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PICList Thread
'[OT] Squelch'
1998\02\27@023930 by Jacques Vrey

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Hi to all

I was wondering, with all you rf fundi's out there, if anyone could
explain to me how the squelch on two-way radio's works. I would
suspect that it would be some type of adjustable low pass or bandpass
filter ??

A simple circuit will also be of great help.

Sorry to push up the noise ratio but, dare I say, I haven't found a
group of more knowledgable people around.

TIA

Regards


Jacques Vrey
Iscor Steel Profile Products
Internal Post Point 74
PO Box 2
Newcastle
2940
South Africa
Tel: +27 (0)3431 48759
Fax: +27 (0)3431 48001
spam_OUTjvreyTakeThisOuTspamit.new.iscorltd.co.za
The views expressed above are not necessarily
those of Iscor Limited.

1998\02\27@035758 by davewave

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A squelch could be thought of as a noise gate
(http://www.rane.com/par-n.htm#noise_gate) that has very fast attack and
release times.

Dave

Jacques Vrey wrote:

> Hi to all
>
> I was wondering, with all you rf fundi's out there, if anyone could
> explain to me how the squelch on two-way radio's works. I would
> suspect that it would be some type of adjustable low pass or bandpass
> filter ??

1998\02\27@042506 by peter

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Jacques Vrey wrote:

> I was wondering, with all you rf fundi's out there, if anyone could
> explain to me how the squelch on two-way radio's works. I would
> suspect that it would be some type of adjustable low pass or bandpass
> filter ??

The quick cheep way is to use the rf carrier level as a trigger
A better way for FM signals is to use the level of static
Below is a scan from a vhf mobile radio handbook I have
The full circuit consists of only 4 transistors and an 741
If anyone wants a scan of the circuit let me know

Squelch

 Under no-signal conditions IC1 output consists substantially of noise,
which is applied to the active
filter TRl3 and its associated components, where a band lying above 15
kHz is extracted. The filtered
noise is then applied, via TRl3, to a temperature compensated amplifier
TRl4, whose gain is set by RV4
(Preset Squelch Control). After amplification, the noise is passed to
the active rectifier IC3, the limiting
characteristic of which cuts off frequency components in excess of 25
kHz. Due to the disposition of D5
amplification is much greater on the positive going half-cycles of input
signal than it is on the negative.
IC3 thus behaves as a rectifier, whose output is smoothed by R80, C79,
and applied to the Schmitt trigger
TR15, TRl6. With TRl5 in conduction, TRl6 is switched off; this in turn
switches off TR8, cutting off TR9
collector potential, and breaking the audio path.

 With a signal present, the noise content of the discriminator output
falls, causing the filter output to
fall. IC3 output falls, and the schmitt trigger changes its state. TR8
is switched on; in turn driving on TR9 to
complete the audio path.


--
Peter Cousens
email: .....peterKILLspamspam@spam@cousens.her.forthnet.gr  phone: + 3081 324450, 380534
snailmail:  Folia, Agia Fotini, Karteros, Heraklion  Crete, Greece.

1998\02\27@043748 by peter

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part 0 353 bytes
> A simple circuit will also be of great help.

Here is the circuit, the text I've posted to the list
It's a zipped windows bitmap

Peter Cousens
email: peterspamKILLspamcousens.her.forthnet.gr  phone: + 3081 324450, 380534
snailmail:  Folia, Agia Fotini, Karteros, Heraklion  Crete, Greece.

Attachment converted: wonderland:squelch.zip (pZIP/pZIP) (000137DB)

1998\02\27@082614 by peter

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Sorry the message with the attachment was meant to
go direct and not to the list
(a computer crash amongst other things broke my concentration
It's hard to think straight while muttering profanities directed to Bill
and Win95)


Peter Cousens
email: .....peterKILLspamspam.....cousens.her.forthnet.gr  phone: + 3081 324450, 380534
snailmail:  Folia, Agia Fotini, Karteros, Heraklion  Crete, Greece.

1998\02\27@110757 by John Shreffler

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There are several different squelch techniques.  One of the most
popular works on the fact that white noise of the "no signal"
variety contains a high amount of ultrasonic noise, whereas
a signal doesn't.

You can also look at the RF signal strength and gate on
the audio if it exceeds a certain amount.

