Operating procedures

Use this page to explain what various functions of a transceiver are and how to use them.

Related wiki pages Apparatus

AF gain

Simply the volume control - like the one at your everyday BC radio.

Passband Tuning

Passband tuning manipulates the tuning circuitry of the transceiver, allowing "close together" signals to be separated - one is rejected by the filter, one is allowed to pass through. It is similar to IF shift, where the IF of the transceiver is shifted slightly to tune in slightly off frequency signals.

Passband tuning and IF shift are used because simply narrowing down the passband would produce muffling of any audio signals. Narrowing down the passband is however the best way to pick out narrow band signals (like CW and narrow band digimodes) from a noisy band.

External link from QST

RF gain

Controls the amount of pre-amplification in the RX before the first if stage is reached. It is very useful in QRN and other noisy conditions where otherwise SSB stations become unreadable. Take the RF gain back and have a QSO that would be impossible with full gain. Some rigs have also a switchable additional RF gain button, that sometimes has also an attenuation function. This in effect just widens the margin of the overall RF gain.

RIT

RIT : Receiver Incremental Tuning, Receive Independant of Transmit is the ability to shift the receive frequency of a transceiver away from the transmit frequency by a small amount. Used in SSB transmissions, it was very useful in older rigs that had frequency stability problems. RIT can also be used to deliberately transmit and receive on different fequencies. This is known as "working split", with the difference in frequencies usually 3khz to 10Khz.

Roofing Filters

A roofing filter is used in an HF receiver and us usually found after the first receiver mixer. It's purpose is to reduce the passband of the first IF (first intermediate frequency) to between 6kHz and 20KHz. This in turn reduces distortion in any amplifier and mixer circuits.

Roofing filters are usually usually crystal types because of the steeper filtering curve that they produce.

Roofing filters can be much narrower than 20kHz when listening to very weak CW signals. In this case the filter may have a bandpass of as low as 250Hz. This also requires that the first IF in the receiver is below VHF, perhaps as low as 9MHz.

See the regular Wikipedia

Squelch

Squelch is used to silence the loudspeaker when no signal is present the frequency of operation. In ham radio this will suppress the general static that remains present on most bands and is heard during breaks in the distant station's transmission. Squelch will not remove noise received simultaneously with valid signal; that may only be done with directional antennas (to block interference from other directions) or filters (to block specific frequencies).

At its most basic level, squelch opens the loudspeaker when any signal is present. Another one of the several forms of squelch used in ham radios, CTCSS, requires that a specific tone be present in a received signal - reducing the probability of a repeater system being triggered by random noise, by adjacent-channel interference or by co-channel interference from DX stations intending to access another repeater on the same frequency in another city.

XIT

Similar to RIT above, but the transmit frequency is changed while the receive frequency is that shown on the frequency display.

Operating procedures
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