Simple SSB-CW receiver with a modified frequency
stabilizer for PSK31 reception and fine tuning.

The simple receiver that has to be stabilized
For the tuning mechanism, no variable capacitor is used but a wooden stick with a shortened winding of copper wire, moving to the VFO coil! The volume control is an antenna coil on a plastic tube that can be moved to- and over the input coil.
A complete description of the original receiver can be found by clicking on the next link: SIMPLE RECEIVER FOR 40 AND 20 METER

The new version of the receiver with fine tuning, 30 meter
band and a connection for the soundcard of the PC.

big diagram

Modified frequency stabilizer for PSK31 reception
The VFO is not stable enough for PSK31 reception. That is very understandable, it is a simple and open, unscreened construction. That is why we have to stabilize the VFO with a frequency stabilizer. That was not possible with a normal Huff & Puff circuit, but it was possible with a modified version.

Modified simple frequency stabilizer and VFO.

It looks like a very normal Huff & Puff frequency stabilizer, but it is a special version. Usually, we use a clocksignal of 10 to 50 Hz in a Huff & Puff stabilizer, so that the VFO signal locks in the same small frequency steps. But the VFO of this simple receiver was not stable enough for such small frequency steps. The frequency varied within 1 or 2 seconds already 10 to 20 Hz. To make it possible to use a Huff & Puff circuit, a much higher clock frequency of approximately 976 Hz is used. Controlling the frequency goes much faster and the VFO can be locked every 976 Hz (8 MHz / 8192). These 976 Hz steps are too large for SSB and CW, but not for PSK31 reception. As the VFO frequency is equal to half the reception frequency, the receiver is tuneable in steps of 2 x 976 Hz = 1952 Hz! These large steps are not a problem at all for PSK31 reception with the PC.

The frequency stabilizer with fine tuning.
big diagram

The modified frequency stabilizer
Much information about Huff & Puff frequency stabilizers can be found on the great site of Hans Summers (search for "Hans Summers" or "G0UPL" to find his site). Nowadays, there are improved versions of the by PA0KSB invented circuit. Here, an older version is used, partly because I had the parts available, but also because it is possible to use this version as a frequency calibrator.

The working principle of this stabilizer is very simple:
In fact it is not a frequency stabilizer but a phase stabilizer. The D-input of a D-flipflop is connected to the VFO signal. The clock input is connected to the clock signal of 976 Hz. On the rising edge of the clock signal, the output is set to the value of the D-input. So a "1" when the clockpulse goes high at the positive part of the sine of the VFO signal, a "0" when the clockpulse goes high during the negative part of the sine of the VFO signal. The output of the D-flipflop controls the VFO frequency via a kind of PLL loop filter.

Reception of the 10 MHz calibration signal without
stabilizer during two minutes. Drift already 60 Hz!

Reception of the 10 MHz calibration signal with
stabilizer during half an hour. Drift only 2 to 3 Hz!

Without frequency stabilizer, the receiver drifted approximately 60 Hz during two minutes, sometimes already 10 to 20 Hz during a few seconds. With frequency stabilizer, the drift during half an hour was only 2 to 3 Hz at 10 MHz! It was very simple to measure the drift. The receiver was tuned to the 10 MHz signal of my simple frequency standard and the frequency of the LF output signal of the receiver was measured with the soundcard of the PC and an audio spectrum analyzer program.

The loop filter
The frequency control is very simple, it has two settings: Full speed upwards or full speed downwards. So a loop filter is required to slow down and damp this extreme frequency control. Choose the values of Rx and Cx so that the control is stable and you do not hear many irritating sounds while receiving CW signals. It is possible to tune in smaller steps when you do use the clock signal of Q14. But stabilization is less good then. Cx has also to be increased to 100 uF and Rx has a value between 68 and 120 ohm. But these values can be totally different in your circuit!
The first experiments were not a success. The solution was simple, I had chosen the wrong values for Cx and Rx, and that was the reason why the frequency control did not work.

The lock indicator with two leds.
Adjust the potentiometer so that they are equal in brightness.
Then the stabilizer is in the centre of the control range.

How to use the frequency stabilizer
When S1 is in the "open" position, the potentiometer acts as a fine tuning for CW and SSB. In the closed position, the VFO synchronizes on multiples of 976 Hz. With the potentiometer, you can tune to the next multiple of 976 Hz and adjust the stabilizer in the centre of the control range by means of the two leds (turn slowly). In the centre of the control range, they have the same brightness. It is funny to see that the brightness of the leds changes when you do move your hand towards the VFO coil. If you move your hand too fast, the VFO unlocks.
With a bad choice of Cx and Rx, the frequency stabilizer locks often (unwanted and bad) on multiples of subharmonics of the clock signal. When rotating the potentiometer slowly, the circuit has to stay locked when one of the leds is almost off and you should not hear too many irritating sounds when receiving CW signals.

