Question about FRS/GMRS Walkie Talkies

C

Cass Lewart

Guest
The inexpensive FRS/GMRS walkie talkies operate on up to 22 channels,
each with selectable 38 subchannels.For example, channel 1 has an
assigned frequency of 462.5625 MHz. The same frequency seems to be used
on all subchannels. If I set one walkie talkie to channel 1, subchannel
0, or a scanner to 462.5625 MHz I can listen to transmissions from
another walkie talkie set to channel 1 and any of the subchannels (0 -
38). However, if the receiving walkie talkie is set to channel 1,
subchannel 10, then the transmitting walkie talkie also has to be set to
channel 1, subchannel 10. My question is how does the walkie talkie tune
out other subchannels?

My feeling is that the walkie talkies use some kind of TRF with an
active low pass filter. BTW is it possible to implement IF type band
pass filters (10.8 MHz, 200 kHz bandwidth, or 455 kHz with 10 kHz
bandwidth) using active components and capacitors without inductors? I
was unable to find any schematics of Walkman or walkie talkie type
receivers.

--
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
_/ Cass Lewart Voice (732) 264-9541 Fax (630) 566-0349/_/
_/ e-mail: rlewart@monmouth.com _/_/_/_/_/_/_/_/_/_/_/_/_/
_/ N 40 deg 23' 41.9", W 74 deg 11' 29.7"/_/_/_/_/_/_/_/_/
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
 
Cass Lewart wrote:
The inexpensive FRS/GMRS walkie talkies operate on up to 22 channels,
each with selectable 38 subchannels.For example, channel 1 has an
assigned frequency of 462.5625 MHz. The same frequency seems to be used
on all subchannels. If I set one walkie talkie to channel 1, subchannel
0, or a scanner to 462.5625 MHz I can listen to transmissions from
another walkie talkie set to channel 1 and any of the subchannels (0 -
38). However, if the receiving walkie talkie is set to channel 1,
subchannel 10, then the transmitting walkie talkie also has to be set to
channel 1, subchannel 10. My question is how does the walkie talkie tune
out other subchannels?

My feeling is that the walkie talkies use some kind of TRF with an
active low pass filter. BTW is it possible to implement IF type band
pass filters (10.8 MHz, 200 kHz bandwidth, or 455 kHz with 10 kHz
bandwidth) using active components and capacitors without inductors? I
was unable to find any schematics of Walkman or walkie talkie type
receivers.
Click to expand...
They are not TRF, they use subaudiable CCTS tone squelch. You can't
have that many narrow band adjacent channels with a simple TRF design.

--
17 days!


Michael A. Terrell
Central Florida
 
"Michael A. Terrell" (mike.terrell@earthlink.net) writes:
Cass Lewart wrote:

The inexpensive FRS/GMRS walkie talkies operate on up to 22 channels,
each with selectable 38 subchannels.For example, channel 1 has an
assigned frequency of 462.5625 MHz. The same frequency seems to be used
on all subchannels. If I set one walkie talkie to channel 1, subchannel
0, or a scanner to 462.5625 MHz I can listen to transmissions from
another walkie talkie set to channel 1 and any of the subchannels (0 -
38). However, if the receiving walkie talkie is set to channel 1,
subchannel 10, then the transmitting walkie talkie also has to be set to
channel 1, subchannel 10. My question is how does the walkie talkie tune
out other subchannels?

My feeling is that the walkie talkies use some kind of TRF with an
active low pass filter. BTW is it possible to implement IF type band
pass filters (10.8 MHz, 200 kHz bandwidth, or 455 kHz with 10 kHz
bandwidth) using active components and capacitors without inductors? I
was unable to find any schematics of Walkman or walkie talkie type
receivers.

They are not TRF, they use subaudiable CCTS tone squelch. You can't
have that many narrow band adjacent channels with a simple TRF design.

Which means that anyone with an open receiver on the channel will hear
Click to expand...
everyone. For that matter, if there are two pairs of the FRS units
within range on the same channel, but using different "subchannels",
they will interfere with each other, though it may not be all that clear
what is happening.

The subaudible tone sent at the transmitter opens the squelch at the
receiver, provided both units are set for the same tone frequency (read
"subchannel"). If they don't match, the receiver will still be receiving
the signal (provided they are set to the same channel) but it won't
go from the receivers detector to the audio amplifier, and it will appear
like there is no signal. But an open receiver will hear it fine, and
won't even be aware of the tone squelch.

Michael
 
: The subaudible tone sent at the transmitter opens the squelch at the
: receiver, provided both units are set for the same tone frequency (read
: "subchannel"). If they don't match, the receiver will still be receiving
: the signal (provided they are set to the same channel) but it won't
: go from the receivers detector to the audio amplifier, and it will appear
: like there is no signal. But an open receiver will hear it fine, and
: won't even be aware of the tone squelch.

: Michael


Hi guys,
thanks for the explanation.I checked it by suppressing the squelch. Now
to my other question - are any of these receivers using active bandpass
filters as IF transformers, if they are superhets. I could not find any
inductors.
Cass
--
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
_/ Cass Lewart Voice (732) 264-9541 Fax (630) 566-0349/_/
_/ e-mail: rlewart@monmouth.com _/_/_/_/_/_/_/_/_/_/_/_/_/
_/ N 40 deg 23' 41.9", W 74 deg 11' 29.7"/_/_/_/_/_/_/_/_/
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
 
Cass Lewart wrote:
Hi guys,
thanks for the explanation.I checked it by suppressing the squelch. Now
to my other question - are any of these receivers using active bandpass
filters as IF transformers, if they are superhets. I could not find any
inductors.
Cass
Click to expand...
Either that, or a DSP to give selectivity.

