Why Does My Varactor Diode Test Circuit Behave Differently on a Breadboard?

[piepermd]

It looks to me like it has RF tuning only on RX, but has two separate switched four channel crystal oscillators, one for TX, the other for RX. It should be possible to replace the crystal oscillator(s) with digital synthesizer board(s). The harmonic multipliers that generate the TX carrier, and the LO for RX, can still be used.

There should be no problem using varactors to tune the small RF signals in the RX path, but you can expect problems generating the higher amplitude RF needed to replace the crystal oscillators. For those, you should look at synthesizer ICs that include a current controlled CMOS oscillator inside a PLL.

Yes, I’m using the Si5351 chip for that

 

[piepermd]

…and it works beautifully!

 

[Baluncore]

Yes, I’m using the Si5351 chip for that

If you can read the AGC voltage with the processor, you can inject a small RF signal into the receiver front end. The processor can do a peak search to optimise the RF tuning, without needing to store three calibrated tables of data. Depending on the time constant of the AGC, which you can switch, the tuning process can take less than a second, whenever you change the channel frequency.

 

[piepermd]

What is the AGC voltage?

 

[Baluncore]

AGC = Automatic Gain Control.
https://en.wikipedia.org/wiki/Automatic_gain_control

The receiver signal strength meter shows the AGC voltage. The AGC voltage controls the gain of the IF amplifier strip. The received carrier voltage is detected at the output from the IF strip and maintains the carrier level at the detector.

If you have the same circuit for the SCR-522 as I do, you will see the meter connections to the second grid of the RF amplifier, in the bottom right-hand corner of the BC-624A receiver block. I think the signal was there called A.V.C. (Automatic Volume Control) and is connected to all IF amplifiers. It is also used to mute the receiver during transmit.

 

[piepermd]

AGC = Automatic Gain Control.
https://en.wikipedia.org/wiki/Automatic_gain_control

The receiver signal strength meter shows the AGC voltage. The AGC voltage controls the gain of the IF amplifier strip. The received carrier voltage is detected at the output from the IF strip and maintains the carrier level at the detector.

If you have the same circuit for the SCR-522 as I do, you will see the meter connections to the second grid of the RF amplifier, in the bottom right-hand corner of the BC-624A receiver block. I think the signal was there called A.V.C. (Automatic Volume Control) and is connected to all IF amplifiers. It is also used to mute the receiver during transmit.

OK, I got it. Let me chew on that a bit. The main problem now is that I am not getting expected results even with the simplest of Varactor circuits.

 

[piepermd]

Are you saying I can send the desired frequency to the receiver’s RF coil at a low level and then do a reverse bias voltage sweep to the varactors to look for a maximum RF voltage at the 288 meter output?

 

[Baluncore]

Are you saying I can send the desired frequency to the receiver’s RF coil at a low level and then do a reverse bias voltage sweep to the varactors to look for a maximum RF voltage at the 288 meter output?

Yes, but I don't know what your 288 meter output is. Maybe a link to the circuit diagram you are using.

I have done it before. You inject a small test carrier as a fake RF signal into the RX front end. It is RF amplified and converted to the IF frequency, amplified along the IF strip, then detected by the audio detector where the slowly changing DC component becomes the AGC voltage. For an AM receiver, the AC component of the detected signal is the audio. By changing the varactor bias and reading the AGC voltage, the processor searches for the best signal peak versus varactor bias voltage.

There seem to be two RF tuned circuits that would benefit from tuning. Then the IF is 12 MHz all the way through, so that does not change. Maybe the local oscillator also needs tuning, but that will depend on how you generate and inject the LO into the mixer.

For normal operation, the AGC must be slower than the low frequency audio, so you can benefit from speeding up the AGC time constant during tuning. You should be able to optimise the RF tuning in less than a second. Once autotune is done, turn off the injected RF and switch back to the normally slow AGC.

 

[piepermd]

The 288 meter output is the meter connections to the second grid of the RF amplifier as you described above.

 

[piepermd]

How do I change the AGC time constant?

 

[Baluncore]

How do I change the AGC time constant?

It turns out the AGC is connected to bias G1 of the RF amp, and the 1st and 2nd IF amps. The AGC is the lowest line on the diagram below the IF strip, that goes to the "AVC", terminal 6 on the main receiver connector.

Just to make it interesting, the receiver IF strip amplifies the signal from RF on the right to the AGC and audio detectors in the same stage on the left. The detected AGC is low-pass filtered by resistor, 262-2 and capacitor 211-C before distribution. I believe that LC circuit sets the AGC time constant.

There is an RC filter at each of the three stage bias connections that allows the AGC bias to pass, but prevents the RF or IF output getting back to the RF or IF input via the AGC line, which would cause it to howl like an oscillator. Hopefully the time constant of those attenuators will be short and can be ignored.

The +HT supply, drawn above the AGC line on the diagram, is also bypassed to every stage, but also has a bigger reservoir capacitor to ground, 206-15. The meter connections appear to be part of the +HT circuit.

