SSB Carrier Null Notes

AB7MY


The K2 SSB carrier balance null is a compromise between USB and LSB. Often in some rigs there is *both* a null pot and a null capacitor to balance out the carrier leakage, but in the Elecraft SSB option only a pot is present. Nonetheless, a carrier null of -40 db or better is typical.

The balance adjustment can be done using only the K2's LED S-meter in the SSBA BAL menu parameter, and it will be adequate for most purposes. This is well detailed on page 17 of the SSB option manual. Don't forget to return the SSBA menu setting to parameter 3 when you're done! I forgot to do this myself !!!

If an external receiver or QRP wattmeter is used, you may be able to get the null even lower. On my WM-2 wattmeter, using its lowest full scale range of 100mW, I can get both the USB and LSB carrier null low enough that it does not even register at this sensitivity. This equates to probably less than 1-2 mW on this instrument. How many db is this below 10-15 watts of SSB PEP? You can do the math. ;-)

Not everyone may be able to achieve this degree of null, but you ought to be able to get pretty close, depending on your building proficiency, test equipment, lead lengths and toroid winding, and of course component tolerances. Be sure to ground the crystal cases before doing the carrier
null adjustment!

The exact level will also vary a bit from band-to-band. Do the null on 40m as stated in the SSB option manual, where the rig's TX strip gain is highest. This gives the most sensitivity for tiny carrier levels.

If you use another rig to do the carrier null, you should attach a dummy load to the K2 and just use the leakage RF signal to the other rig to do the adjustment. Turn off the AGC on the rig used for listening to the K2's carrier, and obtain sufficient signal to produce a half to mid-scale S-meter reading. You can move the antenna lead of the test receiver closer or farther away from the K2's dummy load to get this signal strength.

Every rig is different, depending on stray capacitance and inductances in the K2's circuitry. But you ought to be able to get a low carrier reading that is not normally detectable on the receiving end. You probably will be able to hear the carrier a little on a rig in you shack that is in close proximity to the K2 or its cabling. This is no cause for concern.

The carrier level will *not* be dependent on the setting of the Power control, due to the low carrier level that is essentially independent of the rig's RF drive. So you may leave the Power control at a low setting to do the alignment. Be sure the bottom covers are installed in the K2,
so the null setting will not change after the panels are attached.

The null pot (R1) in my K2 was positioned near the 1 or 2 o'clock setting when the best null was obtained on both USB and LSB.

Additional note from N6KR:

There's some signal leakage *around* the BFO attenuator, which puts a lower-limit on how far down you can go. This leakage can be reduced by about 10 dB by changing the BFO inductor  (L33) to a toroid (45 turns of #28 on a T44-8/90 , which can be purchased from our parts department, support@elecraft.com ). This will improve carrier suppression on sideband. The toroid should be mounted securely underneath the PCB, and you'll have to re-run CAL FIL. (Write down your present BFO frequencies before making the change.)