Passive SSB detector, can it be done?

[neazoi]

Passive CW/SSB detector, can it be done?

Hello, I am looking at a way to detect CW On/Off keying signals (ssb spectrum) passively, without the use of a BFO.
Can it be done?

Wikipedia states that "Though SSB uses substantially less bandwidth and power, it cannot be demodulated by a simple envelope detector like standard AM."

Based on this a simple envelope detector cannot be used. This page https://michaelgellis.tripod.com/mixerscom.html states that the way envelope detectors work is by mixing the sidebands with the carrier of an AM signal.

But if you think of a single diode rectifier, followed by a shunt capacitor in PSU circuits, all it does is to allow for the positive portion of the AC to pass through. Can this somehow be used for detection of an CW signal that is transmitted in the SSB domain? (i.e an ssb transmitter that is keyed on/off).

 

[Borber]

SSB is nothing else but shifted audio spectrum 300-3400Hz to higher frequencies. No passive circuit can shift back SSB to original spectrum which starts with 0Hz.

 

[neazoi]

SSB is nothing else but shifted audio spectrum 300-3400Hz to higher frequencies. No passive circuit can shift back SSB to original spectrum which starts with 0Hz.

What about in the case of a single ssb tone? Can this tone be "rectified" instead of "mixed down" to audio?

 

[Borber]

Re: Passive CW/SSB detector, can it be done?

Single tone or keyed CW transmitted in SSB band can be demodulated by diode envelope detector. It is nothing else but AM 100% modulated with square wave signal.
Ham operators still use BFO to listen to telegraphy signal that modulates CW.

 

[neazoi]

Re: Passive CW/SSB detector, can it be done?

Single tone or keyed CW transmitted in SSB band can be demodulated by diode envelope detector. It is nothing else but AM 100% modulated with square wave signal.

All right, thanks.
So this single tone transmitted as an ssb signal, can be demodulated.
However, in order to be able to listen the demodulated signal as a tone in the audio domain, a bfo would be needed.
Is that correct?


In the case of an SSB voice signal, can a frequency detector be used to detect the frequency shifts of the SSB voice? (raTio detector)
The raTio detector converts frequency changes to amplitude changes, which then can be envelope detected.

I believe the output waveform will be a compressed audio reproduction of the SSB voice signal

 

[Borber]

Frequency detector is useless because it's output on AC packet of signal is DC pulse. No useful information hidden in frequency exists frequency is constant or what is the meaning of variable pitch in audio signal.

 

[chuckey]

For CW, just use diode detector and use this to switch on and off a local audio oscillator.
Frank

 

[BradtheRad]

Normally my ordinary AM radio could not pick up morse broadcasts. However there were occasional times at night when I'd hear morse beeping. I've always assumed it was due to some RF frequency interference beating. I have no idea what the original frequencies were.

On SW, every so often, I would tune to a frequency which had a soft on-off swishing sound. I could tell someone was keying morse, but my radio barely detected it.

Eventually I purchased a radio with BFO. It easily brought in SSB broadcasts (morse and voice). Voices that sounded garbled were finally intelligible.

I have wondered what would happen if I were to broadcast an RF carrier. Would I find the right frequency, to beat with a SSB broadcast, and make it audible?

 

[neazoi]

I have wondered what would happen if I were to broadcast an RF carrier. Would I find the right frequency, to beat with a SSB broadcast, and make it audible?

Absolutely, if the RF carrier is frequency stable. BFO injects an IF carrier, assimmung a superhet receiver), but this can be done in RF input level, before the first RF conversion. After all, what you need, is to somehow replace the missing carrier. It does not matter if it is in RF or IF level.

 

[BradtheRad]

Absolutely, if the RF carrier is frequency stable. BFO injects an IF carrier, assimmung a superhet receiver), but this can be done in RF input level, before the first RF conversion. After all, what you need, is to somehow replace the missing carrier. It does not matter if it is in RF or IF level.

Thanks. My wonderment came from the fact that I heard morse broadcasts on the commercial AM band. I did not modify the radio, nor did I operate an RF transmitter.

At the time I thought a ham was broadcasting audible morse code on a commercial band. By accident I thought.

However I came to understand this was unlikely.

So over the years I decided that, in addition to a ham's CW transmission, there was also an RF frequency coming from somewhere out in the ether, which beat with the CW wave, and produced sound in my radio.

I guess it created a difference frequency. And its frequency was just the right frequency to coincide with the distant station I was listening to.

Furthermore it was at the right frequency to demodulate to audible tones in my ordinary AM radio.

It stays in my memory because I still have recordings of it.

I'm sure the explanation is quite simple.

 

[betwixt]

Your explanations are correct. If you hear the Morse as a tone it is either transmitted as modulated AM (the modulation being the pitch you hear) or you are hearing a beat signal with a nearby carrier. There is another possible reason you can hear the Morse, that is the 'quietening' effect of the receivers AGC. It may alter the gain according to the presence or absence of the carrier and make the background noise change in sympathy with the keyer.

Brian.

 

[drkirkby]

I have wondered what would happen if I were to broadcast an RF carrier. Would I find the right frequency, to beat with a SSB broadcast, and make it audible?

As a kid I built an oscillator that done just that - injected the carrier at the frequency of the signal being received. The advantage of going in at the IF stage is the oscillator can be at a fixed frequency, whereas one needs to change both the tuning on the AM radio, and of the BFO if going in at the RF. But it sure was fun when I first heard SSB on my AM radio. I don't ever recall having any problems with the level of the RF carrier, but tuning it correctly was quite critical, and a slow motion drive was used.

Dave