrower bandwidth. My tuning caps were just a tad too small so I used a center frequency of 150Mhz. You will notice below the peak is at 149.5Mhz. That's because I used a piece of floating PCB onto which the coupling capacitors were soldered. It was getting mechanically depressed during tuning changing the stray capacitance around the filter. 
   

   

   
    
   

   73's
   

    
   

NanoVNA - Inductor Measurements

Starting with a 1.2uH inductor results were immediately satisfying. The measured L and variation with frequency matched the curve shown in the TDK datasheet. And the self resonant frequency was readily measured as 180MHz.

1.2uH Inductor

Let's try an unknown but small 0603 wire-wound chip inductor. Measured as 22nH.

How small can we go? I have some 12nH 0201 chip inductors with factory measurements (sample size, variance etc). Wow. 12nH measured.


Previously I have used a signal generator and test fixture with a measured parallel capacitance
( 112pF from memory ) to measure the resonant LC frequency, then back calculate what L was. This worked very well down to nH sized inductors. With a power detector I was also able to estimate Q by sweeping around the resonant frequency for the -3dB points.

The NanoVNA is now my preferred approach. I prefer the fixture without the 50 ohm resistor. The curves are easier to see and therefore interpret. Either fixture is fast to use and easy to see how L changes with frequency. I can't see how Q can be measured, but A/B comparisons show slight differences. The higher the inductor Q the closer to the edge of the Smith Chart. 

Go on, give it a try. 

NanoVAN Test Fixture - Capacitor results

Here I present two results: measuring a supposedly high Q 110pF capacitor with and without the 50 ohm chip resistor in series.

110pF capacitor in series with 50 ohm chip resistor

Immediately some questions are posed, and some pleasing results. VNA suggest 110pF capacitor is actually 112pF. This is true over most of the HF range but by 54MHz the effective capacitance has already fallen to 109pF. It gets progressively worse after that. Worth knowing in the design of filters and matching networks.

Overall the fixture with a 50 ohm series resistor appears useful.

110 pF capacitor with no series resistor

Keen readers will notice that the 110pF capacitor, tested in this fixture, measured 110pF, not 112pF. That could be because I lost the first capacitor in the shed floor. Sorry, but the time taken to present these results didn't warrant re-measuring the capacitor in the first fixture. ( Seconds to measure and view on the nanoVNA screen, many minutes to collate and present. )

What is interesting is that while the nanoVNA display showed a trace around the edge of the Smith Chart, the software did not. Is this evidence that at the extremes there are errors as data is transferred and manipulated to re-display the trace, or underlying accuracy issues with the NanoVNA? Loss of one too many decimal places? The S2P file I exported with "nanoVNA partner" went to 9 decimal places, however that does not mean the software worked at that level of precision. Displaying answers to 4 decimal places is meaningless without knowing the precision of the underlying calculations.



Viewed on the nanoVNA it suggested by the software the capacitor is useful up to 2m before it departs from being a 110pF capacitor.

At this point both test fixtures have, within reason, results and findings consistent with each other.

I then examined in turn capacitors of 100nF, 10nF, 1.5nF and 150pF. The nanoVNA screen clearly showed where these capacitors departed from being the stated value and confirmed my understanding of why the value and type of decoupling capacitor requires thought. Unfortunately, with no series resistor once the readings are transferred to the PC the data translation/transfer/rounding issues prevent a meaningful display of the results when the S parameter file is later opened with "nanoVNA partner".

However, all is not lost.  The exported S2P file, viewed in RFSIM99, is a close representation of what the nanoVNA screen was showing.

100nF capacitor, no series resistor

The 100nF 1206 capacitor measured 93nF at 50kHz. At 9MHz it was already a 1nH inductor. Not what I expected from a 1206 part. A trip to the workshop to check. Indeed, this seems correct. While the 50 ohm resistor fixture showed the transition was at 18MHz, the difference between the two is within the accuracy I wanted.

For decoupling purposes the series impedance at 36MHz is 0.05R+j0.65R. Above that the decoupling gets worse.

How does 10nF look? A 10nF 1206 "recovered" capacitor was tested. The series impedance at 72MHz is 0.09R+j0.63R, getting worse as frequency rose beyond that.

