1284MHz PLL for 6cm Receive Converter

 I was recently fortunate enough to be given a Mitec Down Converter by VK6UM. 6cm but with a 140MHz IF. 

No problem, or so I thought, let's replace the 1280MHz oscillator with a PLL design generating 1284MHz. That would give a 144MHz IF.

I took a 23cm beacon  board previously described and hacked it around. 

First issue I faced was the PLL lock detection. I used the same MB1507 PLL chip but I increased the reference frequency to 500kHz. I had a few Forth commands that I could use over the serial link to change the frequency. While the VCO tuning voltage would shift according to the frequency I set, the lock detector never went into a lock waveform. After a few days I tried a 250kHz reference and straight away we had lock. 

Some qualitative testing of the PLL output was needed. After a bit of head scratching it occurred to me I could take a 800MHz output from my GSM test set, mix it with the 1284MHz signal, and look at the 484MHz result on the GSM's spectrum analyser. That worked and revealed some 250kHz spurs. I was able to reduce those with another rework of the loop filter values.

The next issue I faced was microphonics. Replacing the loop filter capacitors with tantalums as necessary reduced the level of microphonics when I tapped away with a screwdriver. Listening with an ICQ7a on narrow band FM to the 1284MHz signal was like a road crew sound check. Tap tap TAP TAP.... Useful test but I later realised I had forgotten to replace a 10uF MLCC on the regulator output feeding the VCO. By then it was all boxed up and installed. 

So the microphonics are there for now. I modified the PLL board based on what I had learned and in due course will rebuild the 1284MHz PLL. Hopefully when Larry, VK6UM, checks out the result it will be serviceable.

73's

23cm FM receiver - 1st Mixer Stage

Autumn starts in a few days bringing the promise of cooler days. So back to the 23cm project. I decided, based on what was in the junk box, to use a MAX2681 mixer. Reasonably cheap if you have to buy one I was able to salvage a couple from 900MHz data modems. 

One of the unforeseen bonuses with this part is the input matching at 23cm.

The input impedance is 50-j133 ohms. So a 16nH inductor in series with the input pin will cancel the input reactance. 

Too easy!

73's

23cm FM Receiver - Musings

 All the assemblies for a 23cm FM receiver are coming together i.e.

Once the 1296MHz signal is mixed down to a nominal 446MHz I still have not decided on my approach.

 I am leaning towards the last option, the RDA5807.

I have a number of Tait UHF Orca handheld boards which I could either use as donors for the necessary filters and IC's, or as a working IF strip. These boards probably have faults but after cutting to size I have enough boards to be able to get one working. With a bit of work I could also use the synthesizer to generate the approximate 400MHz signal needed to mix down to the 45MHz IF strip. But this all becomes a bigger project than I had contemplated and the risk is not so much technical as motivational. Will I ever get it finished?

The simplest, most expedient, lowest risk is to secure a working handheld inside the case and move forward. But that does not sit comfortably with me.

The left field final option is the RDA5807. While walking back and forth to the workshop it dawned on me that this single chip FM Receiver would be a real time saver if it worked. I have some experience with these and the risk relates to how well it will work with a narrow band FM signal. If I can get sufficient recovered audio with acceptable noise levels this will be the quickest, most expedient way forward. 

Time for some testing.

73's

Richard

23CM 500mW Amplifier AH101 - duplicated with revised matching

Since I have a lot of these boards I used one to make a test fixture. OSL calibration of the VNA right at the device legs proved feasible since I have some 50ohm RF chip resistors. Fortunately, the measured S parameters were broadly consistent with the tabulated S parameters in the datasheet.

With the device soldered onto hte board I switched between the calibration at the device legs and the calibration at the SMA sockets. This allowed me to infer the electrical length and impedance of the connecting micro-strips, and any strays.The length and impedance were close to the expected value so I was able to confidently rework the matching. 

As you can see below I was able to use a single inductor at the SMA end of the board for the input. I inferred that a microstrip with slightly higher impedance would give a better match. The crazy cutter went mad and the result was just a few dB improvement to a return loss of 21dB. I overshot the mark!

Matching for the output network was placed at the device end of the output microstrip. The capacitor to ground was meant ot be 1.5pF, but I found the slightly smaller 1pF capacitor gave me a better result. Return loss of 24dB was achieved.

That makes two successful amplifier boards and a great learning exercise. I am a lot more confident with the Smith Chart now and RFSim99.  I still have a heap of the AH101 devices so one day I would like to try 4 in parallel using Wilkinson dividers.

 

 

That might have been the end of the matter but I noticed the claimed specifications in the datasheet were not supported by the S parameters. Which should be embarrassing to a manufacturer!

To be fair, this is a discontinued part and the original manufacturer has changed hands several time. So let's look at the TQP7M9102. This is Qorvo's suggested replacement. What we find is a table of S parameters in the datasheet explicitly qualified by: 

"Test Conditions: V CC  = +5 V, I CQ  = 135 mA, Temp. = +25 °C, unmatched 50 Ohm system, reference plane at device leads"

The S parameter file downloaded states:

"The data is for the unmatched device in a 50 Ohm system, 2-port file de-embedded to the device leads."

So my expectation is this should be the same dataset. The best I can say is they are broadly similar. I drew this to Qorvo's attention. The response simply acknowledged the error. With that attitude I can no longer consider Qorvo for commercial applications which means it is unlikely any of their products will ever find a way into my ham projects. Once those AH101's are gone that's it!

 

73's

23CM 500mW Amplifier AH101

 I have not determined why the SXB4089Z input matching was out. After lots of maths and component substitution I suspect the mmic itself is damaged. I will swap it out soon to confirm this.

In the meantime I tested an AH101 device. This was made awkward because the test board was set out with matching networks for the AH1, which are quite different. With some hacking I got it to work. Input return loss was initially 17db but I was able to tune that to under 30dB with a little foil gimmick capacitor. Output return loss was 17db. Gain was 13dB.

 

73's

23cm 500mW Amplifier SXB4089Z

 After calming down over JLPCB's appalling process for dealing with their errors, which is to deny everything and obfuscate to the point I gave up, I populated one of the test boards for the SXB4089Z. My earlier posts here, here and here set out the steps I took to end up with the following:

 

Expected Results

My results did not quite match this. Using the nanoVNA to measure the input and output ports, adjust for the attenuation used and then combining everything into a single file for RFSIM99 to read showed that the output matching was nearly spot on (I will need to change the 1.2pF output capacitor to 1.0pF) but the input matching was all over the place. I repeated the measurements and got the same result.

Actual Results

I am confident that once the input reflections are resolved that it will only take a small tweak to get this working well.

73's

Richard

23cm Amplifier Test Boards are almost here

As many readers will know, the current Covid19 pandemic has significantly increased the time it takes for air freight to be received. But the test boards have all been made and are presently in Singapore.

23cm Amplifier SBB2088Z - More Navel Gazing

It occurred to me some time after I had prepared the PCB layout that I had left out the small input "stability" resistor in the transmission line version. Since the minimum resistance needed to ensure stability was 1 ohm I wasn't too concerned. Just putting 1 ohm into the actual circuit wouldn't change much would it?

Well, it turns out it did.I started from scratch and derived a circuit with a 1 ohm resistor and transmission line matching. I found that the capacitors adjacent to the 50 ohm ports were reduced by around 50%. And the transmission lines were around 20% different in length. Now 1 ohm is a really small amount of resistance and these are significant changes.

So I inserted a 1 ohm resistor into the original transmission line version and compared the two.

While there is a small variation in gain, all versions will be substantially the same in practice. Should I be concerned that 1 ohm made such a difference?

I put the difference circuits giving almost the same result as comforting in that the matching will be smooth and relatively insensitive. I look forward to receiving the boards to find out if this is indeed the case.

73's
Richard

23cm Amplifier Musings - SBB2089Z

While I got myself organized to order PCB's for testing the SXB4089 mentioned in the last few posts, I thought it would be worth looking at some of the other devices. The first of these I turned my attention to was the SBB2089Z, a 100mW device.

What I found, if you study the large image below, was that the bare device was acceptable for 6m through 70cm. However, at 23cm the output was not as good a match to 50 ohms as I had hoped (RL= 8dB ).

As expected, matching to 50 ohms at 23cm introduces loss of gain at other frequencies. There was also some slight chance of instability hence the series resistor in the second version to prevent these.

