What is a Superegenerative Detector and How Does it Work?

[michael1978]

helo

can somebody tell me how does work this circuits please, calculations and the rest

 

[berkeman]

helo

can somebody tell me how does work this circuits please, calculations and the restView attachment 215030

Where did you get the schematic? 'There is probably a basic explanation there. Can you post a link to where you got it?

 

[AlexCaledin]

"... the circuit is basically a single transistor superregenerative RF oscillator with a constant amplitude.
Here we have tried to enhance the design such that the amplitude becomes considerably magnified in order to turn OFF the transistor completely during the oscillations.
This called for an increase in the feedback capacitor and also to use a transistor specifically designed for handling extreme high frequency ranges such as a BF494.

Further modifications include an inductor with the emitter of the transistor, and a capacitor across the emitter resistor of the transistor.

Due to this the transistor is switched ON as soon as the base emitter voltage of the transistor falls significantly, resulting in an abrupt cut off in the oscillations.
However this prompts the emitter capacitor to discharge, allowing the collector current to yet again resume its flow, initiating a fresh cycle of oscillation.
The above happening forces the circuit to flip flop between two situations, oscillator OFF and oscillator ON, resulting a sawtooth frequency of about 50kHz at the output.
Each time the circuit flips across the above ON/OFF states, results in a significant stepping up of the amplitude which in turn constitutes greater amplification of the received signals. The procedure also gives rise to noise but only as long as a station is not being detected."

http://circuitsearch.blogspot.ru/2013/10/make-this-simple-fm-radio-circuit-using.html

- But the MMBT918LT1G transistor is very cheap and 5 times more high-frequency than BF494.

 

[michael1978]

Where did you get the schematic? 'There is probably a basic explanation there. Can you post a link to where you got it?

of course here https://www.homemade-circuits.com/make-this-simple-fm-radio-circuit-using/but i find difficult to understand the capacitor between the emitter and collector?

 

[michael1978]

"... the circuit is basically a single transistor superregenerative RF oscillator with a constant amplitude.
Here we have tried to enhance the design such that the amplitude becomes considerably magnified in order to turn OFF the transistor completely during the oscillations.
This called for an increase in the feedback capacitor and also to use a transistor specifically designed for handling extreme high frequency ranges such as a BF494.

Further modifications include an inductor with the emitter of the transistor, and a capacitor across the emitter resistor of the transistor.

Due to this the transistor is switched ON as soon as the base emitter voltage of the transistor falls significantly, resulting in an abrupt cut off in the oscillations.
However this prompts the emitter capacitor to discharge, allowing the collector current to yet again resume its flow, initiating a fresh cycle of oscillation.
The above happening forces the circuit to flip flop between two situations, oscillator OFF and oscillator ON, resulting a sawtooth frequency of about 50kHz at the output.
Each time the circuit flips across the above ON/OFF states, results in a significant stepping up of the amplitude which in turn constitutes greater amplification of the received signals. The procedure also gives rise to noise but only as long as a station is not being detected."

http://circuitsearch.blogspot.ru/2013/10/make-this-simple-fm-radio-circuit-using.html

hi, i don't understand the capacitor between the emitter and collector, how is working biasing i understand emiter restitance and bypass capacitor i understand, and , tunned circuit i understand,

 

[AlexCaledin]

The emitter has its capacitance as well, the emitter-base capacitance, it may be 10 or 20 pf or so, with those old transistors. So, together with that emitter capacitance, the capacitor between the emitter and collector makes voltage divider such as to apply (via C1 and C4 which are big enough) a part of the "LC" high-frequency voltage across the emitter-base junction of the transistor, without overloading the oscillations in the LC circuit. This feedback is positive, so it makes the whole circuit to oscillate - if the average current, through the transistor, is sufficient - but that current, in a super-regenerator, is not stable and the high frequency oscillation interrupts itself periodically so it has to start anew every 20 microseconds and the process of starting is extremely sensitive to the small input signal and noise.