The 1.5nF capacitor demonstrated why two values of decoupling capacitor are sometimes shown. It would be poor at decoupling until the frequency rose to 72MHz. Above 300MHz the response was erratic so this needs checking.

The 150pF capacitor was unaffected by parasitics until the frequency rose to 234MHz.

I overlaid the results of 3 39pF capacitors of different construction, mica clad, 1206 ceramic and a SMD mica? film capacitor. There was no real difference until the frequencies rose above 300MHz. After this all displayed a similar erratic pattern though it was shifted.

3 39pF capacitors of different construction.

Advantages of 50 ohm test fixture

There is some suggestion in published papers that this fixture should have greater accuracy. I see nothing in the results to date to support this. Perhaps my test fixture for SMD parts was better than hte authors test fixture.

In any event, since you have to build one of these to calibrate the other fixture ( OSL ) then for now I put aside the "accuracy" debate. 

Advantages of the test fixture without 50 ohms

A/B comparisons of decoupling applications at spot frequencies gets easy since for the same frequency A is worse than B if it is further away from the 0+j0 point of the Smith Chart.

Conclusion to date

Either fixture appears useful at characterizing capacitors down to very small values. The lowest I tested was 1.5pF and the results gave me confidence in the value and presence of parasitics.
Accuracy appears degraded above 300MHz but for my purposes will still be adequate.

For A/B comparisons I would use the test fixture without the series resistor since the deviation is easier to spot on the small display.

NanoVNA Test Fixture - Details

Here is a picture of the fixture I am using. I had allowed for several fixture styles. At present I am using two of the fixtures, both single sided. The unused fixtures are double sided but I may never get around to trying them. The results to date suggest single sided is more than adequate.

The 50 ohm chip resistor fixture is shown on the top right hand corner. A tricky soldering job since one pad is underneath, and the other wraps around. Clearly too much solder but the OSL calibration process is hopefully negating the additional stray capacitance that blob is contributing to the measurements. Not obvious is the manual removal of the ground trace that runs under the chip resistor by scratching it off.

To calibrate the fixture I use the chip resistor version. As shown it is open circuit. I use a piece of foil squashed onto the SMA pins with my thumb as the short. The piece of foil is used to connect the pad adjacent to the ground side of the 50 ohm resistor to the untinned areas as the load calibration.

I then use the same calibration settings for both fixtures. Using the calibration for the chip resistor version with the non-chip resistor version has proven to be acceptable.

To use the same fixture without the resistor I sweat soldered a piece of tinned copper foil as shown. The SMD then is placed where the pad for the chip resistor was used to connect to the center pin of the SMA connector. That tiny chip inductor is I think 0201. It was the smallest physical and electrical value I have tested to date, with good results. But more on that later.

I hold the test device in place by clamping it with a wooden clothes peg. Sometimes  I use a piece of bare fiberglass, or other benign material like the fingernail on my thumb, and I have found no material difference in readings by doing this.

Experience with this fixture suggest I should have made the pad slightly larger to more easily accommodate some of the SMD inductors just larger than 1206 size parts. And maybe shaped the ground pad to be closer to make placing the 0201 parts a little easier.

NanaVNA - Test fixture for measuring passives

Recently I've been exploring what the NanoVNA can do. It really is an impressive piece of test equipment for such a small sum. There has been lots of discussion about measuring passives at HF but I was troubled by the lack of evidence that some commentators had actually done what they advocated. Just like fake Chinese transistors, too many online experts write on the basis of what they have read, not what they have done.

And there was very little online information I could find about using the NanoVNA to measure very small values of passive components. What information I could find was at times inconsistent. So I set out to discover for myself what worked. Over the next few weeks I'll post what I found.

But first the starting point has to be the text fixture. I had a PCB house fabricate a few boards so I could experiment. A variety of single and doubled sided layouts were contemplated, some would use a 50ohm chip resistor in series with the passive being tested, some would simply place the passive component across the SMA.

My biggest challenge was presenting the test results. Taking a picture of the screen was impossible given the screen reflections.  "nanoVNA partner" is a small download and worked well on both XP and Windows 10. I recommend it.