I'm going to get two boards fabricated for testing: one for 6-70cm using just the MMIC, and one for 23cm using transmission lines.


73's
Richard

23cm Amplifier - PCB Artwork

After plugging the various parameters into a micro strip calculator I had the line length and width needed for matching. The draft of the PCB looks like this :

If you have this device and would like the artwork in PDF format just ask.

73's

23cm Amplifier Development using RFSim99 - Part 2

Following on from Part 1, here is how I plan to replace the matching netowrks with microstrip.

Step 1 : Input network

1. Remove the LC components on the input network.
2. Press Simulate and read off input impedance of 4.96R+j3.93R
   
   
3. We know that one full revolution of the Smith Chart is 23cm/2. By inspection we can see that we need a transmission line around 62 degrees (135 degrees/2)
   
   
    
4. Insert a 4cm transmission line (62degrees electrical length at 1296MHz) and press Simulate
   
5. Not quite long enough. Trial and error guesswork gives us 4.4cm. The alternative to this inspection approach is to use a program like Iowa Hills Smith Chart which I prefer. But I'll stay on-topic.
6.  Now we need to move along the arc towards the origin with a series capacitor. I started with 6pF, since it was there on the output network. There was little movement so I halved it each iteration. I found 0.82pF was the closest standard value. You can buy these!
   
   
   

Step 2 : Output network.

1.  Repeat along the lines in Step 1. It is a nice coincidence that the output transmission line is the same length as the input line. Since we hit the arc running towards the origin a littel closer to the origin than the input network did, we can use less reactacne. I guessed 1pf. Turns out 1.2pF is better.

Results

We still have stability and good matching. Gain is  unchanged at 18dB. But now we have two standard value capacitors and some PCB drafting to do.

I'll post about the artwork next.

23cm Amplifier Development using RFSim99 - Part 1

I have an assortment of MMIC's recovered from a cellular base station. Since an engineer previously selected these as suitable for a device where linearity matters I thought they would be good contenders for UHF and microwave projects. The SXB4089, a 500mW output device, looked interesting for a 23cm transmitter so I decided to document my approach. This post concerns the steps leading up to  generating PCB artwork for a test board.

Step 1 : Set up RFSim99

1. Load the S2P file into a unmatched schematic and press Simulate
2. If not auto-scaled set the lower frequency to 50MHz via the Graph Limit Setup command
3. Repeat for the upper frequency of 6.05GHz
4.  Start with 500 points and reduce until we get something close to 1296Mhz. 497 is just right

Already we note that matching will be needed on input and output where the return loss is too high (S11=-0.84dB, S22= -2.46dB)

Step 2 : Check for Instability

1. Switch to the Smith chart. S11 and S22 plots show the unstatble region is outside the smith chart boundary
2. With S11 sweep the frequency by dragging the slider. Very small possibility of instability at 630MHz noted.
3. Sweep frequency for S22 display, note large range of unstable loads at 473MHz.

Step 3 : Fix instability

1. I try a range of resistors in series with Port 2. Nothing appeals.
2. Repeat for Port 1. I quickly establish that a minimum of 1.5 ohms brings stability on input and output at all frequencies.  
   
3. Let's be a little conservative and use 2.2 ohms going forward.

Step 4 : Matching

1. Drag the slider so the frequency is 1296MHz.
2. Press Auto Match
3.  Be amazed at how clever Stuart Hyde is. I'd like to meet him to say thank-you in person.
4. A conjugate match simultaneously brings input and output to 50 ohms and updates the schematic
5. Sweeping with the slider still shows no instability

Results:

A stable amplifier with a gain of 18dB matched on input and output to 50 ohms.

You could change the values to he nearest standard value, and press Simulate to see what happens to gain, matching and stability. However, in Part 2 I will cover how I plan to replace the matching networks with micro strip.

1296MHz or 23cm Beacon

A nearby Ham, and a good friend of mine, VK6KDX Jack is working on an EME station for 23cm. Locally our 23cm beacon has been off air for eons. For testing purposes Jack needed a distant signal source. Here then is the 23cm beacon, measuring 10cm x 5cm, under construction:

The beacon is now complete and tested. The ident is in CW every 60 seconds. The PLL chip is loaded at start-up by a STM8S1003F, or STM8S003F, micro running the fantastic STM8 forth with more details here. This micro also gates the amplifier chain to produce the morse ident.

