80m SSB Receiver - Weaver method

Update:

I am living through a record breaking heatwave (6 days above 38c) so it's strictly indoor activity at present. I did waste a few hours uploading the gerbers for the boards shown below to a PCBA bureau. Turns out that with another day or so on my part I could consolidate both boards and by adjusting the components get 5 populated boards delivered for around $20 each (say USD14). All that would be left is a few through hole components. The op-amps were changed to the surface mount version. Tempting? Or is it the heat getting to me?

73's Richard

This is part of a longer term project to build a complete transceiver. Initial testing of the receiver boards shows demodulation and opposite sideband rejection are taking place. I look forward to mounting these boards on a chassis together with the audio amplifier / AGC / squelch board and synthesizer board for further testing and evaluation.

73's

Richard

Using a D2E-S-DC12V to Switch RF at 6M

 

Initial tests are encouraging. Insertion loss less than 0.3dB and both isolation and return loss around 35dB.

Time now to put it on the 6m radio and see how it copes with 50W of FM.

73's

Richard

1 Watt HF Class A Buffer Amp - Part 1

 I'm slowly assembling pieces for an 80m Weaver based transceiver.  It's really hot now in the workshop with summer arriving so I retreat to the house after 9am. The receiver is taking shape so I'm turning my attention to the transmitter while in the house.

Reviewing the 1 Watt Class A buffer I have used in previous projects I found obsolescence had arrived. While I still had some of these transistors in the parts bin it felt appropriate to cast around to see what else was out there.

1Watt HF Buffer - Class A

While this amplifier has served me well I'm going to let myself be distracted and test how this amplifier chain works with transistors readily available today.

Q4 could be a SOT23 device. I've got plenty I can try and it's not worth agonising over. Q5 should be something more substantial to cope with the heat it will dissipate. Easily found I've ordered a couple of types of SOT89 transistors for Q5.

Q6 dissipates around 1.5W so I wanted something capable of handling at least 2W. I think I have found suitable transistors though the package of each is different.

Let the testing begin! 

73's

Richard

Noise in "Jelly Bean" regulators - conclusion

Some rules of thumb emerged from my investigations:

1. If the data sheet makes no mention of a regulators output voltage noise then it's going to be horrible until confirmed otherwise. Maybe only good for digital projects.
   
   
2. Any 78Lxx or LM317 type regulator followed by a  capacitance multiplier will deliver a clean supply voltage for low power analogue circuits. But the voltage drop of the capacitance multiplier needs to be either adjusted out or allowed for.
   
   
3. A TL431 is a worthy alternative, and with a capacitance multiplier will deliver a cleaner supply voltage for demanding applications. 

My new standard for low noise regulation is the TL431 and capacitance multiplier. 

73's

Richard

Noise in "Jelly Bean" regulators - Part 5

 After an eternity I finally got a low noise amplifier inside a metal box that didn't oscillate! So, I quickly compared two regulators on my voltage regulator module:

    Regulator                                            Amplified Noise pp

    78L08                                                200mV

    TL431                                               15mV

    78L08 + capacitance multiplier        4mV

 

These readings would need to scaled to match previous results. But for now I have confirmed, in my mind, how much better a TL431 is over a jellybean regulator.

73's

Richard

GM300 - Replacement Brain ?

In my experience modifying ex-commercial radio gear  for ham use is a mixed bag:

• The Tait 500 was poor when converted to 6m due to powerline interference when mobile
• I have a brand new VHF Motorola GM328 tuned to 2m which appears to suffer interference from broadcast band FM (pcb inductors in a tuned circuit with varactor tuning means low Q so broadband front end with no selectivity).
• Unilab's have been very good
• The Philips PRM80 converted to 2m is perhaps the best mobile radio I have had with regard to coping with pagers. 

Mind you, the Icom and Yaseu mobile rigs I own have been sadly lacking.

So when I opened a Mororla GM300 I was pleasantly surprised to see a radio with shielding over every coil in the receiver front end and a diode mixer. This was a departure from the norm and worth investigation. 

Alas, trying to reprogram the radio for 70cm use was fruitless. After a couple of hours trying every serial interface I had and several versions of the Motorala software I had no success.

Which got me thinking. It's a fairly standard PLL circuit. So I pulled the entire synthesizer/receiver/exciter board from the radio. Powered from the bench using STM8 eForth and a $1 minimal development board I quickly had the PLL loaded up.

Only problem was the oscillators would not lock up oin the required frequencies. Padding with a few surface mount caps was partially successful. The tuning range is only just wide enough.