As far as using the NanoVNA, at least one published paper would appear to advocate a different approach to what I found. Could it be the NanoVNA was better than the equipment used in that published paper?

Lead Acid Batteries - Measurement of Internal Resistance - Results

It's been about 4 weeks now and some observations of the impact pulse conditioning has had on internal resistance and perhaps battery capacity are warranted. More importantly, the need for a more rigorous approach has become apparent.

Over that last 4 weeks the measured internal resistance has fallen from 264 milli-ohms to 175 milli-ohms. While the reduction in the internal resistance tapered off after about 10 days, there is some support for the battery's capacity having increased as well. At the start I was recharging the battery after 3 days on the pulse conditioner. Now it runs for 5 to 6 days before I have to recharge the battery.

So my initial observations are that the pulse conditioner is beneficial. However, there are several issues with the approach and the results cannot be construed as anything other than weak support for pulse conditioners.

The biggest drawback to the approach is the lack of a control battery. With a second battery I could cycle one on the pulse conditioner whilst the other was cycled on a static load. If the static load battery showed little, or no improvement, in internal resistance then there would be much stronger support for the pulse conditioning approach.

Another drawback stems from the lack of automation. I still have to manually changeover the battery at each step of the pulse, charge, measure cycle. So the time between measurements is not constant and the level of charge and discharge varies from one cycle to the next. I clearly need to automate the cycle and let it run unattended.

The final obvious shortcoming is temperature. We are moving towards summer and the average ambient temperatures have increased over the last month. The temperature change could be influencing the results either directly via battery chemistry somehow, or indirectly by it's impact on the voltage regulator which serves at the voltage reference for the D2A conversion. A temperature controlled testing environment would be useful to remove another source of potential error but that is beyond my reach at present.

The biggest obstacle to the full automation with the W1209 board is the need for an additional two digital outputs. I have considered two approaches. The first is using the + and - keys as both inputs and outputs. That would require some careful soldering to insert a resistor, say 2k, in series with each switch. The second approach is a 4017 counter clocked by the pin driving the relay. Then each of the decoded outputs for 1-3 from the 4017 driving a relay to perform each step of the pulse charge measure cycle.

I'm leaning towards the first approach since I don't know if I have a 4017 in the drawer and the first approach avoids any ambiguity over which step the cycle is in.

Lead Acid Batteries - Measurement of Internal Resistance - V3 of Code

​After hours running various tests to determine when the voltage samples should be taken I concluded there was no right answer. Mind you, the sweet joy of using Forth to do this via the serial port was a reward in itself. ( Timing loops could be tested interactively, results displayed on the terminal window, etc etc. )

I settled on a longer delay before sampling the loaded voltage, or Vend in the code, then a short delay before sampling the unloaded voltage, or Vbeg in the  code. Because of the way the voltage depresses over time, then recovers, I doubt there is a right way to do this. But my training in Statistical Process Control and Gauge Capability steered me in this direction.

I now have a testing method that gives me repeatable readings provided each test is run hours apart. So while the calculated internal resistance might not be the value according to some standard it should allow me to discern if my pulse conditioning circuit has any impact over time on the internal resistance.

Over the next few weeks I'll run my trials and see what happens.

Lead acid batteries - Measurement of internal resistance - x4 resolution mod

The initial project used a voltage divider to reduce the measured voltage down to less than 5V. This means the voltage resolution is about 20mV which means the internal resistance measurement for 1 amp of current is 0.020 ohms.

Instead of dividing by 4 with a resistor string, what if we subtracted 10V from the voltage to be measured? This would give us a voltage resolution of about 5mV with an internal resistance measurement for 1 amp of current is 0.005 ohms. This seems like a worthwhile improvement and can be easily achieved.

I had never played with a voltage subtracter before but it proved to work first time. Reaching into the junk box I pulled out a NE5532 dual op amp. I had no reason to chose this device over any other except I had hundreds I had recovered from a couple of boards. A few resistors and it was done.

While I used a NE5532 and 2.4k resistors I don't think there is anything critical about this. Just about any op amp will probably work and the resistors could be anything between perhaps 1k and 100k. As long as they are all the same value.

As before I'll post the updated code over on Hackaday. While this is not yet the complete measurement tool I wanted it will allow me to make some measurements on the impact, if any, pulse conditioning has on internal resistance.