I used parts I had in the junk box. The PLL was recovered from some TV modulator boards I have. The varicap came from another TV demodulator board that I had. The mmics were all recovered too. Except for the final mmic. I had planned to use the AH1 mmic shown in hte phot above which is a 100mW output device. However, both my recovered mmics were broken. So I used an AH101G instead which is a 500mW device. The standing current of 200mA from an 8V supply means 1.6watts is being dissipated which my board wasn't designed for (the AH1 is only 0.75W ). To solve this I used some copper tape to better connect thermally the two sides of the board and create a small fin. I also attached a heatsink to the opposite side of the board. So far, so good. The device hits 75 degrees Celsius and should survive the abuse it will receive.

I'll just have to buy some AH1's and either retrofit one, or build another beacon up. I have a lot of boards left over. Please contact me if you want one. I can furnish a partially populated and programmed board for you.

The code can be found here.

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!

Voltage Controlled Oscillators (VCO) for 23cm

One of my long term projects is to build a 23cm  FM transceiver. This post summarises a few experiments I conducted with a view to making a wideband VCO for testing purposes. The basic variations I tried were all varactor tuned:

1. Lumped LC (or perhaps more like parasitic LC at these frequencies),
2. conventional semi-rigid coax
3. DRO, or dielectric resonator
4. stripline

Version 1
Narrow tuning, layout needed more attention to get a higher frequency

Version 2

Despite my best efforts using a short piece of coax as a tuned circuit I had no real success with this approach. I was able to achieve oscillation but I didn't find this approach for me, a mere mortal, as being something I could reproduce when ever I wanted an oscillator at these frequencies.

Version 3
My DRO attempts were similarly mixed. I could achieve oscillation, but my junk box did run to a huge number of DRO's. I even tried modifying the DRO's found in old mobile phone filters.

I could get these oscillators to work. But just like crystals you never seem to have one on the frequency you want. I ended up achieving a maximum frequency of 1150MHz with this 1600MHz DRO. It seems from my testing of the DRO's I had that in general the maximum frequency when used in an oscillator is around 70% of the resonant frequency of the DRO alone.

What was interesting was how critical the board layout was. Putting a piece of copper foil under hte DRO, thereby extending the groundplane, lifted the frequency of this oscillator from 904MHz to 994MHz. That's about 10%. Placing a via next to this extension lifted the frequency still further to 1105MHz. I never managed any further significant increase in frequency. A raft of fiddling only achieved 1150MHz.

Version 4

Days had passed fiddling with these approaches when I tried a strip line version of something I had seen in "VHF Communications" once and at http://lea.hamradio.si/~s53mv/spectana/vco.html. Almost immediately I enjoyed success. I now had an oscillator I could  build over and over and I found that each time I built one it worked, albeit with some modest alteration to the stripline length if my guess was off the mark.

After my first attempt I decided to dispense with direct coupling the mmic to the transistor in the oscillator. Instead, I used a fourth stripline adjacent to the resonator striplines feeding a mmic to lift the output level. The circuit, shown below, has proved to be repeatable. The stripline length may need to be varied depending on your substrate and varactor properties. The stripline to change is the centre stripline which runs between the collector and base striplines. Shorten the stripline to raise the frequency, or lengthen the stripline to lower the frequency. Easy with a knife or a small piece of copper foil cut into a strip and soldered onto the existing stripline to lower the frequency. If you didn't have any copper foil, you could also use the shield from some thin coax or a piece of thick copper wire.

Apart from construction errors and defective parts this should always work. If you can't get it to oscillate try a transistor with a higher ft.

While I used some 6 pin mmics I recovered from a PCB, you could use just about any mmic that comes to hand if you modified the board layout.

Top layer of Stripline VCO, bottom layer is groundplane. Here the mmic is loosely coupled directly to the oscillator

I built one of these with a sweep generator on the pcb which I will post details of later. The pcb, which when compared to the direct coupled mmic version above, should help explain the principles of the layout.

Regards
Richard VK6TT