Setting Rx/Tx Offset - NORCAL NC2030 Manual

Alternatively, the sound card on a PC can be used in conjunction with audio spectrum analyzer software
to view the level of the audio tone as it is swept across the receiver passband. The goal is to get the best
overall rejection across entire passband. It may be necessary to re-tweak the center frequency rejection
using the balance and phasing adjustments get the best results. The ideal rejection response will look
very similar to the image rejection figure above. The nulls will end up at roughly 250, 350, 550, and
750 Hz as shown in the figure.
After just a few adjustments, a minimum 40 db of rejection is very easy to get. A bit more work will be
need to get 45 db of rejection. With some patience and persistence, it is possible to get over 55 db of
suppression across the audio bandwidth, with deep nulls exceeding 70 db. Stop when you are satisfied
with the results.
All three pots interact on the low, mid and high audio end. The balance pot may need further tweaking
to get good suppression across the entire audio band. If the balance pot ever gives a null that is no
longer very touchy, you might try setting the high and low pots back to mid range and start over again.
Often optimizing the high end will hurt the low end rejection and visa-versa. Sometimes tuning past the
optimum null point will help speed this opposite sideband rejection interactive process.
See the figure above for the simulated sideband rejection. This very excellent rejection is indeed
possible (55 to 70+ db) and has been observed in all the prototypes, but will only be good for a small
portion of the 20 or 30m band as the C/L/C RF phasing section will detune and reduce the overall
rejection. Thus it is best to optimize the 20m version for the center of the tuning range (~14.030), while
on 30m, it might be best to optimize around 10.115 MHz, as there is not as much activity in the higher
10.150 MHz end. In general, the opposite sideband rejection will be over 45 db across the 20 or 30m
band.
When you are done, tune the receiver to the strong signal LSB side. Adjust the volume control to
produce a comfortable listening level, then retune the receiver through zero beat to the rejected
sideband. The signal will be barely perceptible, if it is heard at all.

Setting RX/TX offset

The transmit/receive offset is a matter of individual preference. Here we will set the transmit/receive
offset to suit the preference of the operator. However, even after this has been set, the RIT control can
be used to over ride whatever default offset has been set up in this step.
First locate the trim pot used for the transmit/receive offset, R53, as shown in the picture below. This
pot is mounted on the front left hand corner of the board.
Norcal NC2030 v5 12-18-05
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