Answered! Perplexed by super regenerative receiver phenomenon

[Xena E]

Hi RF Gurus!

A student who had been prospecting for a job, had sent me a couple of little radio receiver projects they had constructed as showcase items to demonstrating their competence at electronic assembly.

One of the radios being a self quenched vhf super regenerative receiver that could tune the normal FM band, as well as covering the lower Aircraft band of frequencies.

The device was tried, working very well for what it was, the circuitry wasn't in the least unusual and a quick Internet search could turn up any number of similar designs. The construction was very competent, however I did note one phenomenon whilst using it that puzzled me a little.

Now, I'm aware of the basic MO of an SRR, but was puzzled that although it could receive and demodulate a signal, it's circuit also jammed any nearby radio tuned to the same frequency, it would in fact make them completely silent.

The question of course is how is that possible? I've only ever come across this method of detection and demodulation by reading about the work of Armstrong, but have never had any practical use for it, so never really studied it.

PS.
I did search this question, that led me to a couple of forums and it seemed that in the world of RF any discussions about these deceptively simple SRR receiver topologies seem to often end in a flame war for some reason, so I thought it would be an ideal topic for the calm folks at EEVblog forum instead.  😃👍

Regards,
Xena.

[Kim Christensen]

Well, it's a fine line between a regenerative receiver and an oscillator. It's basically "oscillating" while receiving a signal so it'll swamp out any nearby receivers.
Was this silencing happening on FM receivers?

[Xena E]

Well, it's a fine line between a regenerative receiver and an oscillator. It's basically "oscillating" while receiving a signal so it'll swamp out any nearby receivers.
Was this silencing happening on FM receivers?

Thanks.
 Yes, on FM receivers.

It seems odd that it can still detect and reproduce the program within the quality limits of the topology, but completely obliterate the reception of other receivers tuned to the exact same frequency.

It's not as if it's local oscillator IF re radiation,  it's signal frequency.

If you do this with a normal regenerative detector the set you are using  will howl and become unlistenable.

The only oscillation I'd seen on the scope from this circuit when I was studying it was the quench frequency which was variable but around 25 - 30 kHz... the scope I was using was my vintage Tek 310 so it was no doubt not displaying the whole story.

It doesn't matter, but things like this annoy me till I know.

X

[IanB]

It's not as if it's local oscillator IF re radiation,  it's signal frequency.

If you do this with a normal regenerative detector the set you are using  will howl and become unlistenable.

It's a long time since I've played with FM radios, but I seem to recall that if you put two receivers close together, you do tend to get whistling when they interfere with each other.

With regard to the receiver in question, what happens if you shield everything but the antenna with a grounded Faraday cage? Does it block the interference?

It seems that the device in question is radiating some kind of jamming signal.

Commercial receivers tend to use a superheterodyne design, where the IF signal is amplified after mixing with the RF. On the other hand, I think a super regenerative design amplifies the incoming RF signal directly, before further processing.

So it is possible that the RF amplifier in this device is somehow re-radiating the amplified RF signal, which swamps out nearby receivers?

[IanB]

See on this website, a claim that super regenerative designs pollute the operating frequency, and cannot pass FCC or CE emissions certification:

https://www.nicerf.com/news/superheterodyne-receiver-or-super-regenerative-receiver-module.html

The super regenerative receiver has the advantages of simple circuit and low cost, so it is widely used. The disadvantage is that the frequency is greatly affected by temperature drift, the receiving sensitivity is low, the anti-interference ability is poor, the stability is poor, there is frequency pollution, and it cannot pass certification.

[KE5FX]

There's nothing really wrong with an LO at the same frequency as the RF.  You'd see the same jamming effect with a homodyne or direct-conversion superhet architecture, given poor-enough LO-RF isolation at the front-end mixer.  These receivers don't jam themselves because their (complex) IF is literally centered at DC. 

With a superregen, the quench frequency would introduce a tonal artifact in the audio, but its frequency is too high to hear.  Nearby receivers are jammed because the quenching cycles generate lots of RF sidebands, but again you don't notice them yourself because the envelope of the oscillation is the signal you're listening to.

See on this website, a claim that super regenerative designs pollute the operating frequency, and cannot pass FCC or CE emissions certification

In practice, this is dealt with by adding one or more RF stages in front of the detector.  Soon enough, you end up with something as complex as a conventional superhet, so there is rarely any point to using a superregen detector these days.

[Xena E]

It's not as if it's local oscillator IF re radiation,  it's signal frequency.

If you do this with a normal regenerative detector the set you are using  will howl and become unlistenable.

It's a long time since I've played with FM radios, but I seem to recall that if you put two receivers close together, you do tend to get whistling when they interfere with each other.

With regard to the receiver in question, what happens if you shield everything but the antenna with a grounded Faraday cage? Does it block the interference?

It seems that the device in question is radiating some kind of jamming signal.

