An On-Frequency contemporaneous Repeater:
Super-regenerative detectors have always held a fascination. They always seem like something for nothing, or a free lunch, but as we all know there is no such thing as a free lunch ! However, they do perform remarkably well for the minimal amount of componentry and current consumption. Not many receivers can boast wbfm and am demodulation of a 1uV signal at a few mA of current with just over half a dozen components!
The detector is based on a typical RF oscillator that is turned on and off at an ultrasonic rate at 25-200 kHz or so. This can be done by a simple RC network incorporated in the emitter /source of the oscillator or by a more complicated outboard square/sine/triangle oscillator. The idea is to turn the oscillator off and on and it will, by nature of its positive feedback ,will tend towards oscillation. Of course, the feedback will continue to build up in a positive way (over microseconds) and the oscillator will build up RF power until it saturates, we all understand this. No doubt you have all read of regenerative receivers and how, by attempting to control the positive feed-back, you can achieve very high amounts of receiver gain with a single active device. If you go too far with the regeneration control, it will burst into oscillation, this is an undesirable feature, as it serves no useful purpose and you can't hear diddley squat with the receiver in that condition! It is now oscillating and generating an undesirable RF signal that will be coupled to the antenna and hence radiated.
I tend think of the operation of a super-regenerative receiver as a regenerative receiver operating in a " sampling mode" Someone ( i.e. the RC circuit or outboard quench circuit) is tweaking the regeneration control up and down at an ultrasonic rate. At some stage of the"ultrasonic twiddling" of the regeneration control, the circuit is going to pass through the rising sensitivity stages of an optimised regenerative receiver!! and signals will be received, In order to hear the demodulated signal, all you need to do is filter out the RF oscillation and the quenching/regeneration frequency with a low pass filter and all that is left is the ultrasonically sampled demodulated audio. A simple low pass RC filter will do this. The Super-regen will only demodulate AM and WBFM ,I suspect the WBFM demodulation is due mostly to slope detection but I have yet to perform a few experiments to prove this using a range of various " Q" tuned circuits , Remember also that as the pre-oscillating circuit builds up energy in its in its positive feedback mode by nature of feedback, the "Q" of the tuned circuit will rise to some maximum limit, ( just like a Q multiplier) I am sure it is this increased "Q" that enables such a simple circuit to slope demodulate WBFM. +/- 75 kHz signals.
NBFM signals +/-5 kHz are not demodulated easily , the "Q" of the tuned circuit is too low at the normal frequencies of interest.However there are experimenters who have devised some circuitry to do this. I have not seen it but would be very interested to see it. It would be interesting to build a super-regen operating at 455 kHz, to see if it could demodulate NBFM ?? a simple IF for a receiver no doubt!! Super-regen detectors by their nature are a form of logarithmic detector and can be used as such exhibiting up to 80 dB range , they thus have an "AGC" performance and are also quite immune to impulse noise! They can also be used as a wide range logarithmic "S" meter with a little bit of extra circuitry ,there is a very good article on this in in VHF Comms ,I will dig it out .
I made up a simple collpits oscillator using a BF960 dual gate mosfet . The 12 volt B+ was fed out via a simple interstage audio transformer, so I could tap off the demodulated audio into a cheap multimedia computer amplified speaker. I controlled the RF grounded (47pf) gate two of the mosfet via a output of a square/sine/triangle function generator. I could vary the type of controlling waveform, its frequency, its amplitude ,switching levels etc ,to see what effect these parameters had on sensitivity , demodulation and oscillator RF power output. I set the L/C circuit to run at around 49-50 MHz , The antenna was a 1/4 wave GPlane on the roof fed with 50 ohm coax. The antenna input was tapped at about 1 turn up the oscillator tuned circuit via a BNC connector. I connected the power, switched on the audio amplifier and the function generator then fiddled with the quench frequency and amplitude of the various waveforms (square,sine and triangle) .(Triangle and sine gave the best performance with square wave quenching coming a long way behind.)
