[RTTY] Frequency skew

Mon Feb 10 17:31:44 EST 2014

Tim
I've made this suggestion before on this reflector.

I can suggest a completely independent way of measuring the frequency of a RTTY 
signal received by your radio and AFSK system.
You will need "Digipan" PSK software (because the waterfall is calibrated in 
fractions of Hz), and a soundcard connected to you radio.
Set the rig to upper SSB, start Digipan as if you were going to operate PSK31.

Let's "measure" the frequency of a RTTY signal...
If you are looking for a station reported on, say, on 3589.100 kHz, set the 
radio dial to 3588.100 kHz, (exactly 1 kHz lower).
The 3589.100 kHz MARK frequency should show up at 1000.0 Hz on the Digipan 
waterfall if your radio is in calibration.
Click on the actual MARK trace and read the exact frequency in the Rx window 
below the waterfall. The SPACE frequency will be
170 Hz lower.
Of course, Digipan will NOT decode RTTY, but  you WILL be able to reliably 
measure the MARK
frequency (1000.0 Hz), which you would then add to the radio dial frequency 
(3588.100 kHz) to get 3589.100 kHz.

Now let's calibrate your radio to WWV...
For completeness, measure the frequency of WWV 5 MHz carrier exactly the same 
way. Set the radio dial to
4999.00000 kHz, and observe the WWV carrier which should be exactly 1000.0 Hz on 
the waterfall if the radio is exactly
calibrated correctly. 4999.00000 kHz + 1000 Hz = 5.000000 MHz.

If you notice a calibration error, just calculate that error in parts per 
milloin, then apply this parts per million correction to the actual
frequency you are measuring.

I use this technique in the Frequency Measuring Tests and get within 1 Hz of the 
right answer.