Lead acid batteries - Measurement of internal resistance - Code

While working on a small hardware modification to improve the resolution of measurements it occurred to me that displaying an internal resistance of 0.2 ohms as 200 was easier to read than 2. I updated the code over on  Hackaday

Debugging a change like this is so easy with Forth, especially when a simple x100 instruction fails because the 16 bit integer maths overflows.

Regards
Richard

ps the hardware mod worked a treat giving a fourfold improvement in resolution. I'll describe it shortly once I've finished the soldering. It might even mean that no modifications to the board are required!

Lead acid batteries - Measurement of internal resistance - Schematic

Once you have modified the W1209 board here is the schematic for the overall project as it stands today.

Regards
Richard VK6TT

Lead acid batteries - Measurement of internal resistance

Hams, listen up. With our short arm deep pockets mentality this is going to really appeal to you. This project takes a cheap assembly, $2 delivered, from China and turns it into a test fixture for measuring the internal resistance of small lead acid batteries.The project as it stands works very well. However, I've already thought of ways to improve it which I will incorporate in due course and report on.

There were two motivating reasons for this project. The first, and a long standing one, was to measure the internal resistance of lead acid batteries to see if some of the rejuvenate, repair or restore ideas I had come across had any objective merit. As a ham I have plenty of small lead acid batteries which I only require occasionally. I periodically float charge them and occasionally I have had to dispose of one via the recycling depot after cells have gone open or short circuit. Recently I began playing with a pulse conditioning circuit and initial measurements of internal resistance suggested there was some lowering of the internal resistance. But manually setting up a test jig and then remembering to take measurements has been a problem for someone so easily distracted.

So I was looking for a way to automate the entire measurement, conditioning and charging process to firmly establish if there was any benefit in pulse conditioning these batteries. Then I stumbled across the project at eForth for cheap STM8S gadgets • Hackaday.io which is one of the most exciting ideas I have seen in recent times. The thought of taking a $2 board, replacing the firmware, and doing something entirely different with it just thrills me. The more I use these gadgets the more ham applications I see.

This project uses the W1209 thermostat board, readily available for under $2. Delivered. Hams, listen up. What I did was make a few small modification to the board:

1. remove the 20k smd resistor next to the sensor connector,
2. throw the sensor in the junk box,
3. add a 10k resistor across the sensor connector (underneath if a smd resistor or remove the connector if using a through hole resistor)
4. add a 30k resistor (or two 15k resistors in series) from the upper terminal of the sensor connector to the +12v rail.

Then I built a 1A constant current load from a 5v regulator and two 10 ohm resistors. You probably have those in your junk box ready for a project like this.

At present my board works as follows:

• after connection to the battery  I hit the "+" key to run the measurement routine. It takes about 1 second and in that time it reads the battery voltage 32 times, summing the result, activates the relay which increase the current drawn by 1 amp and takes a further 32 measurements of the battery voltage summing those results, releases the relay then saves these two sums into eeprom
• I then hit the "set" key  and the display shows me the average of the open circuit voltage, the loaded battery voltage, and the calculated internal resistance.
• If I hit the "-" key the run counter is reset to zero.

The current partially discharge battery I tested this project on had a beginning voltage of 12.8V, a loaded voltage of 12.5V, and an internal resistance of 0.2 ohms.  Why not 0.3 ohms? That's the result of scaled integer maths. The 0.2 ohms is the correct answer. e.g. 12.81 - 12.59 = 0.22. But the display only shows voltage truncated to 0.1v.


My longer term goal is a test fixture that repeats daily a charge of the battery, measure the internal resistance, then applies the pulse conditioning until it is time to start the cycle over again. Then once every 8 weeks I can either dump the data back to the PC by reading the eeprom, or read the data via the display for whatever run I chose.

Over a series of posts I will cover the initial Forth code and subsequent improvements, the circuit for the 1A constant current load, how everything is wired up and eventually the further modifications to improve the resolution.

In the meantime here is what you need to do. Firstly, read the hackaday link above for background information. I really like the programmable power supply. I can see several of those going onto my workbench in due course. The team behind this Forth project need a big thank you for doing such a great job.