 

[michael1978]

The emitter has its capacitance as well, the emitter-base capacitance, it may be 10 or 20 pf or so, with those old transistors. So, together with that emitter capacitance, the capacitor between the emitter and collector makes voltage divider such as to apply (via C1 and C4 which are big enough) a part of the "LC" high-frequency voltage across the emitter-base junction of the transistor, without overloading the oscillations in the LC circuit. This feedback is positive, so it makes the whole circuit to oscillate - if the average current, through the transistor, is sufficient - but that current, in a super-regenerator, is not stable and the high frequency oscillation interrupts itself periodically so it has to start anew every 20 microseconds and the process of starting is extremely sensitive to the small input signal and noise.

So c1 is not coupling capacitor, so c1 and c4 voltage divider, is like this tuned circuit, when finish one oscillation,is zero voltage, during the oscilations LC the emitter resistor is charged through inductor, transistor is of, after emitter discharge through c1 and c4 right? and turn the transistor on, and repeating on off

 

[michael1978]

please can somebody more explain me, for example, what do capacitor c1 and c4, what for values to select,

 

[Averagesupernova]

please can somebody more explain me, for example, what do capacitor c1 and c4, what for values to select,

It appears that they are both bypass capacitors. The size would be determined by the frequency of interest.
-
I have to say some of the component values as well as a few other things seem a bit wonky. Bypass and feedback capacitors in the pF and nF range and they hang a 10 uF on the output.

 

[AlexCaledin]

- do not forget, the amplitude of the high (near 100MHz) frequency oscillation is itself oscillating here! So, the bypass capacitors for the HF are, at the same time, the phase-shift capacitors for that "secondary" oscillation process. Indeed, the RC is always the same, 10nf x 10kOhm, to make the HF oscillation unstable.

 

[michael1978]

It appears that they are both bypass capacitors. The size would be determined by the frequency of interest.
-
I have to say some of the component values as well as a few other things seem a bit wonky. Bypass and feedback capacitors in the pF and nF range and they hang a 10 uF on the output.

can you show me one example, for example 100mhz what is formula of capacitor, i know for bypass 10 OF 100 time smather then the emitter resistor i mean for amplifier, thnx

 

[michael1978]

- do not forget, the amplitude of the high (near 100MHz) frequency oscillation is itself oscillating here! So, the bypass capacitors for the HF are, at the same time, the phase-shift capacitors for that "secondary" oscillation process. Indeed, the RC is always the same, 10nf x 10kOhm, to make the HF oscillation unstable.

thnx but this osicillation is dificult to study amstrong coppits hartley is easy in you mind, but the problem is how it works

 

[AlexCaledin]

... how it works

- when there is some HF oscillation, C5 is getting charged, C4 discharged, C1 discharged with delay. So the average base-emitter voltage is getting less and less. The nanosecond pulses of the transistor current are getting weaker as the LC oscillator is getting damped (but still "sucking" last transistor current pulses via C3). Finally, there's practically no HF oscillation because the base-emitter voltage is too low and cannot be restored immediately (even though C5 is already discharging) because C1 cannot stop discharging immediately being delayed by C4. So the HF oscillation has to start anew some microsecond(s) later and that process of start is extremely sensitive to the input signal.

- the "Up" saw on this (not very accurate) picture is the emitter (C5) voltage, it's used as the output in that variant,
https://refdb.ru/look/1527014.html

 

[michael1978]

- when there is some HF oscillation, C5 is getting charged, C4 discharged, C1 discharged with delay. So the average base-emitter voltage is getting less and less. The nanosecond pulses of the transistor current are getting weaker as the LC oscillator is getting damped (but still "sucking" last transistor current pulses via C3). Finally, there's practically no HF oscillation because the base-emitter voltage is too low and cannot be restored immediately (even though C5 is already discharging) because C1 cannot stop discharging immediately being delayed by C4. So the HF oscillation has to start anew some microsecond(s) later and that process of start is extremely sensitive to the input signal.

