Center Frequency when setting RTTY Filters

[W0YG, Charlie Summers]

Since the ability to set filters when using the RTTY mode is different than
LSB/USB if used for AFSK, what center frequency do you use?  Perhaps I
should rephrase this.  When setting filters in the RTTY mode (the K2
defaults to the SSB filters when in RTTY mode unless RTTY filters are set)
do you use 1000 Hz as the center frequency?

Thanks and 73,

Charlie, W0YG..>>

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[Jeff Stai]

You want to set it to the center of the two tones you are using for AFSK. If 1000Hz is your Mark frequency, and 170Hz is your shift, then you should center at 1000-(170/2) or 915Hz.

You can do this graphically with Spectrogram using the techniques detailed on the Elecraft Builders Resources pages. Set the cursors based on the AFSK tones from your RTTY modulator software or hardware. Then center the filter peak between the two cursors.

hope this helps! - jeff wk6i


At 10:43 AM 10/9/2005, Charlie, W0YG wrote:
>Since the ability to set filters when using the RTTY mode is different than LSB/USB if used for AFSK, what center frequency do you use?  Perhaps I should rephrase this.  When setting filters in the RTTY mode (the K2 defaults to the SSB filters when in RTTY mode unless RTTY filters are set) do you use 1000 Hz as the center frequency?

--
Jeff Stai               [hidden email]
Twisted Oak Winery      http://www.twistedoak.com/


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[Jeff Stai]

Ooops - I knew I would get dyslexic - it's LSB, so the necessary shift to a lower TX frequency requires a corresponding shift higher in the audio applied - so you want to center at 1000+(170/2) or 1085Hz.

sorry! - jeff wk6i


At 11:30 AM 10/9/2005, Jeff Stai wrote:

>You want to set it to the center of the two tones you are using for AFSK. If 1000Hz is your Mark frequency, and 170Hz is your shift, then you should center at 1000-(170/2) or 915Hz.
>
>You can do this graphically with Spectrogram using the techniques detailed on the Elecraft Builders Resources pages. Set the cursors based on the AFSK tones from your RTTY modulator software or hardware. Then center the filter peak between the two cursors.
>
>hope this helps! - jeff wk6i
>
>
>At 10:43 AM 10/9/2005, Charlie, W0YG wrote:
>>Since the ability to set filters when using the RTTY mode is different than LSB/USB if used for AFSK, what center frequency do you use?  Perhaps I should rephrase this.  When setting filters in the RTTY mode (the K2 defaults to the SSB filters when in RTTY mode unless RTTY filters are set) do you use 1000 Hz as the center frequency?

--
Jeff Stai               [hidden email]
Twisted Oak Winery      http://www.twistedoak.com/
Rocketry Org. of CA     http://www.rocstock.org/
Amateur Radio           WK6I ~ Calaveras County, CA


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[Bill Coleman-2]

On Oct 9, 2005, at 1:43 PM, Charlie, W0YG wrote:

> Since the ability to set filters when using the RTTY mode is  
> different than LSB/USB if used for AFSK, what center frequency do  
> you use?  Perhaps I should rephrase this.  When setting filters in  
> the RTTY mode (the K2 defaults to the SSB filters when in RTTY mode  
> unless RTTY filters are set) do you use 1000 Hz as the center  
> frequency?

I went through this a few months ago. Initially, I set things up for  
the Low Tones (1275 / 1445). However, when I went to reprogram the  
KDSP2 filter, I found that it would only move to center frequencies  
that were multiples of 100. This made it impossible to set the filter  
in the center (1360).

Now, you must know that I have my K2/100 modified with the KI6WX  
"1.9" kHz filter. It's really like the old stock filter - about 2 kHz  
wide, but with less passband ripple. If you have a wider filter, you  
might consider using higher frequencies. This places the second  
harmonic higher in the passband, outside the filter range.

After some consideration of the problem, I ended up picking 1500 Hz  
as my center frequency, and placing my tones at 1415 and 1585. The  
next problem I had is that I could center the adjustable bandwidth  
filter on both modes (RTTY and RTTY REV). This was solved by modding  
the BFO to increase the range. Adjusting the BFO range is kinda  
tricky. As you increase the range, the steps the BFO move become  
courser, so there's more opportunity for slight frequency variations  
as you change filters or switch modes. I managed to increase the  
range just enough to get all the passbands centered on 1500 Hz.  
That's all you want -- just enough range to get those frequencies  
set. For this reason, using a frequency like 1500 Hz may be better  
than trying to use the standard high tones.

The adjustable bandwidth filter is a real benefit on RTTY. With just  
a few adjustments, you can get some really nice filters set up for  
digital use -- something that would have cost a lot of money in other  
rigs.

For RTTY, I use the progression OP1, 1.0 kHz, .5 kHz and .3 kHz. OP1  
is good for scanning the bands, 1.0 is good for tuning in a crowded  
band, and .5 kHz pretty much is single-signal. .3 is kinda narrow for  
RTTY, and it will clip some of the signal -- but it does work great  
for PSK31.

For CW, I use the odd progression of 1.0 kHz, .4 kHz, .16 kHz and  
OP1. I don't like tuning around the CW bands with OP1 unless it is  
really vacant, like up on 10 or 15 meters these days.

About the only downside of running RTTY with the K2/100 is that you  
can only use about 20-35 watts. The KPA100 will get HOT very quickly.  
PA efficiency gets really poor at this level.


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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[Bill Coleman-2]

On Oct 9, 2005, at 2:30 PM, Jeff Stai wrote:

> You want to set it to the center of the two tones you are using for  
> AFSK. If 1000Hz is your Mark frequency, and 170Hz is your shift,  
> then you should center at 1000-(170/2) or 915Hz.

If you do this, don't use OP1 as your FL1 filter. The second harmonic  
of your tones will be in the passband when you are transmitting,  
otherwise.

I recommend higher frequencies. I ended up using 1500 Hz as the  
center of my RTTY filter passband.


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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