If you decide to give Forth a try, and I strongly encourage you to do so,  your shopping list is:

• a few of the W1209 modules, pictured below, 
• the programming dongle,
• and a serial interface (USB dongle or MAX232 based device) if you don't already have one.

You'll never touch arduino again and probably end up running amforth or something similar on those those arduino boards like I do.

I will post working copies of my firmware if you simply want to flash the board and not learn Forth. And if all you want is a pre-programmed modified board then you best contact me. 

Satellite Tuner Board becomes a 800MHZ - 1600MHz Signal Generator

I've been so busy on non Ham activities that I haven't had a chance to pursue my Ham projects. As a stop-gap measure here is something I recently finished. While it doesn't neatly fit the goals of my blog to be easily repeatable by the reader it will hopefully inspire you to attempt something similar. If you have one of these satellite receiver boards then I can provide further details and code to help. 

A while back a generous Ham, Fritz, was giving away some satellite tuner boards at the NCRG Hamfest.  The board had a nice TXCO but on closer inspection it also had some nice PLL chips (Si4133) and mmic amplifiers. So I downloaded the relevant datasheets and ruminated on what it could be used for. My ultimate goal is to use one of these to generate a WSPR transmission on 1296MHZ. But you have to crawl before you can walk!

Before modification the PLL section of the board looked like this:

After removing the superfluous controller chip from elsewhere on the board and cutting the board to fit into an enclosure it became a matter of delicately soldering some thin wires onto relevant via's. These wires then went to a new control board I made.

It transpires that my version of the chip did not have the IF Out function. Which meant I couldn't use it for 70cms or 2m. However, each PLL chip had two PLL's and I found I could cover 800 MHz to 1600 MHz in four overlapping ranges, with steps down to 10kHz. The only drawback was I couldn't load the frequencies fast enough to also make this work as a micro-controller based frequency sweeper. Each frequency change took around 50ms to achieve.

In due course I hope to take some photographs of the noise sidebands as the step size, or phase detector frequency, changes. This should illustrate why the largest possible step size should be used if you are interested in signal purity.

The chip can be purchased for around A$10 (US$7) in a TSSOP package which would be easier to work with than the ones I had on the tuner board. Which is why I didn't remove them but used them in-situ. If you would be interested in building one from scratch let me know. I think a signal generator that covered 2m, 70cm and 23cm would be a great project with a cost around A$50 (US$35).

While kits exist that generate frequencies like the ones this project does, there is enormous satisfaction in re-purposing something that was meant to be scrapped.

Regards
Richard VK6TT

Test Equipment Tip for Initial Testing of Transmitter Amplifiers

While many of us start out building transmitter stages and tuning for maximum output, we should quickly learn that over-driving a transmitter stage must be avoided. Even a slightly over driven amplifier stage generates significant harmonics and distortion products. So here is a quick tip on using the oscilloscope to check for the presence of a transmitter stage being over-driven. 

A 10MHz analogue oscilloscope will be fine for looking at HF signals since we are going to measure relative voltages. I am unsure if a low bandwidth digital CRO will work.

Connect your oscilloscope to the collector or drain of the active device. With no drive and the oscilloscope set to AC coupling ensure the horizontal trace is on the centre graticule. The time-base setting is not critical so I use something that gives me a band across the screen with drive applied rather than the actual waveform. Try 1ms per division and adjust to suit your preference.

Now apply perhaps 10% of the drive you expect to use when the amplifier is in operation. Adjust the trace so it sits between say the second graticule above the centre line and the second graticule below the centre line. As you increase the drive you should see the trace touch the third graticule above and below the centre line at the same time.

Adjust the vertical amplifier gain to reduce the signal size on the screen then increases the drive again. Again, you should see the trace touch the third graticule above and below the centre line at the same time. Keep repeating this until you notice that one of the third graticule's above or below the centre is being touched before the other is being touched.

At this point the amplifier is no longer linear. If you have a fast oscilloscope you can increase the time-base speed to observe a few cycles of the waveform. You will notice that when non-linear the waveform peaks that were last to hit the third graticule will be distorted.

