Setting up Filters with a modern Macintosh

[Bill Coleman-2]

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: [hidden email]
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901

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[Leigh L. Klotz Jr WA5ZNU]

I have enough BFO range as described in the manual and am able to reach
4912.97 on the low end.  Unfortunately, while that is low enough for me
to center the RTTY filters on 1000Hz for one of the sidebands, it is not
for the other.  On the "bum" sideband, the narrowest RTTY filters are
centered around 7700-750Hz.  Both CW and CWR filters at 600Hz are ok.

My K2 is #3121 (Rev. B) and is at current revision level on ECOs,
including the KSB2 bandwidth. I built the KSB2 as the current shipping
revision, but with parts from Mouser, as it was a week before the mod
kit came out.

I did some searching and saw some posts from 2003 that said that being
able to reach RTTY filter frequencies of 2200Hz was often possible only
on one sideband, but I am not able to hit even my target 1000Hz mark.

On the other sideband, I have no trouble, and so I am fine on the lower
bands, as I can use R, but on the higher bands, I have to switch
sidebands for digital modes.  That's not a a problem for some modes such
as PSK but it is a problem for MFSK and its variants.

Are other K2s able to center RTTY filters on 1000Hz on both sidebands?  
Would this small cap Bill suggests help mine without affecting
stability?  It looks like I need a 250Hz-300Hz move down on the low
end.  I don't know if that is reasonable and an effect of the wider
KSB2, or if I have some other, undiagnosed problem.

73 es TNX,
Leigh / WA5ZNU

On Sat, 7 May 2005 7:25 pm, Bill Coleman wrote:
> 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.
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[Don Wilhelm-3]

Leigh,

Most if not all K2s will have a problem tuning for a 2200 Hz tone on both
sidebands, but 1000 Hz should be possible with some work on the BFO.

What you are asking to do is to have a BFO range of more than (IF center
frequency +/- 1000 Hz + filter width) - the center frequency is the real
problem - it will depend on which matched crystal set you have installed in
your K2.

You can certainly try Bill's capacitor addition, but be aware that the BFO
could become unstable if the crystals are pulled too far - if you end up
with 'muddy sounding' signals, it is not working for you.

You could also change or pad C173 and C174 (observing the relative size
ratio) to shift your entire BFO range since you have excess range on one
side and accomplish the same thing - note that your BFO high and low
frequencies could be shifted a bit from those indicated in the manual if
your IF center frequency is off to one side of the average.

73,
Don W3FPR

> -----Original Message-----
>
> I have enough BFO range as described in the manual and am able to reach
> 4912.97 on the low end.  Unfortunately, while that is low enough for me
> to center the RTTY filters on 1000Hz for one of the sidebands, it is not
> for the other.  On the "bum" sideband, the narrowest RTTY filters are
> centered around 7700-750Hz.  Both CW and CWR filters at 600Hz are ok.
> ...
>
> On the other sideband, I have no trouble, and so I am fine on the lower
> bands, as I can use R, but on the higher bands, I have to switch
> sidebands for digital modes.  That's not a a problem for some modes such
> as PSK but it is a problem for MFSK and its variants.
>
> Are other K2s able to center RTTY filters on 1000Hz on both sidebands?
> Would this small cap Bill suggests help mine without affecting
> stability?  It looks like I need a 250Hz-300Hz move down on the low
> end.  I don't know if that is reasonable and an effect of the wider
> KSB2, or if I have some other, undiagnosed problem.
>
> 73 es TNX,
> Leigh / WA5ZNU
>
> On Sat, 7 May 2005 7:25 pm, Bill Coleman wrote:
> > 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.
>

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[Bill Coleman-2]

On May 8, 2005, at 2:10 AM, Leigh L Klotz, Jr. wrote:

> I have enough BFO range as described in the manual and am able to  
> reach 4912.97 on the low end.  Unfortunately, while that is low  
> enough for me to center the RTTY filters on 1000Hz for one of the  
> sidebands, it is not for the other.

I ran into this problem, too, when I tried to configure the RTTY  
filters to center on 1500 Hz.

> I did some searching and saw some posts from 2003 that said that  
> being able to reach RTTY filter frequencies of 2200Hz was often  
> possible only on one sideband, but I am not able to hit even my  
> target 1000Hz mark.

It's easy, really.

First, figure out which sideband you're having trouble with (remember  
that the sideband used flips above 20 MHz). Normally, for 20m, you'd  
use the LSB side for RTTY, and the USB side for RTTY-REVERSE (which  
you'd commonly use for PSK)

I had trouble with the RTTY side. My BFO bottomed out at 4912.64 --  
which was low enough for the CW filters at 600 Hz.

> Are other K2s able to center RTTY filters on 1000Hz on both  
> sidebands?  Would this small cap Bill suggests help mine without  
> affecting stability?

Yes. I'd try about 2 pf or so.

The 3 pf cap I used moved the low end down about 800 Hz without  
affecting the high end.

Of course, increasing the BFO range means that the BFO adjustment  
steps are a little wider, so don't increase the BFO range much more  
than you have to.

>   It looks like I need a 250Hz-300Hz move down on the low end.  I  
> don't know if that is reasonable and an effect of the wider KSB2,  
> or if I have some other, undiagnosed problem.

One concern I would have is the use of 1000 Hz signals with the OP1  
filter. One reason RTTY pitches AFSK signals so high is so that the  
harmonics of the audio signal fall outside the SSB filter passband.  
If you are using the OP1 filter in FL1 for RTTY mode, this is the  
filter you'll be transmitting with.

For my filters, 1500 Hz is plenty high to kill any second harmonic,  
whereas the standard frequencies of 2125, 2295 were high enough that  
my filters might actually attenuate it a bit.



Bill Coleman, AA4LR, PP-ASEL        Mail: [hidden email]
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901

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