How does this noise blanker work?

I think you are right. The signal is normally muted by Q3 being conductive until Q2 removes it's gate voltage. I'm not sure the time constants are useful for discerning pulses from other sounds either.

I suspect the name is misleading, I think it might be a 'noise gate' rather than a noise blanker, a circuit that mutes the sound until a certain level is reached with the intention of killing background noise until sufficient audio level is reached. The term 'noise blanker' is usually used to describe a system to suppress impulse interference.

Brian.

betwixt said:

I think you are right. The signal is normally muted by Q3 being conductive until Q2 removes it's gate voltage. I'm not sure the time constants are useful for discerning pulses from other sounds either.

I suspect the name is misleading, I think it might be a 'noise gate' rather than a noise blanker, a circuit that mutes the sound until a certain level is reached with the intention of killing background noise until sufficient audio level is reached. The term 'noise blanker' is usually used to describe a system to suppress impulse interference.

Brian.

Click to expand...

You are right, I just found that it is a noise gate indeed and that explains the extra mosfet.
As I see it, we only need to reduce the RF input signal, only when these short noise pulses come in.
I think the circuit (with different capacitor values) would work for HF if Q2 collecor is directly connected to the RF out line and the Q3, R5, R4, C4 completely missing.

What do you think?

(I am looking for a simple pulsating noise blanker working on the RF, not IF, for my receivers that do not have one)

Noise blankers or noise gates at RF stage increase the noise figure and reduce the dynamic range of the receiver.
But they work well at IF stage or in baseband, stages where could be used discrete circuits or DSP noise reduction algorithms.

vfone said:

Noise blankers or noise gates at RF stage increase the noise figure and reduce the dynamic range of the receiver.
But they work well at IF stage or in baseband, stages where could be used discrete circuits or DSP noise reduction algorithms.

Click to expand...

When the huge pulsating noise makes the receiver deaf (AGC) I think it does not matter much if a noise blanker exists directly in the receiver input. Such a noise blanker though, should be after a preselector, so that the amplifier noise pulses are from in-band and not from other bands. Many times the pulsating noise exists only in certain portions of the HF only.

Saying so, I am talking about receivers that do not have IF noise blankers, because the ones that do have, can deal with this type of noise in a better way as you describe. It's a better something than nothing situation.