[Elecraft] K2, Noise Gen and Spectrogram

[G3VVT at aol.com]

James,
 
Sometimes in addition to a noise generator and a PC with a soundcard  running 
Spectrogram you may need to add a 600 ohm 1:1 isolation transformer  between 
the LS output of the K2 and the PC sound card input. Perhaps I was  
unfortunate in that both my PC and a separate laptop had spikes on the  display from 
harmonics of the AC power frequency. In the UK we use 50Hz AC power  so that was 
present as a very large spike followed by 150Hz and every 100Hz  after that 
through the audio spectrum. With your use of 60Hz AC power in the USA  would 
possibly expect that plus multiples through the audio test range if  afflicted.
 
The use of an audio isolation transformer killed these spikes stone  dead.
 
Don, W3FPR suggested the audio transformers from scrap PC modem cards as a  
possible source of suitable 600 ohm transformers.
 
The Spectrogram program turned out to have other uses for me beyond  checking 
K2s.
Having done a hurried rebuild on a 2m repeater for a semi local repeater  
group, when installed on site the through audio was poor and sounded  muffled. 
With the aid of a VHF noise generator plus my laptop running  Spectrogram was 
able to follow the audio through from the repeater RX audio  output, through the 
logic and right up to the TX modulator. Apart from the  normal RX de-emphasis 
and the blocking of the audio below 300Hz in the  logic to remove the 
incoming CTCSS, the audio was correct right the way through  the system.
 
The problem in the end turned out to be the TX modulation pre-emphasis was  
not functioning. No doubt there are many other uses for the Spectrogram 
program,  but on this occasion saved me a lot of time in fault finding on the  
repeater.
 
Bob, G3VVT