Question about FRS/GMRS Walkie Talkies

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: snipped-for-privacy@monmouth.com _/_/_/_/_/_/_/_/_/_/_/_/_/ _/ N 40 deg 23' 41.9", W 74 deg 11' 29.7"/_/_/_/_/_/_/_/_/ _/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/

Reply to
Cass Lewart
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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.

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17 days!


Michael A. Terrell
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Michael A. Terrell

Which means that anyone with an open receiver on the channel will hear 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

Reply to
Michael Black

: 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: snipped-for-privacy@monmouth.com _/_/_/_/_/_/_/_/_/_/_/_/_/ _/ N 40 deg 23' 41.9", W 74 deg 11' 29.7"/_/_/_/_/_/_/_/_/ _/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/

Reply to
Cass Lewart

Either that, or a DSP to give selectivity.

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16 days!


Michael A. Terrell
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Michael A. Terrell

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

Reply to
Michael Black

Michael A. Terrell ( snipped-for-privacy@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: snipped-for-privacy@monmouth.com _/_/_/_/_/_/_/_/_/_/_/_/_/ _/ N 40 deg 23' 41.9", W 74 deg 11' 29.7"/_/_/_/_/_/_/_/_/ _/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/

Reply to
Cass Lewart

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.

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16 days!


Michael A. Terrell
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Michael A. Terrell

Now

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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."

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Richard Crowley

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