[RTTY] Frequency accuracy of WZ7I skimmer

[Wes Cosand]

Hi Folk:

I have received several emails suggesting that the spots from the WZ7I RTTY
skimmer may be significantly off.  While it is not my aspiration to be a
fiduciary frequency reference, I want to be accurate enough for the
reported signal to fall within your RTTY passband filters.  That is not the
case for some users.

During RTTY Roundup I used QS1R units for 40, 20, and 15 meters.  Each of
the QS1R SDR units send RF to 16 instances of HDSDR.  Each instance of
HDSDR is calibrated using the built in exalted carrier single sideband
calibration facility against WWV.  Audio then goes from HDSDR to
RCKskimmer.  Some users suggested that RCKskimmer must be introducing some
systematic frequency error since spots from all RCKskimmer spotter stations
seemed off.

This week I used both RCKskimmer and MMTTY to monitor the W1AW bulletin
Baudot transmissions on 80, 40, and 20 meters.  The frequencies reported by
RCKskimmer were well within 100hz of the published frequency, Using LSB fed
to MMTTY and observing the crossed ellipse display, the VFO dial frequency
was quite close to the 2125hz difference from the published Mark frequency
one would have predicted.

During Roundup I was using a 25 year old Ten Tec on 10 and 80 meters so
there is greater opportunity for inaccuracy on these bands.  Also
RCKskimmer generates a low number of spurious spots by some mechanism I do
not understand but they are usually out of band.  For any given 2.4 khz
band segment there is some low but significant probability of making an
error when I type the reference frequency into RCKskimmer or into HDSDR
while setting up the receivers.

The W1AW digital bulletins are transmitted at 6 and 9 PM EST during weekday
evenings.  The frequencies can be found at
http://www.arrl.org/w1aw-operating-schedule

If anyone has suggestions for how the skimmer spots can be improved I would
certainly welcome an email.  I am aware of the issue of spots that are
generated by stations in S&P mode.  Hopefully we can improve our skimming
tools over the next few months but I, at least, am having fun with the
present software.  Hope some of you are as well.   ... chuckle....

Wes, WZ7I