View attachment 215603
- the "Up" saw on this (not very accurate) picture is the emitter (C5) voltage, it's used as the output in that variant,
https://refdb.ru/look/1527014.html

thank you ver much i understand

 

[michael1978]

- when there is some HF oscillation, C5 is getting charged, C4 discharged, C1 discharged with delay. So the average base-emitter voltage is getting less and less. The nanosecond pulses of the transistor current are getting weaker as the LC oscillator is getting damped (but still "sucking" last transistor current pulses via C3). Finally, there's practically no HF oscillation because the base-emitter voltage is too low and cannot be restored immediately (even though C5 is already discharging) because C1 cannot stop discharging immediately being delayed by C4. So the HF oscillation has to start anew some microsecond(s) later and that process of start is extremely sensitive to the input signal.

View attachment 215603
- the "Up" saw on this (not very accurate) picture is the emitter (C5) voltage, it's used as the output in that variant,
https://refdb.ru/look/1527014.html

hi is that document also in english

 

[michael1978]

sorry that i make you tired you explain very good, but this space i don't undertand

 

[tech99]

hi, i don't understand the capacitor between the emitter and collector, how is working biasing i understand emiter restitance and bypass capacitor i understand, and , tunned circuit i understand,

May I summarise the operation of the superregenerative detector shown.
From an RF point of view, L1 and L2 are in series in the collector circuit, and the emitter is tapped on to their junction. This causes collector and base to be 180 degrees out of phase, and together with the phse reversal in the transistor, this creates 360 deg phase shift around the feedback loop from collector to base. This makes oscillation at 100MHz possible.
The circuit is also caused to squegg, or regularly stop oscillating, by using the blocking oscillator technique. This happens at about 50kHz, and is caused by the long time constant provided mainly by C1 xR2 and C4xR4. Without C4 x R4 the circuit will not squegg.
Every time the oscillator tries to re-start, its starting condition is very sensitive to any small existing oscillation in the LC circuit, maybe in the order of a microvolt. This signal determines the height to which oscillation can climb during the build-up time.
During resting conditions, the oscillations in the LC circuit are thermal noise, so the receiver gives a loud noise output in the order of 1 volt. Notice that this corresponds to a gain of 120dB, which is very high.
When a signal is received, the noise is subjectively suppressed, and AM demodulation occurs, with an apparent AGC action.
Due to the 50kHz frequency, which is called the quenching frequency, the receiver has a bandwidth of nearly 100kHz and also responds at several side frequencies of the 50kHz quenching oscilation.
The circuit can be improved by using a separate quench oscillator and by providing better adjustment of regeneration, but this requires increased complexity..
#

 

[michael1978]

May I summarise the operation of the superregenerative detector shown.
From an RF point of view, L1 and L2 are in series in the collector circuit, and the emitter is tapped on to their junction. This causes collector and base to be 180 degrees out of phase, and together with the phse reversal in the transistor, this creates 360 deg phase shift around the feedback loop from collector to base. This makes oscillation at 100MHz possible.
The circuit is also caused to squegg, or regularly stop oscillating, by using the blocking oscillator technique. This happens at about 50kHz, and is caused by the long time constant provided mainly by C1 xR2 and C4xR4. Without C4 x R4 the circuit will not squegg.
Every time the oscillator tries to re-start, its starting condition is very sensitive to any small existing oscillation in the LC circuit, maybe in the order of a microvolt. This signal determines the height to which oscillation can climb during the build-up time.
During resting conditions, the oscillations in the LC circuit are thermal noise, so the receiver gives a loud noise output in the order of 1 volt. Notice that this corresponds to a gain of 120dB, which is very high.
When a signal is received, the noise is subjectively suppressed, and AM demodulation occurs, with an apparent AGC action.
Due to the 50kHz frequency, which is called the quenching frequency, the receiver has a bandwidth of nearly 100kHz and also responds at several side frequencies of the 50kHz quenching oscilation.
The circuit can be improved by using a separate quench oscillator and by providing better adjustment of regeneration, but this requires increased complexity..
#

thank you very much, now i understand c1 and c4, i don't want to bother you, but is difficult to use e seprate quench oscillator for beter adjustment