With sawtooth quenching at between 100-200 kHz and full 12 volt amplitude I could hear 1 uV easily from the HP8640 signal generator at 50 MHz. I then connected the antenna connection to the Agilent E3044 Spectrum Analyser and saw the signal carrier at 50 MHz at a level of +10 dBm !! and its 100-200 kHz sidebands either side falling away for some distance , This is one of the reasons why super-regens lost popularity mostly because of the potential for noise re-radiation and interference to others. 10 mW into a good antenna can go a long way in free space too. In the good old days and even now, it is desirable to put an RF amplifier onto the front end between the oscillator tuned circuit and the antenna, mainly to use the reverse isolation of the amplifier to minimise the oscillator re-radiation. (you will find it doesn't contribute a great deal of an improvement of the receivers sensitivity.)
Which brings me to my main interest in the undesirable re-radiation of the super-regenerative receiver . The simple circuit can and does act as a low power repeater!! To prove this I took a Maxon SX-49 headset transceiver pair that has an open field range of 350-400 meters with the antennas up on both sets. I retuned the super-regen to the Maxon radio channel of 49.890 MHz (remember that these are NBFM sets ). I could use my AR3001 scanner receiver on NBFM to pick up the maxon radios at 49.890 directly. I removed the antenna from the maxon and placed the scanner across the room with its whip antenna disconnected and it could not hear the Maxon transceiver when I called . I connected up the super-regen to its antenna and turned it on. You could see the noise on the scanner "S" meter but the mute on the scanner was silent.(nothing heard) (I sought of expected it because these FM mutes operate on a FALL in noise on receipt of a signal) so I turned on the little maxon with no antenna and placed it close to the breadboarded super-regen that connected to the outside 1/4 wave antenna, operated the PTT and talked. Low and behold the scanner unmuted and my speech appeared from its speaker !. I turned the super-regen off and the scanner fell silent again . I connected the spec An to the antenna input of the super-regen receiver and measured +8-10 dBm of output , not a bad RF power output level for short range application , The next test was obvious ,how far is it transmitting? Well I put the maxon microphone on vox mode,on top of the Broadcast radio speaker, switched everything on, jumped in the car, tuned the ICOM 706 to 49.890 MHz and there it was!! clear as a bell,(also in about 3 other places either side of the carrier due to mixing of the quenching frequency sidebands) . I went for a drive around the local region and found the useful readable signal area range was about 2 Kms radius (using the 80m H.F. whip for receiving 49 MHz ) and that was down at street level in the urban environment. With a range like that , +10 dBm and on a poor antenna , no wonder it is advocated to place a RF preamp between the antenna and the oscillators tuned circuit for isolation purposes.
Although the super-regen cannot demodulate NBFM it does still lock to the frequency it receives and hence tracks it re-radiating the NBFM received signal in the process. Very useful I thought !, how will its 1 uV receive sensitivity coupled with 10mW RF output system go as a short range repeater! to extend the range of a pair of maxon headset transceivers (10mW into 15 inch base-loaded whips and about 1 uV sensitivity)??? I should be able to a least double the range they can operate using this simple repeater between them. Food for thought!
Just in case you are wondering about the Plessey patent, its a functional design for a repeater that transmits and receives on the the same frequency at the same time! very cleverly thought out too. plessey Electronics UK actually produced a military version called the Groundsat . It is a Back pack synthesised receiver transmitter covering 30 to 80 MHz with three selectable power outputs 10mw 100 mW and 1 w , although the patent covers all modes of transmission, FM is the only practical mode the repeater uses . The secret is in its direct conversion receiver, where the receiver Local osc is also the transmitter oscillator . it works like a high power PLL demodulator , where the demodulated audio remodulates the receive oscillator and is amplified and transmitted out the antenna. Because the transmit signal coming back around into the receiver is identical to the local oscillator signal, there is no difference other than a fixed phase shift . the mixer product is D.C ,this can be removed by capacitive coupling, the true received signal is the wanted signal . Somewhere I have the USA Patent office No.4134068 , you could look it up on the patent website!
Now I see some clever Dick has managed to get a super regen to demodulate DSSS , What the essentially do is to mix the spreading code with the quench frequency , so the receiver also performs a correlation function , of course there is feed back to make the despreading code track the incoming DSSS
check this excellent article on super regens !! http://www.eix.co.uk/Articles/Radio/Welcome.htm
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