You can back off the drive until both the positive and negative peaks are moving in unison, which I find easier to discern with a slow timebase setting rather than a few cycles being displayed on the screen. That is the limit for linear operation of the stage.

Spectrum analysers and other nice test gear allow you to measure how non-linear your transmitter is. But isn't it easier just to avoid over-driving the transmitter to begin with? Hopefully this tip helps you avoid over-driving your transmitter stage giving you a nice clean signal when you finally go on air.

Regards
Richard VK6TT

Testing my Noise Source and 2m Band Pass Filter - update

Yesterday I caught up with Jack, VK6KDX's, and enjoy the pleasure of chatting about his projects while drinking his coffee. A great way to spend a few hours.

One of the activities we pursued was looking at the output spectrum of noise sources on some test gear Jack owns. Jack demonstrated his noise source and it works very well for testing 23cm filters. We then had a look at my noise source on the spectrum analyser. It had peaks and looked horrible. It occurred to me this morning that the reason for that was most likely stray RF being coupled into the noise source and being amplified by the mmic chain. I still haven't put my noise source into a metal box and the power supply decoupling could be greatly improved.

Despite the shortcomings of the test set-up it did allow me to see the 2m Band Pass Filter in action. A small shoulder on the left of the response curve could be either stray RF coupling or a result of this being a no-tune filter. But I'm really happy with both the noise source and the filter. Especially the filter. It's always pleasing when you build something that confirms your measurement technique for the inductors is really good. No trim-caps, no squeezing or expanding coils. Measure, design and build. Worked first time with no tweaking.

The spectrum analyser was set to 146MHz, 20Mhz per division. We didn't check or adjust for any drift in the spectrum analyser itself. As expected the response falls off faster on the high side. If you haven't read how I measure nH inductors then check out the post http://vk6tt.blogspot.com/2017/01/measuring-small-inductors.html.

Regards
Richard VK6TT

Noise Source Part 1 - Construction

I've heard of several local hams buying noise sources from China in recent days. I had read that they were based on this circuit found here and a quick look at the pictures in the listing supports this. I wondered how hard this could be?

I knew I had too many MAR03 mmics in the parts draw which I now never use. A quick Google and I came across an alternative approach here. So I quickly put a string of mmics into the pcb software and came up with a schematic and board as shown below:

 

I used whatever 1206 size inductors were in my parts bin for I1 to I4. I used a selection of values just in case this avoided some weird response in the output spectrum. One of the improvements I made was to put an attenuator on the output. It appears often overlooked but not all mmics have a good match to 50 ohms on the output port. The ERA5, used in the first example, is very good and would only see a small improvement with an attenuator. But since I didn't have one to hand I used a small amount of attenuation to bring this project closer to a 50 ohm output impedance.

I went up to the workshop and etching a board as follows:

I print an outline layer of the pcb and cut a piece of Cricut vinyl to suit.

I then remove the backing from the vinyl and print the layer onto it. It helps when you print the outline if you put a crease on the bottom corner of the paper to mark which way it goes back into the laser printer.

Then I iron this onto my pcb. Normally large black areas give me trouble because laser printers struggle to print dense large areas. So I tried something different today. I had just received some eBay sheets of gold foil transfer. In theory you put the grey side onto the toner and apply heat and pressure. It looks a bit rough but let's see what happens after etching.

I pulled the board out of the bubble bath and my initial concern was the patches of toner showing. Perhaps the toner density would prove too light and some etching of the groundplane areas would occur?

Only one way to know. I wiped the board with acetone to get this:

Apart from a few minor blemishes, particularly where the insulation tape crept up towards some tracks, everything looked good. The gold foil is definitely part of my standard procedure from know on.

So I fixed the minor issues and build this. Initial testing shows it works but I need to visit someone with better test gear than mine to verify this. As always if you need more details please ask.

I will report back soon on the testing.

Regards
Richard VK6TT

AD8307 Power Meter - Mk2

I previously discussed the Mk1 version of this AD8307 power meter which had a x10 enhancement. One day the meter movement got damaged when I was trying to re-glue the scale. That was when I rebuilt it into the case you see above. I took the opportunity to avoid drilling a hole for the pot and to streamline the whole setting up process in use by revisiting how the reference voltage for the differential amplifier is generated.