 

[rootone]

There's no harm in using two transistors, they are cheap.
https://www.google.ie/search?q=two+...AhUYM8AKHZ3DAC0Q9QEIKjAA#imgrc=WxtaQ5VSt9ki0M:

 

[michael1978]

There's no harm in using two transistors, they are cheap.
https://www.google.ie/search?q=two+transistor+radio&client=firefox-b-ab&dcr=0&tbm=isch&source=iu&ictx=1&fir=WxtaQ5VSt9ki0M%3A%2CkH_xqfNZCm6Z4M%2C_&usg=__yFze9x9s2QpJvSx6YfQNP7x-E5Y=&sa=X&ved=0ahUKEwiD_4O05OLXAhUYM8AKHZ3DAC0Q9QEIKjAA#imgrc=WxtaQ5VSt9ki0M:

hello is beter than with one transistor?low noise, is this regenative i don't see a coil;-)

 

[tech99]

hello is beter than with one transistor?low noise, is this regenative i don't see a coil;-)

This is a superegenerative detector. In principle, any oscillator circuit can be used, and the circuit is caused to go into and out of oscillation at a low frequency, called the quenching frequency, by using a lot of feedback plus some relatively long CR time constants. An advantage of the circuit is that it provides very high gain without the need for critical adjustment - the RF oscillator is just left "flat out".
In another type of circuit, called a regenerative detector, we do not use a quench frequency, but manually adjust the positive feedback so that the oscillator is on the verge of oscillation. This requires a very stable construction, and careful adjustment of the amount of feedback and the gain. When properly constructed and adjusted, the circuit is sensitive enough to detect thermal noise and has a much smaller bandwidth than the superegenerative circuit. The circuit can also be used gently oscillating, when it becomes less critical to adjust and very sensitive, and is able to demodulate CW and SSB.
If the superegenerative circuit is used with a separate quench oscillator and a regeneration control, it is possible to reduce the spurious side responses and even to obtain an adjustment where the receiver is quiet with no signal. (This was the principle of the WW2 IFF transponder system used with Radar).
All these ideas involve a degree of analogue expertise and green fingers - not often found.
The superegenerative receiver was used in 1930s to provide public telephony to islands, including between the Hawaiian Islands and to the Western Isles of Scotland.

 

[michael1978]

This is a superegenerative detector. In principle, any oscillator circuit can be used, and the circuit is caused to go into and out of oscillation at a low frequency, called the quenching frequency, by using a lot of feedback plus some relatively long CR time constants. An advantage of the circuit is that it provides very high gain without the need for critical adjustment - the RF oscillator is just left "flat out".
In another type of circuit, called a regenerative detector, we do not use a quench frequency, but manually adjust the positive feedback so that the oscillator is on the verge of oscillation. This requires a very stable construction, and careful adjustment of the amount of feedback and the gain. When properly constructed and adjusted, the circuit is sensitive enough to detect thermal noise and has a much smaller bandwidth than the superegenerative circuit. The circuit can also be used gently oscillating, when it becomes less critical to adjust and very sensitive, and is able to demodulate CW and SSB.
If the superegenerative circuit is used with a separate quench oscillator and a regeneration control, it is possible to reduce the spurious side responses and even to obtain an adjustment where the receiver is quiet with no signal. (This was the principle of the WW2 IFF transponder system used with Radar).
All these ideas involve a degree of analogue expertise and green fingers - not often found.
The superegenerative receiver was used in 1930s to provide public telephony to islands, including between the Hawaiian Islands and to the Western Isles of Scotland.

thanks