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K2 USB/LSB Receive Sound
Nov 18, 1999
As has been discussed recently, the two sidebands may sound quite different on receive without careful alignment. Here's the "official" Elecraft guide to making both sidebands sound approximately the same.
First, a couple of facts:
1. The 7-pole filter on the SSB adapter is NOT the source of the asymmetry on receive. The asymmetry is due to the 2-crystal filter between the IF amp and the product detector. Its passband may be shifted slightly below that of the SSB adapter's filter, favoring one sideband over the other. (It favors the case of the BFO below the filter, i.e. LSB on 17 m and below and USB on 15 m and up. This leaves 17/20 m USB sounding different.)
2. There is virtually no detectable asymmetry on *transmit*, only on receive. On transmit only the 7-pole filter is involved, and it has a very symmetrical passband.
Procedure for equalizing the way the two sidebands sound:
1. Using CAL FIL, set up the BFOs as described in the revised SSB adapter manual:
BF1 = 4913.5/LSB and 4916.3/USB
BF2 = 4913.5/LSB and 4916.1/USB
2. Exit CAL FIL. Select 40 meters (to obtain a lot of receiver noise), LSB mode, and FL2. (Always operate using FL2 rather than FL1, since it allows the BFOs to be optimized for receive. FL1 is automatically used on SSB transmit.)
NOTE: In a future revision of the K2 design, we may eliminate C180, the 10 pF capacitor in the 2-crystal filter. You may wish to try the steps below both with and without C180 in the circuit. Experiments so far favor removing C180.
3. Switch to CW mode and select a moderate bandwidth filter (FL = 0.70-1.20).
4. While listening to band noise, cycle between LSB, USB, and CW using the MODE button. If the pitch of the noise is much higher on USB than on LSB, adjust L34's slug clockwise. (This shifts the passband of the second crystal filter upwards.) Compare LSB and USB again. Continue adjusting L34 until the pitch of the noise with LSB and USB is about the same.
5. Now switch to CW. You may find that L34's present setting makes CW sound a little weaker than with the original setting. If this is the case, turn L34 a half-turn counter-clockwise. The goal is to find the best compromise between LSB, USB and CW.
6. Every K2 is a little different. You may find that moving the BF2 settings as much as +/- 150 Hz from the recommended settings provides optimal results.