 

[piepermd]

Yes, I can see what you mean on the schematic. So from exactly where do I send the voltage to the microprocessor to look for a peak when sweeping the bias voltage to the varactors? I still don’t know how to change the time constant, and I don’t know what you mean by the “+HT supply”.

 

[Baluncore]

So from exactly where do I send the voltage to the microprocessor to look for a peak when sweeping the bias voltage to the varactors?

You will need to check the range of the AGC voltage by injecting a big and a small test signal for the selected channel. That safe voltage you give to the processor will be derived from the AGC voltage, since that sets the gain of the strip, needed to overcome any off-tuned RF circuits. As you tune the RF, the AGC will change.

I still don’t know how to change the time constant,

I would disconnect capacitor 211C and replace it with something three orders of magnitude smaller, while watching the AGC line with an oscilloscope. If higher frequency audio modulation appears on the AGC line, then you have shortened the time constant. When you inject a sample RF signal without modulation, the AGC will change quickly to show the gain of the receiver.

You might use a relay to change the TC during tuning. If the voltage was low enough, I would consider a FET of some sort.

and I don’t know what you mean by the “+HT supply”.

+HT is the high tension positive supply, being hundreds of volts. It supplies the positive voltage to the (anode) plates of the vacuum tubes, usually through an RC low pass filter, then through the primary of the tuned coupling transformer. The transformer secondary drives the grid of the next stage. Prepare to be confused, the signal flows right to left in that circuit diagram.

 

[piepermd]

Thank you SO much for your help with this project thus far! I’ll keep you posted…

 

[Baluncore]

I assume you have checked your Private Messages.

 

[piepermd]

Yes, that was great! Thanks again.

 

[piepermd]

My receiver has been somewhat modified, using some of the ideas from the BC-624-AM and BC-624-C circuits. I wasn’t sure how much of this had been implemented in my radio but I spent the last two days going over it and it appears to match the mods published for HAM conversion many years ago. Here is a schematic showing the mods. I’m not sure how this might affect using the AVC voltage for tuning.

 

[Baluncore]

I’m not sure how this might affect using the AVC voltage for tuning.

If you have an S-meter, then you have a voltage and current signal proportional to signal strength. If that can be converted to a low voltage, it can be digitised by the autotuning processor during the peak search.

 

[piepermd]

I do have the S-meter installed

 

[piepermd]

Can I just use a pair of resistors as a voltage divider to decrease the voltage for input to the microprocessor pin? Do I still need use a relay to switch the capacitor to change the time constant of the AVC circuit?

 

[Baluncore]

Your attached circuit became blurred when attached as a file.webp
We need some way to prevent the forum corrupting schematics.

Maybe you could email me the original diagram as an attachment, or provide a link to the web discussion with the diagram.

 

[Baluncore]

Can I just use a pair of resistors as a voltage divider to decrease the voltage for input to the microprocessor pin?

Now I see the circuit, the 250k S-meter pot, used I assume to adjust FSD of the S-meter, is such a divider chain and the voltage on the wiper contact will be at the lower end of the voltage range. Check out that wiper voltage with an oscilloscope. The S-meter is connected to the VT-207 plate circuit and the +HT supply, which is not good for A-D converters.

I am not yet sure how the AVC on/off switch works, or what sets the time constant. I will look at that in 8 hours when I get back.

Usually the AGC will be automatic when listening to several different transmitters, but when listening to only one, the AGC voltage is set with an RF gain pot, so the detected voice is clear, and the background noise does not rise to fill the quiet gaps in speech.

 

[Tom.G]

The top of the 250k S_METER pot is the source of AVC voltage, negative-going with increasing signal strength.

The AVC voltage is tapped off thru the resistor going to the left of that junction. The AVC time constant appears to be 15ms, if i'm reading the blurry values correctly of 1.5Meg and 0.01uF.

The switch above those two components switches the AVC line to ground (OFF) to disable AVC, and to the 15ms RC filter (ON) to enable AVC.

The left half of the dual diode tube, 12H6, is the AM detector, the right half is the Noise Limiter, whose threshold varies with received signal strength.

Hope this helps!
Tom

 

[piepermd]

Outstanding, thank you! I just need to be sure that the maximum voltage at the analog input pin is less than 5V. I imagine the voltage will vary quite a bit depending on the strength of the test signal applied to the the RF coil and this will need to be optimized as well. What I really like about this approach is that so many variables are eliminated, and I would no longer need to use a calculated bias voltage based on a theoretical model of the varactor’s behavior but can use how the circuit responds under real conditions.

 

[piepermd]

The top of the 250k S_METER pot is the source of AVC voltage, negative-going with increasing signal strength.

The AVC voltage is tapped off thru the resistor going to the left of that junction. The AVC time constant appears to be 15ms, if i'm reading the blurry values correctly of 1.5Meg and 0.01uF.

The switch above those two components switches the AVC line to ground (OFF) to disable AVC, and to the 15ms RC filter (ON) to enable AVC.

The left half of the dual diode tube, 12H6, is the AM detector, the right half is the Noise Limiter, whose threshold varies with received signal strength.

