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11-11 of 11 messages
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Previous
Page 2 of 2
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RE: RF Detection Circuit
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Reply
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by WB6BYU on May 20, 2009
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Mail this to a friend!
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Actually that may complicate the design: do you want to
silence ALL receivers in the room when someone transmits
on ANY frequency?
If so, it isn't hard at all. As I explained before, a
simple diode detector, driven either from its own whip
antenna or from the antenna connected to the receiver,
will give a DC output on strong signals. Think of a
simple field-strength meter with a diode detector and a
meter. If it can deflect a meter, it can drive an op amp
or comparator circuit to operate a relay or squelch
circuit.
But that may not be very popular if someone using a cell
phone or a radio that isn't being monitored can shut down
all radio communications without warning.
If you want to ONLY squelch receivers on the operating
frequency it becomes much more difficult. Yes, you can
build a tuned circuit into the diode detector, but it
will have a pretty broad bandwidth (probably 10 or 20
MHz or more at VHF.) To make it frequency specific
you could tap off the RSSI output found on many receiver
circuits and run that to a voltage comparator. When
the RSSI voltage is above a preset level, drive the
squelch circuit to shut of the audio.
The problem will be to find the right comparator level,
however. It has to be high enough that normal communications
don't get clipped, but low enough to respond to an HT
on very low power. Of course, with outside antennas
it would also respond to an HT on the roof or perhaps
a mobile in the parking lot, which wouldn't otherwise
cause feedback problems.
To really make it work you'd need a separate receiver
tuned to the same frequency but with a small inside
antenna so it only responded to signals in the room.
But that wouldn't be too convenient for a scanner.
So there are two different ways it can be done, but
neither is without practical problems. You'll have
to decide which best fits your circumstances. The
second option will require some internal modifications
to all the radios involved, though it might be as simple
as adding a small circuit and soldering 4 wires to the
existing circuit board. Some radios, however, don't
have an RSSI output on the IF/detector, so would need
more circuitry.
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