Super regenerative receiver quench investigation

 Mike Pinfold ZL1BTB

 

 

 

 

 

 

 

 

 

 

 

 


I was investigating super regenerative receivers by looking through the patents on the web when I came across a patent for a super regen that can demodulate NBFM ,
We all know super regens can demodulate AM and they slope detect WBFM but they cant effectively demodulate NBFM without super careful of adjustment of quench frequencies and the waveform shape . Neal brown was granted a patent US3337807A on 29 Aug 1967 for super regenerative amplifier detector to which he added a fm demodulator to demodulate the quench frequency .

If you look it up on the web you can read all about it

test setup ,polyakov basic super regen self quenched with Quad demod board connected

So I decided to investigate the claims of the Patent , I decided to use a PLL tone decoder as an FM demodulator running at nominally a quench frequency of 20 Khz , having had previous experience with NE567 as an FM AM demodulator at 455 Khz .. 20 Khz is achieved with a 4K7 variable resistor and a timing capacitor of 0.039 mfd .

I decided to make a self quenched super regen at around 25 Mhz using a grounded gate J310 Jfet oscillator configuration , I tuned the quench to be at about 20 Khz using the trim pot across the quench capacitor . I fed the unbypassed audio along with the quench frequency directly into the ne567 with its pll osc trimmed to about 20 Khz and picked off the audio out of the FM or AM pins of the NE567 and fed it to my bench top amplified speaker .

 

 

 

 

 

 

 

 

 

 

                           20kHZ QUAD DEMOD Circuit diagram

I used an Agilent signal generator as the signal source for the super regen detector , AM was a 1Khz tone at 80% mod and FM 1 Khz at 3KHz deviation for NBFM

 

 

 

 

 

 

 

 

circuit of poliakov super regen

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NE567 PLL as AM/FM demodulator with grounded gate super regen at 25 MHz

I did have a play with the regeneration and quench frequency pots as there is a lot of interaction between them to optimise demodulation .I was able to monitor audio directly from the receiver

itself and to compare it to that demodulated via the NE567 ,

With careful adjustment you can demodulate 3 Khz deviation NBFM and listen directly but it is very fiddly to achieve and im not sure if the oscillator is still operating in the normal quenched mode

The recovered A.M audio level from the NE567 fm detector is much much louder than the direct demodulated audio from the super regenerative detector , direct level was xxx mV verses yyy mV
of demodulated AM via the FM detector . (Yet to measure)
Neal brown did make the comment that the modulation of received signal is impressed onto the quench frequency in the super regenerative mode and this appears to happen ' So to check my measurements,
I also built up a standard quadrature FM demodulator tuned for 20 Khz ,preceded by a single untuned amplifier stage , I fed the unbypassed quench frequency into this demodulator and received the same
 result as the NE567 , the large audio improvement demodulating the approx 20 Khz quench frequency

I looked at the quench frequency with an oscilloscope connected to the top of the quench capacitor with and without AM modulation

 

 

 

 

 

 

 

 

 

 

 

 






 

 

 

 

 

 

 

 

 

 

 

 

 




There is definately deviation of the quench frequency with AM modulation so I  decided to feed the quench frequency into the spectrum analyser to confirm the presence of quench waveform modulation , this is what I saw without and with 80% AM modulation 1 Khz tone .

 

 

 

 

 

 

 

 

 


 

NE567 PLL Decoder for AM/FM at 20 KHz

 

  
   25MHz 1KHz 80% AM into receiver, quench showing characteristic FM display

 

                  25 Mhz carrier  only -80 dBm

So it appears that the AM received modulation definitely ends up by frequency modulating the quench waveform . Hence the excellent audio output level from the quench fm demodulator when compared to the "normally detected" audio output

As a matter of intrest the best sensitivity was hearing the 80% modulated test tone very noisey at -100 dBm ….. 2.24 uV in a 50 ohm system without an rf preamp ahead of the superregen. Going to try an rf preamp to see if I can improve this sensitivity figure and also isolate the oscillations from radiating out the antenna

I will next try an Externally quenched super regenerative receiver where I will use the same NE567 AM/FM decoder circuit for detection ,but I will use the square/triange wave form from the 20 Khz NE567 VCO to gate the rf oscillator in and out off "quenching" and see if the synchronous detection capability of that setup enables modulation to be achieved .

I noticed a potential problem for FM demodulators in super regen receivers that are self quenched is that the quench frequency varies with rf amplitude and regeneration voltage , so a fixed frequency demodulator like a quadrature form ,will not cope with quench frequency drift , the PLL can cope over a wider range , so I decided to try a pulse counting FM demodulator since the quench frequency can be down around 20 Khz or so where this type of demodulator can give a good level of audio output because of the large ratio of change in pulse width to carrier frequency . There are a number of pulse count demodulator circuits on the internet most of them incorporated in domestic band wbfm receivers , where the demodulated Intermediate frequency is between 200 KHz and 80 KHz . For WBFM at 75 Khz deviation the down converted 200 to 80 Khz I.F. ( depending on the design) Gives good signal to noise and fidelity response .The fact that the quench frequency may wander around somewhat doesnt make a huge difference to the demodulated audio level .
I built up a simple 2 transistor circuit copied from the web (vk2zay and others ) of a limiter and a pulse counting ( integrating) Monostable the output of which is sent the a low pass filter to obtain the wanted audio

 

 

 

 


 

 

circuit of pulse counting FM demodulator

What I did for my circuit that operates at a lower frequency , was to multiply the critical components like the capacitors by the scaling factor of I.F frequency eg 200 divided by my circuit frequency of 20 ie ratio of 10 ... so 50 pf at 200 Khz becomes 500 pF at 20 Khz . Optimally the monostable should really have two stages of I.F amplification ahead of it , but since I was not going for fidelity here ,I only used one to test the principle .. well it worked .
The super regen I tested it on had a quench amplitude of about 300 mV ,so that was a resonable quench level to start with , I got the "pulse counting" circuit going and played around with component values to optimise the simple circuit I had .The recovered audio was very good and light years ahead of the usual source of audio in the super regen I was using , I put this down to the efficiency of the FM demodulation process rather that the usual envelope detection process to obtain recovered audio .

 

I suspect the performance of the super regenerative detector gets let down by the audio chain after it I suspect the high amplitude of the quench frequency plays havoc with the audio chain , perhaps a well performing low pass filter, active or passive is required at the demodulated output .

What makes me think this is the spectrum analyser sweep of the audio output of the poliakov receiver from 100 Hz to 10kHz .This shows the RF test signal of -100 dBm at 10.96 MHz, modulated with a tone of 2Khz with very good signal to noise ratio. !!

 

 

 

 

 

 

 

 

 

 




This a reasonable signal that should sound clearer than it does I will try a better low pass filter to see if I can get rid of the extraneous noise and improve acoustic performance

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


I had built up some time ago a third order low pass active filter of the sallen and Key, the circuit from VHF Comms 4/1969 "Active audio Filters D.E Schmitzer DJ4BG so I pressed it into service ,it rolled off sharply above 3 Khz it worked extremely well and it enabled me to now make s/n measurements with a S/N meter

 

 

 

 

 

 

 

 

 

 


 

 

2 KHz  modulation tone  AM 80%  mod   good  s/n  visually !

 

 

 

 

 

 

 

 

 



The receiver is building up from random noise there is no coherent signal to replace the noise so random noise predominates ??

 

 

 

 

 

 

 

 

 

 


This could correlate with the receiver spending slightly less time building up from random noise as there is now some coherent rf stimulating oscillations in the tuned circuit hence less noise is seen.????

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