Hope this helps!
Tom

It helps, thanks! I didn’t realize the voltage is negative, though. That tends to wreak havoc on a microcontroller input pin. There are some ways around it, but I think it will make the circuitry more complex.

 

[piepermd]

I think I can do it with an inverting opamp without a rail to rail supply…

 

[Baluncore]

It helps, thanks! I didn’t realize the voltage is negative, though.

There is a difference between a control voltage being negative with reference to common ground, and a positive AGC voltage that falls as the applied signal increases in strength.
A voltage may become "more negative" as it falls, without actually being negative relative to a reference voltage.

It is wise to use a Zener diode and series limiting resistors to prevent the AGC signal sensed, from exceeding the analog input range.

You need to measure some voltages to get an idea of what changes are needed.

Do you have a link to the Ham mods to the BC-624 ?

 

[piepermd]

There is a difference between a control voltage being negative with reference to common ground, and a positive AGC voltage that falls as the applied signal increases in strength.
A voltage may become "more negative" as it falls, without actually being negative relative to a reference voltage.

It is wise to use a Zener diode and series limiting resistors to prevent the AGC signal sensed, from exceeding the analog input range.

You need to measure some voltages to get an idea of what changes are needed.

Do you have a link to the Ham mods to the BC-624 ?

“Negative-going” was an unfortunate turn of phrase. A decreasing positive voltage is obviously not “more negative” or “negative-going”. Two hours of my life wasted by that, but I’m glad I don’t have to worry about it. Since the AVC voltage decreases with increasing signal, am I not then looking for a dip rather than a peak? Here’s the whole book:

https://radionerds.com/index.php/File:Surplus_Radio_Conversion_Manual_Volume_1.pdf

 

[piepermd]

It appears from my further reading that Tom is correct- the AVC voltage is negative in polarity with respect to ground due to its rectification by the 12H6 diode and becomes increasingly negative with an increase in signal from the last i-f transformer (294). This added negative bias is applied to the control grids of the previous stages to decrease their sensitivity. Thus I must look for the greatest negative value of the AVC voltage when applying my bias sweep to the varactors. My apologies!

 

[piepermd]

I hope I didn’t offend anyone with my comments. I didn’t understand how the AVC voltage could be negative and this led to some frustration on my part. I won’t make that mistake again! The advice here has been extraordinarily helpful and I am very grateful for your time and expertise. I hope you will continue to assist with my project.

 

[Baluncore]

The circuits provided lack all the normal voltage information needed to quickly service the equipment. I see the 12H6, 12AH7 and 9002 all have a hard grounded cathode. When I see the cathode of a VT tied hard to ground, I look more deeply into the design to find out when, if, or how the grid goes negative. Those circuits are probably being used as switches, not as linear amplifiers.

Thus I must look for the greatest negative value of the AVC voltage when applying my bias sweep to the varactors.

I really wonder how far you can safely extrapolate from the many interpretations of the existing documents, before you make an actual measurement of a real voltage.

 

[piepermd]

Many thanks for your reply! I am finishing the power supply for the receiver so I can make some tests. I have cleaned and deoxidized all of the tube pins and sockets after testing the filaments for continuity and applied voltage to the heaters and they all glow. Once I have the B+ voltage online I can apply an LO signal directly to the 225 harmonic oscillator coil and a carrier signal 12 MHz higher to the 221 antenna coil. I have already removed the mechanically variable capacitor mechanism that was installed for the ham mod. Am I correct in thinking that I will still see the AVC voltage fluctuate with changes in the applied RF signal level? Since I’m applying a relatively pure signal at the front end, no tuning is required to test the AVC voltage? I imagine that the amplitude of the signal passed to the IF strip from the front end will be somewhat less because the tuning circuits are not in resonance. For the AVC voltage, do I probe between the S-meter adjustment pot wiper and chassis ground?

 

[Baluncore]

For the AVC voltage, do I probe between the S-meter adjustment pot wiper and chassis ground?

The S-meter circuit is difficult to model in that loop. The 12AH7 looks like it does a square-law conversion for the S-meter scale only, with the wiper used to calibrate the meter FSD. No matter how the S-meter components are adjusted, or change over time, the AVC voltage will remain a steady indication of the signal strength, so measure the voltage on the AVC line with a high impedance voltmeter.

 

[piepermd]

That’s easy, because the AVC line goes right out to the Cinch-Jones connector pin.

 

[Baluncore]

That’s easy, because the AVC line goes right out to the Cinch-Jones connector pin.

And it is hard, because you need a very high impedance voltmeter.

The AVC line is driven through a low-pass filter that employs a 1M0 series resistor.
A digital voltmeter, with a typically 10M0 input resistance, will only read 10/11 = 91% of the actual voltage from ground.

You must compute the actual voltage by multiplying the meter reading by 11/10 = 1.100, and then maybe re-measure the AVC voltage relative to a low impedance voltage closer to the computed value, (maybe from a 10k pot). You are looking to make a zero current, null voltage measurement, then measure the low impedance reference voltage you used.

The AVC high resistance will have implications to the design of the circuit you employ to condition and convert the AVC voltage to the A to D converter input range.