Optimal quench frequency

[mstechca]

I have found the following information from google groups on Superregen receivers:

Believe it or not, you can build an FM receiver with only one active
device: the circuit
is called a "superregenerative detector". The active device (tube or
transistor)
oscillates at two frequencies: the signal frequency (e.g., 88 to 108 MHz)
and a lower
frequency called the "quench" frequency. Generally, the quench frequency
is twice
the highest modulation component in the signal frequency (say, 40 to 100
KHz for FM
broadcast reception).

You can build the circuit with one active device (called a self-quenched
detector) or
two (one device oscillates at the signal freqnency and the other at the
quench frequency.

The quench frequency modulates the signal-frequency oscillations,
suppressing
signal-frequency oscillations in one state and allowing the oscillator to
fully oscillate another, second state. In between the two states, the
signal-frequency
oscillator balances on the verge of oscillation, and incoming signals (or
random
noise) pushes the circuit into oscillation.

Thus, the superregen is capable of large amounts of gain, but the
signal-freqency
oscillation gets reradiated from the detector, making the circuit
unpopular with
onther sensitive receivers using that portion of the radio spectrum. You
can
add an RF amplifier stage, but then you're up to two or three active
devices.

Now look at this part:

Generally, the quench frequency
is twice
the highest modulation component in the signal frequency (say, 40 to 100
KHz for FM
broadcast reception).

is the "highest modulation component" equal to the highest frequency that can be heard? and if so, why 40 to 100Khz instead of just 40Khz?

Also, what is the "highest modulation component" for TV signals?

 

[audioguru]

Hi MStechca,
If the quench frequency is less than double the highest modulation frequency then you will hear a new "beat" frequency that is the difference between the two frequencies.

FM stations broadcast stero subcarrier frequencies up to 53kHz and frequently broadcast 67kHz SCA music for stores and restaurants. The 67kHz is also FM, so its highest frequency is about 74kHz. If the quench frequency is only 75kHz then you will hear 1kHz to very high interference in the audio.
Some FM stations broadcast stock market data or something up to 100kHz. So maybe the quench oscillator should be set to 120kHz to avoid hearing nterference.

TV stations also have multiplexed stereo sound, so they also broadcast subcarrier frequencies up to 53kHz. I don't know if they broadcast other stuff above that, they probably do.

Now here is another problem with a super-regen to add to its list:
1) Transmits RF interference.
2) Has high distortion and poor treble because the positive feedback makes its single tuned circuit to have a bandwidth too narrow.
3) Has poor stations selectivity because it has only a single tuned circuit.
4) Is very easy to overload with stong local stations.
5) Poor sensitivity for FM because it must be tuned to the side of a station's carrier for slope FM detection.
6) Very noisy when the signal is strong.
7) Beat frequency interference between its quench oscillator and the stations modulation frequencies.
:cry: :cry: :cry: