undetectable product detector?

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neazoi

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Hi,
The stories about the old galena receivers that could not be detected (because they lacked a local oscillator) are known.
I wonder if a product detector could be made somehow, so that it could detect SSB or CW signals without the need for a local oscillator?
I am thinking of a TRF design with a product detector instead of a envelope detector.
 

Direct Passive Conversion of AM or skirts of filter on FM are possible with hot-carrier, Schottky or pin diode, but obviously Adjacent channel Interference (ACI) will suffer from difficulty achieving stable high Q resonators.

But for the crystal pickel jar radio in mid60's I heard radio stations 200km away, with crystal earbud and no batteries as a kid, thanks to Dad.
 


My question was really about product detectors, if they could be made without requiring a local oscillator.
Precending amplifiers and selectors could be used but I am asking about a product detector without a LO.
 

I believe a product detector always requires a local oscillator (LO) running at a much higher level then the input carrier.
Product-to-Sum Formula = Sin Fc sin LO = 1/2(cos ( Fc -LO) - cos (Fc + LO) ). cos Fc-LO = the beat note in CW or SSB, cos (Fc + LO) is the second harmonic which is filtered out.
Frank
 
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    neazoi

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neazoi said:
My question was really about product detectors, if they could be made without requiring a local oscillator
Click to expand...

The detection by local oscillators is possible because there is the highest frequency on the board, and the diffusion power of a signal on the air increases as increase the frequency of it oscillation, so that if there is no internal circuit performing at least a sudden change in the electromagnetic field, it will be difficult to detect it.
 
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    neazoi

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Even a DSP (Digital Signal Processing) SSB demodulator needs an oscillator to recover the carrier suppressed at the transmitter side.

**broken link removed**

A Direct Conversion Receiver (DCR) in fact use also a BFO which is the local oscillator, that has almost the same frequency as the RF frequency (with audio/baseband difference).
 
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    neazoi

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The word PRODUCT refers to multiplying two signals, so you MUST use a local oscillator. Or in some situations, the "local" signal is generated by a near interfering station. CW and SSB simply needs two frequencies to get a product you can hear as the beat.
 
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    neazoi

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The last diagram in this page http://www.techlib.com/electronics/allband.htm Seems promising
Click to expand...
As promising as a detector radio.

Even if your radio signal is strong enough to be detected by this simple circuit and you don't need more selectivity than provided by a single tuned circuit, it can't demodulate SSB or CW.

The word PRODUCT refers to multiplying two signals.
Click to expand...
Exactly.
 
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    neazoi

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Not true. But performance is poor with a "diode" switch unless a very high Q front end filter exists to isolate the desired carrier. All diodes conduct in one direction more than the other, so that circuit on last page uses a lightly biased diode to detect the sum and difference frequencies while the bandwidth limit of the simple x10 amp rejects the 2*f result leaving the DC modulation being detected. Thus works for mV levels of signals but well below diode noise levels for uv signals.

Thus better design uses an LO and IF filter or two or three in some cases or direct conversion with LO when using ultra high Q SAW filters.
 


I thought that the light biasing of the diode is only there just to improve the sensitivity of the diode. I have never seen biasing diodes to make it able to detect SSB-SC, this is confusing.

I think the last circuit uses a local "BFO" but at the audio (or ultrasonic) region. I think this local BFO interacts with the audio signal chopping it down, or interaction with the voltage level in the case of unmodulated carrier. I think that is how it operates but I am not sure?
 

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