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CSQ
- Thread starter 74IN
- Start date
Carrier squelch is based on a comparator having an adjustable reference. ie. the "squelch control".
The comparator is looking at input signal derived agc voltage & comparing it to the squelch control voltage.
When the agc voltage rises above a certain level, the comparator circuit changes the CSQ line.
Noise squelch is based on high pass filtered audio noise present at the output of an FM discriminator/demodulator.
The hi pass filtered noise is rectified and fed to a comparator.
During no input signal operation, the noise squelch produces the most "noise".
As signal increases, the hi-pass filtered audio level decreases due to "limiting" inherent in fm det.
At some point, the lowering of rectified hi frequency noise will fall below a preset and change the comparator.
Carrier squelch and noise squelch are used in parallel to minimize falsing of the squelch circuitry.
Together, these circuits qualify an incoming signal as a wanted signal or just noise spikes.
Each circuit will have different attack / decay times.
In some cases, the noise squelch circuit also provides a function usefull in a scanning receiver.
The fm detector used in noise squelch can also determine if a signal is centered on frequency.
If the signal is outside a given bandwidth, the receiver continues to scan even if the same signal could be demodulated by the AM detector.
The comparator is looking at input signal derived agc voltage & comparing it to the squelch control voltage.
When the agc voltage rises above a certain level, the comparator circuit changes the CSQ line.
Noise squelch is based on high pass filtered audio noise present at the output of an FM discriminator/demodulator.
The hi pass filtered noise is rectified and fed to a comparator.
During no input signal operation, the noise squelch produces the most "noise".
As signal increases, the hi-pass filtered audio level decreases due to "limiting" inherent in fm det.
At some point, the lowering of rectified hi frequency noise will fall below a preset and change the comparator.
Carrier squelch and noise squelch are used in parallel to minimize falsing of the squelch circuitry.
Together, these circuits qualify an incoming signal as a wanted signal or just noise spikes.
Each circuit will have different attack / decay times.
In some cases, the noise squelch circuit also provides a function usefull in a scanning receiver.
The fm detector used in noise squelch can also determine if a signal is centered on frequency.
If the signal is outside a given bandwidth, the receiver continues to scan even if the same signal could be demodulated by the AM detector.
It's a carrier with nothing "riding" on it like a tone.
I copied the following:
A simple carrier squelch or noise squelch operates strictly on the signal strength of the signal, such as when a television mutes the audio or blanks the video on "empty" channels, or when a walkie talkie mutes the audio when no signal is present. In some designs, the squelch threshold is preset. For example, television squelch settings are usually preset. Receivers in base stations at remote mountain top sites are usually not adjustable remotely from the control point.
In devices such as radiotelephones (also known as two-way radios), the squelch threshold is set with an adjustable knob marked squelch. This setting adjusts the threshold at which signals will open the audio channel. Backing off the control will turn on the audio, and the operator will hear white noise if there is no signal present. The usual operation is to adjust the control until the channel just shuts off - then only a small threshold signal is needed to turn on the speaker. However, if a weak signal is annoying, the operator can adjust the squelch to open only when stronger signals are received.
A typical FM two-way radio carrier squelch circuit takes out the voice components of the receive audio by passing the detected audio through a high-pass filter. A typical filter might pass frequencies over 4,000 Hz (4 kHz). The squelch control adjusts the gain of an amplifier which varies the level of noise coming out of the filter. The audio output of the filter and amplifier is rectified and produces a DC voltage when noise is present. The presence of noise creates a DC voltage which turns the receiver audio off. When a signal with little or no noise is received, the voltage goes away and the receiver audio is unmuted. Some applications have the receiver tied to other equipment that uses the audio muting control voltage as a "signal present" indication.
I copied the following:
A simple carrier squelch or noise squelch operates strictly on the signal strength of the signal, such as when a television mutes the audio or blanks the video on "empty" channels, or when a walkie talkie mutes the audio when no signal is present. In some designs, the squelch threshold is preset. For example, television squelch settings are usually preset. Receivers in base stations at remote mountain top sites are usually not adjustable remotely from the control point.
In devices such as radiotelephones (also known as two-way radios), the squelch threshold is set with an adjustable knob marked squelch. This setting adjusts the threshold at which signals will open the audio channel. Backing off the control will turn on the audio, and the operator will hear white noise if there is no signal present. The usual operation is to adjust the control until the channel just shuts off - then only a small threshold signal is needed to turn on the speaker. However, if a weak signal is annoying, the operator can adjust the squelch to open only when stronger signals are received.
A typical FM two-way radio carrier squelch circuit takes out the voice components of the receive audio by passing the detected audio through a high-pass filter. A typical filter might pass frequencies over 4,000 Hz (4 kHz). The squelch control adjusts the gain of an amplifier which varies the level of noise coming out of the filter. The audio output of the filter and amplifier is rectified and produces a DC voltage when noise is present. The presence of noise creates a DC voltage which turns the receiver audio off. When a signal with little or no noise is received, the voltage goes away and the receiver audio is unmuted. Some applications have the receiver tied to other equipment that uses the audio muting control voltage as a "signal present" indication.
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