Super-regenerative reciever - Page 1
Forum section: electronics / beginners

author: csheldon
author_rank: Contributor
timestamp: May 31, 2017, 12:36:17 pm
content: Hello everyone.I was reading and studying about super-regenerative recievers lately and stumbled upon this old article form ELECTOR magazine.http://jap.hu/electronic/rf/el9805.pdfI'm having trouble understanding how this particular one reciever works. I am familiar with the basic theory of super-regenerative recievers, but I'am no expert. Could anyone care to explain how exactly this recievers works.-How it diferentiates beetween the signal and random noise if VF oscilator oscillates wheter there is signal or not-The emitter capacitor for self-quenching is missing, why, and how the self quenching happens?-What is the purpose of two 1N4148 diodes and c5 and c6 capacitor dividers-what does c7 do?-What are those two opamp circuits, especially the right one. I suppose the left one is a low-pass filter

author: csheldon
author_rank: Contributor
timestamp: June 01, 2017, 07:36:34 am
content: Is this maybe the wrong section of the forum to ask this questions? If yes, Could an administrator move this thread to a more suitable section

author: csheldon
author_rank: Contributor
timestamp: June 02, 2017, 05:07:18 pm
content: Anyone?

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 02, 2017, 06:18:51 pm
content: As far as I can see the quenching capacitor is C6, C7 makes the thing oscillate at the receive frequency.IC1A is a low pass filter, I think IC1B is there to get the mark/space from the transmitter back at a comfortable level.The diodes are in the biasing circuit, maybe to get the correct setpoint for the quenching or maybe also to preprocess the output signal, not sure.C5 - maybe needed to make the thing oscillate, because the coil is not like it is normally from collector to supply?

author: Audioguru
author_rank: Super Contributor
timestamp: June 03, 2017, 02:27:44 am
content: It is a simple regen circuit, not a super regen that quenches the oscillations. It is almost oscillating which makes its gain very high so that any signal in its wide bandpass will overload it.The first opamp amplifies the puny received signal then the second opamp is a high gain comparator to make a digital output signal.

author: Paul Price
author_rank: Super Contributor
timestamp: June 03, 2017, 01:30:36 pm
content: This is a super-regenerative receiver.It is always oscillating at a frequency tuned to match the center frequency of the transmitter.C7 feeds back RF from collector to emitter to create RF oscillation. The value of the capacitor must be carefully chosen to provide just enough feedback for a weak oscillation. It cannot be too large or too small in value or the circuit will not oscillate at the RF freq. C7 cap creates an optimal phase shift that does not cause the transistor to saturate or clip the RF oscillation produced, which at the same times ensures the transistor also remains correctly biased to best operate as a low-freq. audio/data amplifier..The two diodes rectify a small amount of the oscillating voltage developed from the L/C resonant circuit connected to the collector tuned to the transmitter's freq.The collector oscillator RF voltage is fed to the two diodes though C5 and is rectified to create a type of FM slope detector. The rectified voltage signal is the detected signal, a low-freq voltage which is then also amplified by this same transistor to produce an amplified audio/data output signal at the collector and this detected signal is output to the next stage through R9 to the first op-amp acting as a pulse data demodulator.This receiver circuit is quite immune to noise because it's oscillation must change by a FM(or RF freq. shift) signal to produce a demodulated output and secondly, because the receiver only begins to detect signals very close to the self-oscillating tank freq tuned to the transmitter's freq. and is therefor not responding to noise outside this tuned narrow freq. range.The first op-amp is a sensitive comparator that detects small DC level changes from the quiescent voltage both inputs of this op-amp are biased at. When no transmission is detected, the voltage at the inputs are slightly different and cause the output of the first op-amp to saturate. Any fast signal(data received) signal will cause this op-amp to saturate to the opposite rail, depending on the demodulated instantaneous signal direction. It only takes a few mV of detected signal to produce a rail to rail output voltage change.The very high sensitivity of this data-recovery circuit demands that small nose pulses will also create an output and these undesirable detected pulses must be rejected and that is the job of the second op-amp.The second op-amp is a pulse-width discriminator that filters out data pulse widths that are too short in width to be the transmitted data pulse shape. In other words it is a combination circuit, a low-pass data filter by being a pulse-width discriminator. R15 and C10 have a T=RC time constant that defines the minimum output pulse width of the digital recovered data received.

author: Paul Price
author_rank: Super Contributor
timestamp: June 03, 2017, 02:20:19 pm
content: There a few creative designers out there that somehow create an electronic circuit which can so best use the fewest parts and yet achieve remarkable results in performance that cannot sometimes be achieved even by much a more expensive, larger in size and many-more component count approach.A super-regenerative receiver like this one is a great example of this.If there can be beautiful poetry masquerading as circuit design, this is a best example.

author: csheldon
author_rank: Contributor
timestamp: June 03, 2017, 04:17:55 pm
content: Agree. This is indeed masterpiece. Thanks for your response!

author: csheldon
author_rank: Contributor
timestamp: June 03, 2017, 04:55:37 pm
content: Could you please explain what is the role of R8, C6 and emiter inducotr

author: Paul Price
author_rank: Super Contributor
timestamp: June 03, 2017, 05:53:41 pm
content: The emitter inductor creates a higher impedance and phase-shift that properly couples the feedback from the collector to emitter capacitor. This creates an RF impedance greater than the emitter resistor alone to develop the feedback RF voltage necessary for oscillation. Putting a resistor from the emitter to ground functions to stabilize a common-emitter amplifier and also sets gain if it is not bypassed by a capacitor(which would be used if it is desired to define the low-frequency 3-db point freq. response freq. rolloff while also permitting signals in the higher portion of the low freq. spectrum of interest to be amplified at the highest gain. In this case an emitter resistor bypass capacitor would cause the gain to be set wrong as it would be set higher than optimal for this super-regenerative circuit to function.)Since the effect of the inductor is only at the HF osc. freq., it does not undesirably lower the gain of the common-emitter amplifier function of this transistor circuit that is needed to be set high enough to amplify the low-level, low-freq. detected signal.The two resistors in question bias the transistor as a common-emitter low-freq. amplifier even as it is also oscillating at a VHF/UHF freq. as well. Their effect is to set the base voltage(and thus base current) of the transistor which, by also setting the voltage drop across the emitter resistor, sets collector current and thus the collector voltage to be somewhere centered between the voltage rails so it can swing up and down without clipping and thus amplify without distorting the low freq. detected signal. This is just like a conventional common-emitter audio amplifier stage would be biased. The ratio of the emitter resistor to the collector resistor to V++ sets the fixed gain of this transistor also functioning as a low freq. common-emitter amplifier.

author: Paul Price
author_rank: Super Contributor
timestamp: June 03, 2017, 06:48:06 pm
content: Diode D1 is shown reversed in this schematic.

author: csheldon
author_rank: Contributor
timestamp: June 10, 2017, 05:39:52 pm
content: I have a follow - up questions on this topic about super-regenerative recievers.There is a schematic of another version of super-regenerative reciever in the attachement. These are those 433 MHz modules available on ebay.Few things are confusing me on this schematic:-Where is the quenching capacitor - wh it isn't connected parallel with the R7,-What is the purpose of C5, C7 and C9-Why is the signal from the antenna preamplifier, through R4 coupled the why it is...why not couple it to the base- is this a common emiter or a common base configuration, it is very difficult to tell-How does the quenching happen and how to determine the frequency of quenching?-What is the purpouse of C12 and R14..never seen a parallel RC filter?

author: csheldon
author_rank: Contributor
timestamp: June 10, 2017, 06:45:11 pm
content: Also one aditional question about the antenna preamplifier in the schematic from the previous post.How it works, why is it wired without emiter degeneration, what class is it, and where can i learn more about this kind of circuit?Much appreciate your help

author: csheldon
author_rank: Contributor
timestamp: June 11, 2017, 10:40:09 am
content: We all live in digital age, funny how many of us don't know a thing about this analogue stuff, seems all this stuff just works by magic

author: Circlotron
author_rank: Super Contributor
timestamp: June 11, 2017, 12:40:47 pm
content: Quote from: csheldon on June 10, 2017, 05:39:52 pmR8 & C12 are a low pass filter.C8 blocks DC while passing audio signal.R14 gives opamp input a ground reference otherwise it would drift everywhere because C8 is an open cct to DC.