At this point I have a proof of concept. Driving the LED display on the head looks straightforward. But I am hampered by only having a partial circuit of the radio. If you can help me it would be appreciated. 

73's

Voltage Regulation - Distributed or Lumped?

I'm getting tired of the semiconductor supply disruptions which manifests itself as price increases and supply shortages. Even humble voltage regulation is being impacted.

As an example of the voltage regulation quandary consider the synthesizer board in my present project, a HF SSB transceiver based on the Weaver method.  In the past I would have slapped a 7805 linear regulator on the chassis for heat-sinking and used wires to connect it to the PCB. But this project is being designed with modular PCB's sized 100mm x 33mm since this fits into a readily available extruded aluminium tube. So less wiring from the chassis to pcb's inside tubing is better.

An AD9850 based synthesizer with micro, mmic and switchable outputs draws some 150-180mA. Now I can use a 7805 type linear regulator in a TO-252 (DPak) package but I have run out and the replacement will cost around $1. It could have cost as little as $0.30 but the wholesaler is out of stock. 

However, I could use 3 separate regulators on the board and that would cost around $0.15-$0.20. But my choices are much wider and the heat to be dissipated is spread over 3 regulators. My parts bin probably means I wont have to buy any regulators, a good thing, if I go down the distributed path.

Will normal ever return? After 12 months of the supply disruptions I am starting to wonder.

73's

IC-R8500 Repair

Was glad to help a good friend, VK6UM Larry, with the repair of an IC-R8500 recently. After some digging around it was clear this was a later version than the schematics I could find online applied to. The fault was the blinking display, a characteristic of a PLL not locking up. 

A few minutes with my ICQ7A used as a sniffer confirmed VCO_A had no output. A few voltage measurements and it appeared the special Icom chip, IC3, was not enabling the power to the VCO. Knowing that chip was likely to be unobtainable, and for thoroughness, I checked the main micro was sending instructions to the synthesizer board. 

I was able to confirm both shift registers, 4094's, were receiving pulses. But the shift register feeding the special Icom chip had no signals on the output pins that drive pins labelled "CON0" through "CON3". It seemed a reasonable assumption that these pins mapped to the output pins enabling the VCO's. So a replacement shift register was obtained.

Upon installation there was no change. A closer examination showed the data pulse was too low in amplitude to be considered a logic high. It transpired, after a fair amount of thought and measurement, that the data line was being loaded by a 130 ohm resistance that should not have been there. The fault was traced to a capacitor array on the main board having broken down somehow. Once removed from the PCB no amount of cleaning the defective part could remove this unwanted resistance, though it did change in value. 

A replacement part could not be found. So in desperation something was fabricated in-situ with 1206 22pF caps and fine wire. Partial success. The radio is now working within narrow segments of it's wideband coverage.

IC-7000 and the 10 Ohm Thin Film Resistor Fix for Instability

I had a request from VK6EH recently for a 10 ohm "thin film" resistor to allow him to make this modification to his radio. After discussing the pro's and cons of thin film resistors and standard surface mount resistors we concluded a "thin film" part was not going to be any different from DC to 430 MHz when used in series with the gate. I don't know where this recipe originated but it reminds me why everything you read has to be treated with scepticism. Peer review is not the same as something is just repeated ad nauseam.

I can't recollect seeing a datasheet for a surface moiunt resostir that wasn't a film construction.So how bad is a standard off the shelf 10 ohm 1206 resistor with regard to parasitics? Since 1nH has 1ohm of reactance at 160-MHz (that's how I deal with stuff in my head), at 430MHz 1ohm of reactance requires around 400nH. (F increased say 2.5 times, so inductance divided by 2.5).

I measured an ordinary 10 ohm 1206 resistor:

 

I can't see how the small parasitics present in a 1206 resistor would make any difference at 430MHz. At HF they parasitics are truly small.. I don't have any 0805 or 0603 size10 ohm  parts to compare this with but I suspect they are even better with regard to parasitics. 

 I'm calling the need for a special thin film resistor busted. And looking at some of the information on the web about these failures is like reading tea leaves and hearing mumbo jumbo. 

The PD55015 is a very rugged part. I dragged the S parameters into RFSim and immedialtely saw that at 50MHz it is potentially unstable. However, the 10 ohm resistor is only a marginal improvement. It might be enough given the radio's remaining circuitry was excluded from the analysis.If I was doing this I'd tend towards 22 ohms and any SMD part that fitted.