{Original Message removed}

1998\02\27@160044 by Martin McCormick

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       Just to complicate things a bit, the noise squelch is one of
several good types of muting systems.  There is also the CTCSS or
Continuous Tone Coded Squelch System which various manufacturers call by
various trade names such as Motorola's PL or Private Line.  This uses a
low-pitched note which is more of a deliberate hum that is injected in to
the audio at any one of many different frequencies between about 50 and
200 HZ.  The receiver may have both a noise squelch and a CTCSS squelch
and you won't hear a thing unless there is both no noise and the right
frequency of hum.  The  older CTCSS-equipped receivers had metal reeds that
resonated at the CTCSS frequency in use, but modern receivers have digital
filters and maybe even a PIC or to in their squelch circuits.  There are
even digital squelches in which a very low-speed data pattern is placed
on the carrier along with the audio.  The pattern is about 10 frames
per second and contains a digital pattern that is unique to all the radios
that should be in communication with each other.  The pattern is even such
that the receiver need only detect that it has reached the same place in the
frame in which reception started in order to open the squelch.

       Then there is the tone-burst squelch in which the receiver
listens for a ping or beep of audio at the start of the transmission to
open the squelch and then has a conventional noise squelch to reclose it
when the signal leaves.  These have problems if there is an interfering
signal which is strong enough to open the squelch because it will stay open
after the desired signal leaves.  Also, if the desired signal is not clear
and should drop out for an instant, the noise squelch will reset and the
rest of the transmission is lost because there is no tone burst to reopen
it.

       Sorry for the rather detailed answer to a quick question, but I
think that PIC's are just the thing for these type circuits.  One could
even decode the digital squelch signals with a little prefiltering of the
signal to get rid of the voice or other audio that would make simple
measurement techniques not work well.

Martin McCormick

1998\02\27@195414 by Mike Keitz

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On Fri, 27 Feb 1998 09:36:14 GMT+2 Jacques Vrey
<EraseMEjvreyspam_OUTspamTakeThisOuTIT.NEW.ISCORLTD.CO.ZA> writes:
>Hi to all
>
>I was wondering, with all you rf fundi's out there, if anyone could
>explain to me how the squelch on two-way radio's works. I would
>suspect that it would be some type of adjustable low pass or bandpass
>filter ??

Most squelches are operated by noise.  As any user of a FM two-way radio
knows, the weaker a signal is, the noiser the recovered audio becomes.
Some of the more astute users have noticed that the "SQUELCH" control
adjusts the maximum noise (or minimum signal) that will be accepted.

The noise isn't just limited to what is heard through the speaker.  At
the output of the FM detector, the noise due to a weak or no signal
extends well above the audio range.  A good radio will filter this
high-frequency noise out of the signal that the user must listen to, but
the squelch circuit only observes this noise.  The squelch circuit
monitors only high frequencies because it is known that they will not be
intentionally transmitted.  The transmitter contains a low-pass filter so
only audio up to about 3 KHz is transmitted.  The squelch circuit
examines only frequencies above 10 KHz or so.

With this background, consider the crude ASCII block diagram below:

FM detector ---> Vol. control ---> LPF ---> Amplifier ---> Speaker
            |                                  ^--------------|
            ->Sql. control ---> HPF --> Env. det -->Threshold -

The signal from the squelch control is filtered to pass only the high
frequency noise, then rectified (envelope detector) to deduce the overall
noise level.  If the noise is higher than a certain threshold, the
amplifier is typically shut off to silence the speaker.  Of course any
part of the audio chain could be interrupted instead.

Usually the threshold is constant, and the squelch sensitivity is
adjusted by controlling the noise level input to the squelch processing
chain.  If the squelch control is set high, only a very slight amount of
noise will be enough to exceed the threshold and turn the speaker off.
If the squelch control is turned down, then no noise enters the squelch
chain, so the speaker stays on even if no signal is received.

Many enhancements are often made around the threshold circuit, for
example adding hysteresis or making the time constant (of the implied LPF
that is part of the envelope detector) vary with the signal level.  These
can help prevent weak signals form "chopping" on and off.

It is very simple to adjust this type of squelch remotely with a PIC.  No
digital pots, DACs, etc. are required.  Anyone besides me know how?


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1998\02\28@210730 by Dave Mullenix

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Jacques Vrey <jvreyspamspam_OUTIT.NEW.ISCORLTD.CO.ZA> asked:

> was wondering, with all you rf fundi's out there, if anyone could
> explain to me how the squelch on two-way radio's works. I would
> suspect that it would be some type of adjustable low pass or bandpass
> filter ??

Close.  They actually use a _high pass_ filter that passes noise that is
above voice frequencies.  When this noise is detected, the speaker is turned
off.

To see how this works, listen to a scanner with the squelch turned off.
When no signal is present, there's loud white noise coming out of the
speaker.  Some of this will make it through the high pass filter and shut
the speaker off.  As soon as a signal appears, the noise disappears and the
speaker is turned on.

Dave Mullenix, N9LTD

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