[R-390] "Visual" alignment with a computer and a sound card...

[John KA1XC]

>
> Using the proper settings in Spectogram, you will see a perfect
> representation of your receiver's passband when the receiver is hearing
> just band-noise. <snip>

Perfect, well not quite.
Not to quibble but the above statement would be true if you were using an RF
spectrum analyzer and looking at the actual 455 KC IF; then the spectrum
mask would be a representation of the IF passband.

In the case of the audio soundcard analyzer you are looking at an audio
spectrum which is the result of the detected IF signals, and this depends to
some degree on the type of detector in use. To keep things simple what will
be coming out of the sound card will be a composite of the LSB and USB power
present in the IF response and there is no way to distinguish it at this
point.

What this means is that if you see an unwanted dip in the audio spectrum you
really can't be sure which side of the IF passband is causing it, so it's
actually possible to over-compensate the wrong side of the passband response
in an effort to make the detected audio response look flat on the audio
analyzer. Of course if you happen to adjust the correct side of the passband
to begin with then things would work out better. Perhaps by manually tuning
the receiver across the calibrator signal and watching the carrier meter you
could get a better idea of which side of the passband might be causing
trouble and then proceed from there.

So the swept IF method of measurement is really the only accurate way to
measure a passband response, as long as it is set up properly.

73,
John
KA1XC