--
16 days!


Michael A. Terrell
Central Florida
 
Cass Lewart (rlewart@shell.monmouth.com) writes:
: The subaudible tone sent at the transmitter opens the squelch at the
: receiver, provided both units are set for the same tone frequency (read
: "subchannel"). If they don't match, the receiver will still be receiving
: the signal (provided they are set to the same channel) but it won't
: go from the receivers detector to the audio amplifier, and it will appear
: like there is no signal. But an open receiver will hear it fine, and
: won't even be aware of the tone squelch.

: Michael


Hi guys,
thanks for the explanation.I checked it by suppressing the squelch. Now
to my other question - are any of these receivers using active bandpass
filters as IF transformers, if they are superhets. I could not find any
inductors.
Cass
Click to expand...
But like I said in a previous iteration in the other newsgroup,
you might not be able to tell by looking if the components are
surface mount. I have the board from a small cellphone in
front of me, and there's no sign of an IF transformer. Any coils
would take careful inspection to uncover. It does use a crystal
filter, and of course visually it's only the third lead that
denotes it as a filter and not a crystal.

I've taken apart some small cellphones and 900MHz cordless phones
where the IF filter is even less obvious. Sadly, they aren't handy
so I can't refresh my memory.

IF transformers are used for coupling, but they also provide selectivity.
But, they require alignment and any time something can be adjusted
there is the risk that it will shift (or be fiddled with) with time.
You can get better results with a crystal or ceramic filter, though
the filter might cost more. (It's all a tradeoff, since mass produced
things bring down cost, and paying a bit more for a filter is offset
by not having to adjust it.) Ceramic filters have been pretty common
in most broadcast radios, for thirty years. IN communication receivers,
ceramic or crystal filters have been virtually the norm for most of
the same period of time. With such filters in place (and they can be
pretty small and as surface mount parts might not be easily identifiable),
the IF transformers are only needed for coupling; careful design does
away with them, and then radios don't look like the average transistor
portable from thirty years ago.

Michael
 
Michael A. Terrell (mike.terrell@earthlink.net) wrote:
: Cass Lewart wrote:
: >
: >
: > Hi guys,
: > thanks for the explanation.I checked it by suppressing the squelch. Now
: > to my other question - are any of these receivers using active bandpass
: > filters as IF transformers, if they are superhets. I could not find any
: > inductors.
: > Cass

: Either that, or a DSP to give selectivity.

: --
: 16 days!


: Michael A. Terrell
: Central Florida


Hi Michael,
can DSP be used at RF frequencies? My recollection is that it was mostly used
at audio frequencies, though science marches on. I have the unsubstantiated
feeleing that the small FRS and walkman type radios use an oscillator, which is
at the same frequency as the incoming signal and the beat is then limited by an
active low pass filter. I may be wrong but as I said I could not find any
schematics.
Cass
--
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
_/ Cass Lewart Voice (732) 264-9541 Fax (630) 566-0349/_/
_/ e-mail: rlewart@monmouth.com _/_/_/_/_/_/_/_/_/_/_/_/_/
_/ N 40 deg 23' 41.9", W 74 deg 11' 29.7"/_/_/_/_/_/_/_/_/
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
 
Cass Lewart wrote:
Michael A. Terrell (mike.terrell@earthlink.net) wrote:
: Cass Lewart wrote:
:
:
: > Hi guys,
: > thanks for the explanation.I checked it by suppressing the squelch. Now
: > to my other question - are any of these receivers using active bandpass
: > filters as IF transformers, if they are superhets. I could not find any
: > inductors.
: > Cass

: Either that, or a DSP to give selectivity.

: --
: 16 days!

: Michael A. Terrell
: Central Florida

Hi Michael,
can DSP be used at RF frequencies? My recollection is that it was mostly used
at audio frequencies, though science marches on. I have the unsubstantiated
feeleing that the small FRS and walkman type radios use an oscillator, which is
at the same frequency as the incoming signal and the beat is then limited by an
active low pass filter. I may be wrong but as I said I could not find any
schematics.
Cass
Click to expand...
Microdyne uses them at 90 MHz for a 50 to 90 MHz wideband IF in the
RCB-2000 telemetry system I worked on, along with FIR filters to select
the final IF bandwidth.


--
16 days!


Michael A. Terrell
Central Florida
 
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:3FD5E8C7.FE0750BA@earthlink.net...
Cass Lewart wrote:

Michael A. Terrell (mike.terrell@earthlink.net) wrote:
: Cass Lewart wrote:
:
:
: > Hi guys,
: > thanks for the explanation.I checked it by suppressing the squelch.
Now
: > to my other question - are any of these receivers using active
bandpass
: > filters as IF transformers, if they are superhets. I could not find
any
: > inductors.
: > Cass

: Either that, or a DSP to give selectivity.

: --
: 16 days!

: Michael A. Terrell
: Central Florida

Hi Michael,
can DSP be used at RF frequencies? My recollection is that it was mostly
used
at audio frequencies, though science marches on. I have the
unsubstantiated
feeleing that the small FRS and walkman type radios use an oscillator,
which is
at the same frequency as the incoming signal and the beat is then
limited by an
active low pass filter. I may be wrong but as I said I could not find
any
schematics.
Cass

Microdyne uses them at 90 MHz for a 50 to 90 MHz wideband IF in the
RCB-2000 telemetry system I worked on, along with FIR filters to select
the final IF bandwidth.
Click to expand...
The WR-G303i model of the "WinRadio" says "... the entire final intermediate
frequency stage and an all-mode demodulator are entirely executed in
software, running on a personal computer."
http://www.winradio.com/home/g303i.htm
 
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