73's

Noise in "Jelly Bean" regulators - Part 4

I again revisited the concept of a low noise audio amplifier. The excellent Op Amp noise calculator suggested that the NF of my amplifier was 26dB. Now that got me thinking: pretty any random mmic has a NF of below 6dB. So why not use a mmic rated to work from DC upwards as a front end?

I didn't have much luck finding any prior work in using a mmic as an audio amplifier. But it did trigger the recollection that Direct Conversion receivers use low noise audio amps and that "Solid State Design for the radio Amateur" had used an all transistor amplifier in a conceptual DC receiver. 

That evening I sat in front of the TV with a calculator and during the commercial breaks reworked the biasing for use with a 9V battery and reduced the overall voltage gain to 1000. That should allow me to see input noise levels down to perhaps 5uV. I ended up with the following:

 

Unfortunately my amplifier was a very good oscillator when the input was shorted or terminated in a low ohm resistor. I haven't been able to stop the oscillations so a different amplifier is going to be built.

73's

Relay Life for Relay #2 - Degradation Found

While the abused relay performed surprisingly well (see here) it did start to degrade. After 6,449,686 cycles the contacts did not open for some 63 cycles. The evidence for this was the declining voltage across the contacts as the constant current regulator warmed up and current limiting set in.

After this point there were numerous "runs" where the voltage across the contacts was much higher than average. 

If relay performance is considered as good then degrading to failure, at what point has it failed? Apart from the periods of degraded performance the relay is still switching reliably at 8.4 million cycles.

In light of the degradation seen at 6.4 million cycles I went back through the data. If the symptom of contacts not opening correctly is a decline in the voltage across the contacts over time, then the relay's first, albeit brief, period of degradation started at 44,296 cycles. 

It would appear that the relay is still fit for the intended purpose of TX/RX changeover. Now it's time to sort our the isolation and loss characteristics.

73's

9V 1Watt Led Driver for single Li-ion cells

 Recently I stumbled across the MT9284 led driver IC. The datasheet shows it can be used for strings of 7 white leds. There is a suggestion in the graphs that the maximum led current is perhaps 20mA. 

I also had some 1 watt white leds by Bridgelux, BXEN-65E-13H-9B-00-0-0. These are approximately 9V at 110ma. 

Can the MT9284 drive one or more of these? Yes. I have tested to date one Bridgelux device at currents up to 80mA. The efficiency has varied from 83% to 92%, depending on led current. 

Nice and bright! I'm going to update my collection of homebrew pocket torches. I find as I get older that the part markings are getting harder to read and a bright light source that can illuminate the part from different angles makes a big difference.

More soon.

73's

Noise in "Jelly Bean" Regulators - Part 3

I repeated the exercise with a 78L08 and noise fell from 480mVpp to 30mVpp. Two surprises - the 78L08 was less noisy than the LN6206. I had expected it would be noisier since higher output voltages tend to have more noise for the 78Lxx parts. And apart from the 78L08 being quieter than I expected my test apparatus is not perfect. 

I had expected that the capacitance multiplier would reduce noise by the same factor as it did with the LN6206 ( 30 fold, 1800mV / 60mV ). But in this case I was only getting a 16 fold reduction (480/30). I checked and the unterminated amplifier was producing ~1.5mVpp on the CRO. However, when I put a 75 ohm termination on the input BNC the noise increased to ~30mVpp. Which requires more investigation.

Still, the capacitance multiplier makes a substantial difference and will become a standard inclusion on future analogue projects.

Flush with some sort of success I put a TL431 onto the test board with no capacitors anywhere in circuit and with an approximate output of 8 volts. 

There was no change in the 30mV of noise on the CRO. So while I cannot quantify how quiet a TL431 is, it appears to be a substantial improvement on the 78L08 alone. I am also unable to determine if the TL431, with capacitors, is an improvement. Or indeed if a capacitance multiplier is worthwhile.

But I'm not beaten yet.

73's

Noise in "Jelly Bean" Regulators - Part 2

On the second attempt my low noise amplifier for measuring regulator noise worked. It's based on a dual low noise op-amp with an overall gain of 300 (x10 followed by x30) and no signal noise of a few mVpp. The very thorough noise calculator I used suggested I should have a better noise floor so further investigation is needed. 

I quickly checked a LN6206 regulator to confirm the approach and saw 1.8Vpp of amplified noise on the cro. That works out at around 6mVpp for the regulator which I might have been able to estimate with the cro itself. But now I can experiment with different noise reduction techniques and relative comparisons between regulators without having to estimate differences at that low level.

