[Elecraft] [Bulk] Re: [Bulk] [K2] Loose screws on KPA100 Q1 and Q2

[Bill Coleman]

On Mar 1, 2011, at 9:30 PM, Don Wilhelm wrote:

> My recommended RTTY filter set settings are - FL1 = The same as the SSB FL1 settings - that gives you the full waterfall display - FL2 = 1.00, FL3 = 0.70, and FL4 = 0.40.  Those narrow filters are centered at 1000 Hz (change the KDSP2 RTTY filter centers to match), and set the MMTTY mark frequency to 910 Hz.  When tuning PSK31 signals in the midst of QRM, use the VFO knob to tune the desired signal to the vicinity of 1000 Hz on the waterfall, and then if you need to narrow the IF filters to alleviate QRM, it is a simple matter of tapping the XFIL button.

Don,

As much as I hate to contradict your expertise -- are you sure it is wise to use such a low center frequency for AFSK RTTY? (Although it does more easily fit the limited BFO range of the standard K2)

Part of the reason RTTY tones are traditionally so high (2125/2295) is to put any harmonics or other distortion products outside the audio passband. With tones centered around 1000 Hz, the second harmonic, and much of the energy of the third harmonic is well within the standard SSB passband.

Also, if tones are centered around 1000 Hz, then the mark frequency would be 915 Hz (1000 - 170/2), and the space frequency 1085 Hz.

I chose 1500 Hz as a center frequency since it put the harmonics outside the audio passband. (I use a rather narrow SSB filter - about 2 kHz wide) A center frequency on 100 Hz boundaries allows KDSP2 filters to be properly centered. I did have to make a minor modification to give the BFO enough range to allow the crystal filter to be set at this point.

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
Web: http://boringhamradiopart.blogspot.com
Quote: "Not within a thousand years will man ever fly!"
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