So, I sent this long note about how to adjust image rejection, and at
the end, Yahoo asked for my password again, and the message never
showed up here on the group. I am assuming that message went into
oblivion somewhere. Pardon me if this ends up as a duplicate.
Tony has fixed the transmit low power issue so more SR RxTx units
should be heard on the air. My RxTx is around 1/2W. Anyone want to
have a QSO, SR-to-SR?
Measurements I have taken show the transmit image rejection of my unit
being 40db, the FCC rules are 43db down. I was able to adjust the
image rejection in PowerSDR for SSB, but I cannot get them to "take"
for CW. Fortunately, there is a simple fix: adjust the audio mixer
line or speaker outs to compensate for gain variances. After making
the simple adjustment outlined below, my image was down better than
I am using a laptop on the bench with a Creative Audigy 2 ZS notebook
PCMCIA audio card. It has a Creative audio mixer instead of the normal
Windows sound mixer, but the basic procedure will work in both cases.
Begin by transmitting about 2-5kHz off center/LO frequency. My RxTx
unit is around 7,055,000MHz, so I often use 7,052,000MHz for testing.
You did set the SR LO frequency in the PowerSDR software, right?
Adjust both line out channels down a tiny bit, I use 95% level for both
left and right speakers on the Creative mixer advanced tab under
speaker volume controls. Under Windows mixer, you can adjust the
sideways "balance" control for the line out control under playback. It
only takes a very small adjustment to drop the image down - mine is a
1% difference between left and right.
While in transmit, key down, look at the image with a spectrum analyzer
and adjust only ONE speaker level (or the balance control) slider for
minimum image level. If you don't have a spectrum analyzer, you can
use another receiver - see below. This minor adjustment should not
affect total power output. You might want to write down the positon or
level, as sometimes PowerSDR plays with the mixer level adjustments
From what I have seen). Your analyzer needs to ahve a fairly good RBW
and VBW - I used 300Hz to see the separate signals.
If you do not have a spectrum analyzer, you can use a separate receiver
with a meter. First, you must figure out what frequency the image will
Find the RxTx LO on the receiver and note it's frequency. This should
not be hard, as I can hear the LO from my unit even when it's into a
Next, move the SR Tx frequency to a know frequency, about 2-5kHz off
the LO. Again, I use 3kHz roughly - the PowerSDR adjusts the freq for
sidetone, etc. I use 7,052,000Mhz. Briefly transmit on that frequency
(into a dummy load), and tune the signal in on the other reeiver. Not
it's frequency and level. Now, subtract the actual transmit frequency
from the LO freq. That is the offset between the two. Add that offest
amount to the LO frequency to find the image frequency. For example,
if the LO is at 7,055 and the Tx is 7,053, then 7,055,000-
7,053,000=3,000. Then for the image: 7,055,000+3,000=7,058,000.
Now, tune in the image frequency on the other receiver, start
transmitting on the RxTx again, and tweak the mixer adjustment (as
mentioned above) for lowest image signal level on the other receiver.
Once you have it minimized, check the actual transmit leve to make sure
it did not change much.
You can verify the mixer adjustment by trying this nulling of the image
signal at several frequencies that the RxTx covers. Remember, that if
you move the transmit frequency to above the LO, reverse the math
(subtract the LO freq from the tx freq, the subtract the difference
from the LO to find the image freq).
If your setup is stable (signals don't change as you walk around the
shack or other strange things happen), you can make rough measurements
of the image rejection, LO leakage, etc, by using the S meter on the
other receiver. I haven't done this, as I have an analyzer. There was
a message a while back on this groutp about how to calculate the
various signal amounts by using the S meter (6db per S unit, I think).
You can use a higher Tx frequency, maybe around 10-15kHz, if your
analyzer does not have a narrow VBW or RBW. Keep in mind that not all
sound cards work as good at higher frequencies, however. Using a
better card, this should not be an issue. Using a motherboard sound
chip may have some problems at higher freqs.
The above should make your RxTx meet FCC specs for image rejection.
There shouldn't be any problem meeting 2nd and 3rd harmonics either,
they are way down.
Again, before I made this simple adjustment, my image rejection was
only about 40db down. After adjusting only the differential gain
between channels, image rejection went to over 53db.
BTW, this same adjustment can be used for gain errors in receive as
well, but not phase. Either feed a signal generator into the RxTx in
at about 3-5kHz off center/LO freq, or try to find a signal already
there. Then adjust the Line-in balance control to null out the image
Pardon any spelling errors, my keyboard skills seem to be going
Hope this makes out of my computer, and I hope this helps the RxTx
users out there.