[Elecraft] Spectrogram - Again

[Tom Hammond NØSS]

John, K7JG, wrote:

>I was having a lot of trouble aligning my K2's crystal filters even though
>I was using the N0SS noise generator and Spectrogram. Starting from the K2
>manual settings for the filters, lots of peaks of variable heights were
>appearing everywhere across the spectrum no matter the filter settings even
>though the radio seemed to be working. After a lot of fiddling, two things
>became apparent. I was setting both the N0SS noise generator and the K2 AF
>gain too high. Using the output pot to set the noise generator just above
>receiver noise on my headphones and then setting the K2 AF gain to minimum
>and increasing it slowly, the desired shapes began appearing. I'm guessing
>my computer's soundcard was clipping even at moderate audio levels and the
>FFT processing of the resulting "squared" waves in Spectrogram resulted in
>all those interesting peaks.

John makes an EXCELLENT point here... especially with regard to the use of 
the AF GAIN on the K2.

While an output level pot on the noise generator may be a worthwhile 
addition for many (the generator DOES have quite a bit of output), the MOST 
important control when performing a CAL FIL alignment using noise (from ANY 
source) and Spectrogram is to NOT overdrive the input to your sound card. 
Overdriving the sound card input WILL (almost guaranteed) result in a bunch 
of spurious artifacts appearing on the Spectrogram display.

Use of the 60dB scale, and adjusting your AF gain to the point that the 
peak signal level displayed on the screen doesn't exceed 40dB should work 
well. Adjusting the AF Gain to produce a 30dB trace may work even better.

When using Spectrogram, NOTE that there is a slider bar along the RIGHT 
edge of the Spectrogram screen. This slider should be run to the top end of 
its range before you begin establishing an AF Gain level setting.

Thanks for the reminder, John.

73,

Tom Hammond   N0SS