UCW and LCW

From the Ten-Tec Reflector October 27, 2006

On Upper Sideband an audio note is heard when a signal is ABOVE the "effective BFO frequency" and in order to bring that note to zero beat, one must tune upward, lowering the pitch of the audio note until it is zero. If you continue tuning upward, past zero beat, the note will rise again as you are now detecting the signal in the lower sideband, if the BFO is inside the IF bandpass. Or in most modern receivers with fixed BFO frequencies that are on the edge of the IF bandpass, it will disappear having gone outside of the IF filter(s) bandpass.

On Lower Sideband an audio note is heard when a signal is BELOW the "effective BFO frequency" and in order to bring that note to zero beat, one must tune downward, lowering the pitch of the audio note until it is zero. If you continue tuning downward, past zero beat, the note will rise again as you are now detecting the signal in the upper sideband, if the BFO is inside the IF bandpass. Or in most modern receivers with fixed BFO frequencies that are on the edge of the IF bandpass, it will disappear having gone outside of the IF filter(s) bandpass.

By "effective BFO frequency" I mean the frequency at the antenna input that results in a zero beat. This is equal to the frequency of the one and only local oscillator in a direct conversion receiver. In multiple conversion receivers it is a combination of one or more local oscillators and the BFO. In some receivers, due to the conversion scheme, the input signal at the antenna is inverted so that USB at the antenna is LSB in the IF.

Some older receivers down convert from the input signal to the the IF on higher bands and up convert from the input signal to the IF on lower bands. In these receivers the direction you turn the main tuning knob to go up the band may be opposite on some bands. The common use of 9 MHz IF in early SSB rigs is what resulted in the amateur convention of using USB on 20 meters and up, and LSB on 40 meters and down. The BFO was set for USB operation, in the IF, and when up converting 40 and 80/75 meter signals to the IF they get inverted. I believe that early Drake 4 series rigs are an example of this, having dial markers going in both directions on the VFO, and a NOR / OPPOSITE sideband switch instead of LSB/USB.

I guess I went off on a bit of a tangent. The short answer is that in USB the audio note decreases as you tune upward.

DE N6KB