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Local Oscillator in PMR 446MHz walkie-talkies

Posted: 18 Dec 2017 18:59
by KPC850
Local Oscillator in PMR 446MHz walkie-talkies -

I have a pair of PMR446 Walkie Talkies - Twintalker 4710DCP - bought in Lidl some years ago, I discovered that the Local Oscillator (LO) operates 21.4MHz below the received frequency (to create a 21.4MHz IF) and can be received in the vicinity.

so for example when one of these walkie talkies is receiving PMR Ch1, which is 446.00625 MHz, the local oscillator would be 424.60625 (nearest the scanner used can get to that is 424.605) and the 'carrier' radiated to a extent on that frequency is receivable in the vicinity.

(BTW as a power saving measure, the carrier is heard pulsing on and off when the scanner is muted and just monitoring the channel - this of course as a battery saving measure, though it means you may miss a fraction of a second before an incoming transmission is heard)

Is the above (LO at 21.4MHz below received frequency) common for many other PMR 446 radios, or are others usually different (to the Twintalker 4710 DCP)?