PPRuNe Forums - Climax Operation

I understood this years ago... I don't anymore! This explanation is from my notes, written in the days before 8.33 kHz spacing which I guess finishes the possibility of offset carrier operation. I remember it is to enable more than one ground station to improve cover on a single channel. 'Provision exists in some areas for offset carrier operation, often referred to as Climax operation. This means that transmissions may be offset from the designated (pilot selected) frequency by plus and minus 5 kHz if a 2 carrier system is used, by + and - 7.3 kHz if a 3-carrier system and by other precise values if more than 3 carriers are used. The only concern for the radio engineer is that if these offset carriers are used a beat note is produced in the airborne Rx at 10 kHz ( or 14.6 kHz) when two (or 3) carriers are received at the same time.
The squelch circuit usually samples the the signal (speech) and the noise usinfg filters below and above 8 kHz. So for normal reception valid signals will have more detected frequencies below 8 kHz ( 300-3000 Hz speech) than above. A simple amplitude comparator will therefore open a gate to allow the speech through to the final amplifier. However if offset carrier operation is occurring beat notes above 8 kHz will will be produced after detection, simulating more noise than usual thus muting the audio o/p - squelch.
To overcome this VHF comms receivers have both signal-to-noise squelch and carrier squelch circuits. If the carrier is greater than 20 microvolts at the aerial then the squlch gate is opened even if beat notes are present. No problem to the pilot since he has selected the particular assigned offset frequncy and wants to hear the received signal.