Commercial receivers tend to use a superheterodyne design, where the IF signal is amplified after mixing with the RF. On the other hand, I think a super regenerative design amplifies the incoming RF signal directly, before further processing.

So it is possible that the RF amplifier in this device is somehow re-radiating the amplified RF signal, which swamps out nearby receivers?

Thanks.

Yes most definitely radiating signal frequency.

The topology works at two frequencies.  Signal and quench.

As I understand it, the incoming signal excites the tank circuit of a quenched oscillator and the oscillations increase until reset by the quench: in this case it's self quenched by starvation of the bias.

The resultant variation of the quench envelope is crudely integrated which corresponds to the incoming signal amplitude.

For the FM signal it is slope detecting one side of the fm envelope.

It's the fact that it obliterates the other receivers reception whilst being unaffected itself is the point of interest.

These seem quite antisocial things to use.

There's nothing really wrong with an LO at the same frequency as the RF.  You'd see the same jamming effect with a homodyne or direct-conversion superhet architecture, given poor-enough LO-RF isolation at the front-end mixer.  These receivers don't jam themselves because their (complex) IF is literally centered at DC. 

With a superregen, the quench frequency would introduce a tonal artifact in the audio, but its frequency is too high to hear.  Nearby receivers are jammed because the quenching cycles generate lots of RF sidebands, but again you don't notice them yourself because the envelope of the oscillation is the signal you're listening to.

So... perhaps we could say that it in some way ' doesn't hear it's own RFI ?

[A.Z.]

as for silencing other receiver, it's due to the oscillator leaking to antenna, a way to avoid that is to place a simple common gate JFET stage at the antenna input, such a stage will offer several advantsges since will give impedance match, buffering, preamplification and I/O isolation

[T3sl4co1l]

Consider the autodyne detector: it oscillates, but the oscillation is influenced by an outside signal.  In particular, the oscillator, having a regenerative characteristic, plus a nonlinear characteristic that limits amplitude to some saturated value, effects a mixing process, which includes the oscillator self-mixing to DC.  Conversely, other signals input to the oscillator, get converted down to baseband; thus one can be used as a combination PLL (tracking LO) and demodulator for AM.  Slope detection of FM works just as well.

Superregen isn't quite the same thing but it averages out the same way more or less.

With a preamp, or some manner of isolation between antenna and receiver, emissions would be suppressed, but since it's an oversimplified design, substantial compromises must be made.  Clearly the center frequency is within the tracking range + bandwidth of the other (proper FM) receiver, and thus stomps the real signal.

Tim

[Xena E]

With a superregen... Nearby receivers are jammed because the quenching cycles generate lots of RF sidebands, but again you don't notice them yourself because the envelope of the oscillation is the signal you're listening to.

So... perhaps we could say that it in some way ' doesn't hear it's own RFI ?

I think the penny has just dropped   

The SRR doesn't hear it's own RFI because it isn't oscillating at signal frequency when it samples the wanted signal.

Is that it? 

X

[IanB]

I would guess, going back to the opening post, it might be appropriate to ask the student what they can say about the design and origins of that circuit, what limitations it has, what compromises it makes, why it might have been attractive originally, and why it is rarely used today?

[Xena E]

I would guess, going back to the opening post, it might be appropriate to ask the student what they can say about the design and origins of that circuit, what limitations it has, what compromises it makes, why it might have been attractive originally, and why it is rarely used today?

Thank you Ian.

The work being applied for with the company was for a job that we call 'wireman'. It is a collective term for the semiskilled assembly technicians, who are expected to come to the work with already developed skills in soldering and component recognition.

Normally first contact with applicants for these openings is by letter or emails with CV attached. If it is thought they are worthwhile having further contact, they are then invited for a face to face interview, and practical aptitude test.

This person, (an 18 yo) was unusual as they sent in the two projects with their details and their ed qualifications saying that it was in lieu of proven prior experience. Each project was well made, and quite thoroughly documented including the origins of the designs, operation, and practical applications.

I recommend them to the company HR for further consideration.

I wanted to get a handle on this observation of the SRR circuit behavior to have something to examine the candidates response when they were asked the same question, (in the event they are invited for an interview).

Thankful for everyone who has replied, it's appreciated!

Regards,
X

[janoc]

This is why regens are banned for most receivers since sometime after WWII - they radiate interference in-band and are only used for things like cheap remote control receivers and such where it doesn't matter as much.

You better stay away from anywhere near airband with this gadget unless you want unwanted attention from the authorities.

[radiolistener]

yeah, although super-regenerative receivers have a simple design and high sensitivity, they tend to emit very strong parasitic signals through the antenna, especially at the receiving frequency. In the aviation band, this can cause serious interference with critical communication and navigation systems. Due to these characteristics, such a receiver is not suitable for operation in this band. To avoid potential issues, it’s better to use more safe circuits, such as super-heterodyne receivers, which don’t produce such emissions.