I started by trying a capacitance multiplier.  I used a 10k resistor, a 1uF tantalum and a general purpose npn transistor. That reduced the noise from the LN6206 from 1.8pp to 60mVpp. That is a large reduction in regulator noise with really no effort. 

 73's

Noise in "Jelly Bean" Regulators - Part 1

In the course of investigating the retrofit of a low noise RF pre-amp to an old 2m mobile, RT85, a random thought from nowhere made me stop and consider regulator noise. 

My goto part for 3 volt regulators in microcontroller projects is the LN6206. Cheap and reasonably low quiescent current. However, there is no specification of the output noise voltage. 

Since I had seen a TL431 on a PCB earlier in the day the device was foremost in my mind. I wouldn't need a pre-regulator to reduce the voltage to the LN6206 which is a 6V maximum input device. However, the TL431 datasheet I had also made no mention of noise.

After a bit of searching I became cynical of the claims the TL431 is low noise. I found just one objective assessment of noise which helped reassure me. Going from 50 nV/√Hz to noise in a 0-100kHz bandwidth and adjusting for gain from the 1.25V band gap voltage of the TL431 to 3V returned 38uV. Which is almost identical to the 40uV output noise voltage of the L78L33C. 

I'm not sure that qualifies as low noise, since a LT3094 is specified as 2 nV/√Hz but costs over 50 times more! I note in the web page above that an additional capacitor could be expected to reduce the output noise voltage considerably. Back of the envelope suggest under 10uV.

Can we do better? It appears so. A simple one transistor circuit after the regulator should make a large difference. 

Verification is the hardest part. I could not find a low noise amplifier circuit that I could re-create with my junk box. My attempts to build a low noise amplifier in a shielded box are yet to bear fruit. 

My whole approach to electronics is to re-use previous work until the need for improvement arises. While trawling through the internet I saw numerous statements that low noise regulators are vital to oscillator performance. This is not something I had considered beyond decoupling supply lines. So it appears the need for improvement exists!

73's

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

Relay Life for Relay #2

After some 10 days of continuous operation the test board flash memory was full with over 2 million cycles of data at a constant 2 Amps using a constant current generator. Below is a picture of the actual relay, nearly destroyed when it was removed from its original PCB.

 

I chose this recovered relay because it was damaged. It was surprising that despite the abuse during removal the results, below, show there was no real degradation over 2 million cycles at the rated contact current capacity. Perhaps 30 volts at 2 Amps would give a different result compared with 12 volts at 2 Amps. But for now I am completely satisfied that this relay, of which I have recovered dozens, will be a suitable basis for use as a 50 watt 6m changeover relay.

 

To put this in context the ADC returns a reading of 214 for 0.685volts. For each cycle 16 readings (the graph was wrongly label as the sum of 8 readings) of the voltage across the relay contacts is taken, summed, and stored for the analysis. So a sum of say 200 means there was 40 mV across the terminals. With 2 Amps passing through the contacts that means the resistance was 20 milli-ohm (0.020 ohm).

That is probably enough from me on the subject of relay life. I'm satisfied that operated within the specified ratings a relay, even when recovered, is not going to wear out. All you have to watch out for is  relays re-printed with over-stated specifications sold by the Chinese on Ali-express etc. Similar to the ludicrous ratings printed on 18650 lithium batteries they sell.

73's

Relay Life - Tests going forward from 5 Million Cycles

After a cumulative 5 million cycles in total the increase in contact resistance is conclusive. Shown below is the final 3.1 million cycles.

 OK, so I was surprised to lean just how robust these inexpensive Chinese relays are. But I also learned what failure looks like and by using various statistical measures I have concluded that going forward I will:

1. measure the voltage across the contacts and save only the sum of 4 readings. This gives me just as good a measure of increasing resistance as 8 readings.
   
2. Opening and closing times have no strong correlation with contact resistance so I will not be recording them any more.
3. Measurement will start at the same time reading 6 is currently taken. The average of readings 1-5 are statistically different to the last three readings, but that difference is decreasing i.e. reading 1 is more different than reading 5 when both are compared to reading 8. Without the RC network this effect is still present. So perhaps this is a heating effect of passing current through the contacts?

Which all means the external flash memory chip I used can now records over 2 million cycles. Which is fortunate because my interest is switching to these 2Amp signal relays for use as a 50 watt 6m changeover relay.

 

I will test one of these shortly at a constant 2 Amps using a constant current generator. These should fail relatively quickly at 2Amps. I might then redo the test with the RC network with an expectation they will fail really quickly. But all I need is 10,000 cycles to meet my goals. 

73's