I was never happy with the meter being driven off-scale until the pot had been adjusted. This time, I used a small micro to generate the reference voltage with a D2A based on a R2R ladder network. I wrote some code so that at power up, a form of reset, the micro started increasing the reference voltage until a threshold was reached. Then it simply stopped. Now I had automated this part of the process my Q measurement procedure became:

1. find the peak response, 
2. flick over to meter to the x10 setting, 
3. turn the power off and back on, thereby resetting the micro and generating a useful reference voltage
4. then to check the Q I look for a response 30dB less than the peak shown on the meter.

This made it even easier to measure -3db frequencies when looking at inductor Q, crystal parameters or trying to get that last 0.1dB improvement when adjusting filters.

Since the R2R network is only 4 bits, the reference voltage to the differential amplifier has 16 steps and I chose the upper resistor on the network to compress those 16 steps into the range needed.

 

Naturally you're welcome to the code and board design. Get in touch and I will shoot it across. I still haven't worked out how to make it available from my blog.

Regards

Richard VK6TT

AD8307 Power Meter

Here is what my current AD8307 power meter looks like. I built this into a "box within a box" to enhance shielding. Inside the case is the battery, a lipo cell, and it's charging regulator. When I want to charge it I plug it into a 12V power supply. The remaining RF circuitry is built into a shielded compartment at the rear of the front panel bnc. It's basically 99% air inside the case!

I use a pretty much standard circuit for the AD8307. I think it was Wes Hayward, W7ZOI
and Bob Larkin, K7PUA that first promulgated this in QST, June 2001. I dispensed with offset and calibration adjustments since I was quite happy with AD8307 as it stands.

However, it occurred to me one day that being able to zoom into the response would be really handy when looking for the 3dB down point when trying to measure inductor Q with my fixture, here. While it would be easy to add a dc amplifier, the input signal would have to be adjusted so that the amplifier did not saturate. My modification was to add a differential amplifier based on an op amp so that rather than adjusting the RF levels, one could twist the pot until the reference voltage to the differential amplifier allowed for the amplifier to avoid being saturated.

The circuit for the first iteration of this is shown below.

My method for sweeping inductors in my test fixture was:

1. find the peak response, 
2. flick over to meter to the differential amplifier output, 
3. adjust the pot to bring the meter to approximately 0dBm, and 
4. then to check the Q I look for a -30dB response on the meter.

This was much less guess work on my part as to where the 3dB down frequencies were. And if you were trying to peak something it was much easier to see the peak.

I made some further improvements and will blog those soon.

Regards
Richard VK6TT

23cm Swept Oscillator

If you read my earlier post, here, I covered the background to a 23cm VCO I built. While this vco is likely to be used in a transceiver one day, it also became part of a useful piece of testgear - the swept oscillator. While there are other ways to align filters, using a swept oscillator rates highly on the bang for buck scale.

 Principle behind use

Principle behind use

The basis concept behind the use of the swept oscillator is shown above. The frequency output of the swept oscillator is connected to a radio or a stand alone filter. Somewhere from the radio a response signal, perhaps the AGC line, is picked up and fed to the Y axis of a CRO. The CRO doesn't have to be fast, since the signal has been detected and the rise time of the detected signal is slow.

The sweep speed has to be fast enough to create the persistence of vision of the CRO's trace, but not so fast that the detector has not had time to react. A sweep of 25 to 40 times per second works for me.

My implementation

At 2m and 70cm I took the expedient path and hacked a commercially made oscillator. But at 23cm I had no alternative but to build a swept oscillator. A picture of my swept oscillator, working but not quite finished, is below. The maximum sweep is from 1230MHZ to 1406MHz.  You will notice the piece of copper foil I soldered onto the middle stripline to lower the frequency range it was covering.

The three controls are sweep speed, sweep width and centre frequency. The bnc connector connects via a patch cable to the X axis of the CRO. The N connector is where the RF frequency is sourced. I use an external attenuator of at least 15dB to ensure the filter or radio thinks it is connected to a 50 ohm source.

VCO
I covered this previously, here, so I'll just repeat the schematic.

