[Elecraft] Setting up Filters with a modern Macintosh

Bill Coleman aa4lr at arrl.net
Sat May 7 22:25:06 EDT 2005


I know many of you out there set up your filters using Spectrogram on a 
Windows PC, but some of us only have (or simply prefer to use) 
Macintosh computers.

I recently had to completely reconfigure my filters, as I added the 
RTTY mode. In order to accommodate my chosen center frequency of 1500 
Hz for AFSK RTTY, I had to modify the BFO to get sufficient range. I 
had already added 47 and 100 pf caps across C174 and C173, respectively 
when I made the A->B mods and the BFO stability mod. I changed these to 
56 and 120 pf, but this did not move the BFO frequencies much at all. 
Then I added a 3 pf cap from X3/X4 junction to ground. This moved the 
lower frequency about 800 Hz without affecting the upper frequency.

Of course, this meant my filter settings were now incorrect. I had 
previously used an ancient MacOS 9 computer and MacCRO to set up my 
filters. I recently went searching for software that could be used from 
MacOS X. I ended up using two different programs.

First, I used CocoaModem (http://homepage.mac.com/chen) by Kok Chen. 
This is an excellent piece of software -- a RTTY and PSK modem 
implementation. Chen started it a bit over a year ago, and has made 
great progress. CocoaModem was the reason I was adding the RTTY mode.

CocoaModem has a number of features, including a spectrum analyzer in 
the RTTY Monitor window, and also in the Config window. The spectrum 
analyzer doesn't have any averaging -- so it is a bit difficult to 
determine the final shape of the filter. It's certainly good enough to 
adjust the filters. By entering in mark and space frequencies on the 
main RTTY window, you can place markers in the spectrum analyzer 
window. For adjusting OP1, I used 400 and 2400 Hz. This worked out well 
for my "1.9" kHz filter. If you only need one marker, just enter the 
same frequency for both mark and space.

To help line up the filters between modes (CW and CW-R, LSB and USB, 
RTTY and RTTY-R), I used a second piece of software.

For this, I used iSpectrum 
(http://www.dogparksoftware.com/iSpectrum.html) by Dog Park Software. 
While it isn't as adjustable as Spectrogram, it features limited 
averaging, a marker, and a waterfall.

After I had done the initial alignment with CocoaModem, I used 
iSpectrum to check the symmetry between the filters in opposite modes. 
What I did was to let the audio play for about 10 seconds, then switch 
modes for another 10 seconds. I could easily spot discrepancies in the 
waterfall view. A few small adjustments to the BFO frequencies and now 
the complementary modes offer little, if any, change in noise "color" 
when you switch between them (using the CW REV key).

In the end, this was a much easier filter alignment than I had done 
previously using MacCRO. I recommend this software to anyone trying to 
align the K2 filters with a Mac.


Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901



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