Cheers and 73
Kai, KE4PT

On 2/10/2014 3:56 PM, Shoppa, Tim wrote:
> Here's my little Monday morning quarterbacking on Skimmer performance this past weekend. I was wondering if maybe my VFO or frequency math (when doing AFSK) was out of whack, because skimmer spots were most often 200-400Hz low. Pretty soon I got in the habit of just clicking about 300Hz or so above any skimmer spot and most often found this to handle the skew, but I also began to notice that not all skimmers were offset by the same amount, sometimes clicking 400Hz high was a bit better. Manually entered spots, while they had more callsign busts than the skimmer busts, seemed to have less of a systematic offset in frequency. So I also thought to myself, how can I check what other guys and the skimmer thought my frequency was?
>
> I looked at the peak of my most prolific run in the contest, 3589.9/3590.0kc in the 03:55 to 04:28 timeframe the first night (hint, my station is most effective on 80M, wow is 80M a joy). Many of these QSO's are already confirmed in LOTW (hint, RTTY contesting has extremely high confirmation rates in LOTW). If they are confirmed in LOTW, I can often see the frequency of the other guys VFO too (not all ADIF's have to contain this frequency but for computer-connected rigs it will be very common). Here's my QSO's, with my VFO dial and the other guy's VFO dial where I know it. You can see I slide up about 100Hz in this run to minimize close-in QRM.
>
> N3QE S57UX 2014-02-09 03:55:58 80M RTTY 3.58991 his VFO 3.58982
> N3QE W5EW 2014-02-09 03:57:11 80M RTTY 3.58991 his VFO 3.5899
> N3QE AB4SF 2014-02-09 03:57:36 80M RTTY 3.58991 his VFO 3.58989
> N3QE KS4L 2014-02-09 03:59:37 80M RTTY 3.58991 his VFO 3.5899
> N3QE N0EKM 2014-02-09 04:00:14 80M RTTY 3.58991 his VFO 3.5899
> N3QE LZ2ZG 2014-02-09 04:01:04 80M RTTY 3.58991 his VFO 3.58983
> N3QE VE6SQ 2014-02-09 04:02:48 80M RTTY 3.58991 his VFO 3.58988
> N3QE K8MU 2014-02-09 04:03:30 80M RTTY 3.58991 his VFO 3.58988
> N3QE AA0AW 2014-02-09 04:04:30 80M RTTY 3.58991 his VFO 3.59105
> N3QE KD2A 2014-02-09 04:06:00 80M RTTY 3.58991 his VFO 3.58988
> N3QE N3PPH 2014-02-09 04:06:22 80M RTTY 3.58991 his VFO 3.5887
> N3QE N2QT 2014-02-09 04:13:29 80M RTTY 3.58991 his VFO 3.58989
> N3QE VE3KI 2014-02-09 04:14:07 80M RTTY 3.58991 his VFO 3.58988
> N3QE WB2SXY 2014-02-09 04:16:27 80M RTTY 3.58991 his VFO 3.58838
> N3QE W4EE 2014-02-09 04:17:10 80M RTTY 3.58991 his VFO 3.58892
> N3QE EA2KU 2014-02-09 04:20:20 80M RTTY 3.59000 his VFO 3.590
> N3QE KA9MOM 2014-02-09 04:27:21 80M RTTY 3.59000 his VFO 3.58999
> N3QE K6UFO 2014-02-09 04:28:35 80M RTTY 3.59000 his VFO 3.590
> N3QE KN3A 2014-02-09 04:28:55 80M RTTY 3.59000 his VFO 3.58997
>
> If you look at "my VFO" vs "his VFO" where both are known, maybe my VFO reads like 30Hz high. There's obviously some with random offsets of up to a kc or more out there.
>
> Now look at the RCKSkimmer spots in this same timeframe (fetched from downloads at reversebeacon.net<http://reversebeacon.net/>):
>
> 20140209.csv:DL9GTB,DL,EU,3589.7,80m,N3QE,K,NA,CQ,17,2014-02-09 03:47:05,45,RTTY
> 20140209.csv:WZ7I,K,NA,3589.5,80m,N3QE,K,NA,CQ,26,2014-02-09 03:49:05,45,RTTY
> 20140209.csv:KB9AMG,K,NA,3589.7,80m,N3QE,K,NA,CQ,25,2014-02-09 03:49:53,45,RTTY
> 20140209.csv:WZ7I,K,NA,3589.6,80m,N3QE,K,NA,CQ,22,2014-02-09 03:54:10,45,RTTY
> 20140209.csv:WZ7I,K,NA,3589.6,80m,N3QE,K,NA,CQ,22,2014-02-09 03:59:24,45,RTTY
> 20140209.csv:N2QT,K,NA,3589.8,80m,N3QE,K,NA,CQ,20,2014-02-09 04:03:50,45,RTTY
> 20140209.csv:WZ7I,K,NA,3589.6,80m,N3QE,K,NA,CQ,25,2014-02-09 04:04:15,45,RTTY
> 20140209.csv:DL9GTB,DL,EU,3589.8,80m,N3QE,K,NA,CQ,23,2014-02-09 04:10:32,45,RTTY
> 20140209.csv:KB9AMG,K,NA,3589.8,80m,N3QE,K,NA,CQ,27,2014-02-09 04:12:38,45,RTTY
> 20140209.csv:KB9AMG,K,NA,3589.9,80m,N3QE,K,NA,CQ,26,2014-02-09 04:19:23,45,RTTY
> 20140209.csv:N2QT,K,NA,3589.9,80m,N3QE,K,NA,CQ,27,2014-02-09 04:24:05,45,RTTY
> 20140209.csv:WZ7I,K,NA,3589.7,80m,N3QE,K,NA,CQ,22,2014-02-09 04:25:05,45,RTTY
> 20140209.csv:KB9AMG,K,NA,3589.9,80m,N3QE,K,NA,CQ,26,2014-02-09 04:26:18,45,RTTY
> 20140209.csv:HA6M,HA,EU,3593.7,80m,N3QE,K,NA,CQ,12,2014-02-09 04:28:47,45,RTTY
>
> Note that several skimmers are systematically low by 100-200Hz and one by closer to 300-400Hz. That last one is way out in left field. The systematics seemed to change from band to band.
>
> None of this means that the skimmers are broken. It just means that we have to apply a little observation when using them to increase our band awareness.
>
> Tim N3QE
>
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