Ramp Generator
I have used this ramp generator many times. It was based on something I saw at Hans Summer's website but with a small correction. It took me a long time to work out why, at the highest point of the waveform, the voltage did not reverse but instead momentarily spiked to the supply rail. It was caused by something called phase reversal and is the result of the op amp driving a capacitive load.

The fix was a small value of resistance, 330 ohms, in series with the output of the op amp to isolate the capacitance.Works a treat now.

Sampling a response

There was a time when sampling the rf coming out of a filter with a diode detector was the norm and it wasn't possible to worry about the response being linear or logarithmic.

Today there are logarithmic detectors that work on low level signals. The AD8307 is useful up to, and including, 70cm. At 23cm I use the AD8314. I will write these projects up soon because my implementation of these logarithmic detectors has enhancements I have not seen anywhere else.

If I am sweeping a stand-alone filter then the output of the filter, with perhaps 10dB of attenuation to improve the match to 50 ohms, is connected to the AD8314 chip. The output drives the Y axis of the CRO.

Where I am trying to sweep a filter already in a radio then I modify my approach. I haven't done this at 23cm yet because I don't have a radio to align. But I have aligned numerous 70cm and 2m radio's with the following approach and I expect it to work at 23cm. I have a dud bnc patch cable with the dud connector cut off. This open end of the coax becomes my sniffer probe. I simply probe around the radio after the filter until I see a response on the CRO. Then I align the filter. I am not making a direct connection. The AD8307 or AD8314 are sensitive enough that capacitive coupling is all that is needed.

PCB
I have a pdf of the artwork if you would like to build one. I don't know how to put this into the blog so a jpeg image is shown below to whet you're appetite. Note that it is not actual size!

Measuring inductors from 10nH to 10uH

Update - Even though this approach is valid, I now simply use the NanoVNA with a test fixture that is included in the calibration. It allows me to measure the effective inductance at the intended frequency of use.  Covered in the early posts with the Test label.

Here is a test fixture I use to measure small, down to nH size, inductors. I got the idea from the book Solid State Design.

Provided your power meter shows a loss of at least 20dB with no inductor connected then the capacitor between the bnc connectors and the floating pcb are small enough in value. When Cg is small then the peak detected is more pronounced and the measurement of Q is more accurate.

In my implementation I use 2 bnc sockets from a panel which are connected to a small piece of blank PCB supported above the ground plane.I glued the floating PCB down with some plastic spacers between it and the PCB underneath. My reasoning was this would reduce the impact of temperature variations on the capacitance by putting an air dielectric between the substrate and the groundplane. I haven't determined if it really makes any practical difference so you might find this unnecessary.

Each bnc connector is connected to this supported pcb via a gimmick capacitor. I used a 2cm or so length of miniature coax. I should have built this with the inner to the bnc connector, the shield to the floating pcb. You can see from the photo below I got one of the small pieces of coax transposed. You could use a gimmick capacitor from twisted wire. If you were using a larger known capacitor then you might find a really small value disc ceramic is suitable.

From this pcb to ground I have a nominal 100pf capacitor. The value isn't critical since you can measure the actual capacitance with a capacitance bridge or meter. I use my digital LC meter which appears suitable for measuring C but I can't rely on it when measuring L since it works at low frequencies. If you can't measure capacitance then use a tight tolerance capacitor and assume a few pf extra.

When I checked with my capacitance bridge I had 109pf from the supported pcb to ground. That forms the C in the well known f = 1 / (2PI sqrt(LC)) equation. I then sweep with my signal generator on one port, and my power meter on the other. At the peak response I solve for L. This allows me to measure nH inductors with ease. Did you know that capacitance is essentially constant with frequency whereas inductance changes with frequency? Measuring an inductor at 1kHz  gets you close, but I prefer measurements at RF frequencies.

By sweeping the frequency to find the points 3dB down I can estimate the Q! The unloaded Q is equal to the resonant frequency divided by the difference between the two frequencies where the response is 3dB down. I will post a separate blog soon about this aspect.

For inductors of 800nH and upwards I have an alternative instrument I can use which measures the capacitance at several megahertz using a clock module. It works well despite only providing a subjective measurement of Q.

The fixture described does work really well and since I strive to build things that do not need tweaking it reduces the uncertainty around measured inductance.

Regards
Richard