A VCO without varicap
Usually we do use a varicap to control the VFO frequency. But we can do that without a varicap.
Fine tuning can be done by just varying the voltage of the base of the transistor T2 via a 1M resistor with a 10k ohm potentiometer. The tuning range is approximately 60 kHz and I cannot explain why, but it is almost equal on all bands! This method of varying the frequency without using a varicap is used to control the VFO frequency by the frequency stabilizer!

Free frequency marker generator! Just touch
a pin of the 74HC4060 with a screwdriver.

Free frequency marker generator
When you do touch pin 7 of the 74HC4060 with a screwdriver, you will hear a tone on multiples of 500 kHz. Perfect for tuning to 7, 10 and 14 MHz.
Touch pin 4 with the screwdriver and you will hear a tone on multiples of 125 kHz. This can be useful when tuning to 10125 kHz and 14125 kHz.
And touching pin 6 is all you have to do to have a signal for tuning to 14062.5 kHz. Tune a little lower and you will receive the QRP signals on 14060 kHz.
And tuning to 7030 kHz can be done by touching pin 14.
You can adjust the 8 MHz oscillator exactly by changing the values of the 47pF capacitors.

When the receiver is connected to the soundcard of the PC, you do not
need the final LF stage of the original receiver (right side of the blue line).

Connection to the soundcard of the PC for PSK31 reception
The signal of the 1st LF stage can be connected to the microphone input of the soundcard. Switch on the microphone preamplifier of the soundcard. The second LF stage for the headphones of the original receiver is not required anymore.
The audio range goes to approximately 3 kHz. You can increase it for experiments with DRM or certain SDR programs. Replace the 47nF capacitor by 4700pF and the 10nF at the collector of T3 by 1000pF.

With the frequency stabilizer, PSK31 reception is good. With the lock indicator (the 2 leds), you can check if the stabilizer is within the control range. For reception of SSB and CW, the frequency stabilizer is switched off. The frequency steps are too large for SSB and CW and the stabilizer causes still some weak but irritating audio sounds. But the VFO is stable enough for the reception of CW and SSB. The fine tuning works perfect, tuning to SSB stations is very easy.

Reception of PSK31 with the MultiPSK program of
Patrick F6CTE goes excellent with the frequency stabilizer!

QRP beacons
Already on the first evening after adding the 30 meter band to the original 20 / 40 meter band version, it turned out that the receiver is not so bad: The QRP beacon SK6RUD on 10133 kHz (0.5 watt!) could be received. Not so very strong, but all information, including the link to the website was heard!
Also the 2 watt beacon of IK3NWK on 10140.8 kHz is often heard.

ARGO, a program for the reception of QRSS beacons with very low power
To make a QSO with very low power, that must be very interesting for us, QRP'ers! A few radio amateurs are active with transmissions with very low power, less than 100 mW. And they do make QSO's with all continents with such low power! That is possible by using a very low CW speed in combination with a special computer program such as ARGO. The CW speed is very low, 1 dot lasts 1 to 3 seconds. The frequency stability of the transmitter and receiver has to be very good. But this simple receiver with frequency stabilizer is certainly very stable!
Unfortunately, there were no active stations. But I could use a professional RF generator to test how much better such a QRSS signal can be received compared to normal CW signals.

Reception of QRSS signals with the ARGO program.
The 31Hz line is the weak testsignal.

And the results were indeed amazingly! The testsignal of the RF generator is the white line at 31 Hz. All other lines are interference signals of the PC or other signals that could be received with disconnected antenna The left part of the 31 Hz line is very bright, but was already so weak, that only slowly transmitted CW signals could be decoded by ear. The center part is 10 dB lower, inaudible but perfectly visible. The right part was even 20 dB weaker than slowly transmitted morse that just can be decoded by ear. Conclusion: A 50mW QRSS signal can do the same as a 5W QRP transmitter with 100x more power!

It is very nice to play with this program. I made a connection for a frequency counter at the frequency stabilizer, so that it is possible to measure exactly the reception frequency. Also the souncard has to be calibrated. My card had an error of approximately 20 Hz at 1 kHz. For the calibration, you can use the 976 Hz clocksignal. Touch this signal with you finger and you will see it on your ARGO screen.

And again we had much fun with a very simple radio circuit!