author: csheldon
author_rank: Contributor
timestamp: June 11, 2017, 01:37:05 pm
content: I'm sorry, I was refering to C12 in parallel to R14

author: Audioguru
author_rank: Super Contributor
timestamp: June 11, 2017, 02:05:25 pm
content: R14 biases the (+) input of the opamp at 0V so this first opamp is a rectifier and cannot pass audio. But it might be an AM detector?Also, the second opamp is shown as a comparator with some hysteresis so it cannot pass audio. But this comparator is drawn wrong. A comparator has its input fed through a resistor to its (-) input, not directly to its (+) input as shown here.Therefore the signal might be digital and is not audio.

author: chris_leyson
author_rank: Super Contributor
timestamp: June 11, 2017, 02:23:41 pm
content: Back in the early 90's I worked on a few super-regen receiver designs for various battery powered transponders 173MHz, 418MHz, 433MHz and 458MHz. In all cases the oscillator was externally quenched with a sawtooth waveform because the performance of self quenching oscillators was not consistent. Simple receivers just had some bandpass filtering in the front end and no preamp. The biggest disadvantage of super-regen receivers is their lack of selectivity and poor adjacent channel rejection. One of the 433MHz receivers used a 418MHz SAW oscillator to down convert to 15Mhz followed by a 15MHz super-regen for AM detection.

author: csheldon
author_rank: Contributor
timestamp: June 11, 2017, 02:26:01 pm
content: I would appreciate if we could stick with the main questions about super-regenerative reciever part of the circuit, as I am more concerned about gaining know?edge about that

author: chris_leyson
author_rank: Super Contributor
timestamp: June 11, 2017, 02:42:00 pm
content: Just saying externally quenched super-regens are a lot easier to design and build.

author: Paul Price
author_rank: Super Contributor
timestamp: June 11, 2017, 09:33:08 pm
content: The second schematic shows a another super-regenerative receiver, but I would judge that this circuit is poorly designed and although it probably does work, it probably doesn't work as well as the first circuit despite having many more components.However, the reason this circuit works at all is probably due to the large amount of components clumsily hooked together and fiddled with to achieve some low level of performance, and this circuit probably only works at all within a narrow range of signal conditions.Looking at the design of Q1 working as a common-emitter RF pre-amp and also the overal circuit, I would wager this entire circuit would show very noticeable varying performance if you had a chance to compare several identical units.This circuit seems to be poorly designed, a result of trail and error design, doesn't look like the work of an experienced RF design engineer. It is a kludge.If you choose to study super-regen circuits, I would suggest you find a better circuit to study.

author: csheldon
author_rank: Contributor
timestamp: June 12, 2017, 10:24:46 am
content: This schematic in the attachement below was the best I could find, I think I pretty much understand how this works, but it is unclear to me, if this thing oscillates with or without the signal present, and those oscillations build up with the help of thermal noise, and quench signal is always present at R1 and C2 junction , how can we detect the ON/OFF signal at that junction, and how wer know there is something really being recieved, and it's not thermal noise?

author: Audioguru
author_rank: Super Contributor
timestamp: June 12, 2017, 02:06:34 pm
content: The circuit looks similar to the receiver circuit in a very cheap remote controlled toy car. They operate on 27MHz.

author: Paul Price
author_rank: Super Contributor
timestamp: June 12, 2017, 04:20:59 pm
content: This latest circuit is another variation of a super-regen receiver. The carrier freq. RF feedback from collector to emitter by a small capacitance C3 causes this circuit to be a grounded-base oscillator.It oscillates continuously at the RF freq.A frequency shift in the received signal causes the frequency of oscillation of the receiver to try to sync with the received frequency and this frequency shift causes the emitter voltage to vary slightly and this shift in voltage can be amplified to form a digital output pulse.Forget about this idea of quenching,it is not necessary for this super-regen receiver to quench to detect FM or FSK digital signals.The "on/off" of a digital pulse is transmitted as a frequency shift and this shift at the super-regen receiver manifests itself in a small voltage change at the emitter which, when amplified by a comparator stage becomes a logic-level on/off bit. In this way a tiny change of emitter voltage is the receiver analog output, and this is not initially an on/off pulse logic-level pulse.After signal conditioning this small voltage change becomes a bit detected by being converted into a logic level pulse and forms a data stream,with logic-level voltage swings at the output of the receiver comparator stages. In this way a data packet sent as a stream of small RF freq. shifts becomes a transmitted digital signal bit stream.