FREQUENCY MATCHING BETWEEN STATIONS WHEN OPERATING FSK441 (A VISUAL METHOD)
BY RAD HANDFIELD-JONES ZS6RAD
The WSJT manual is quite clear on the need to ensure that two stations in QSO on FSK441 are on the same frequency and recommends the use of a high-resolution frequency counter (minimum of 100Hz) to determine the accuracy of the transceiver readout. For those without access to a frequency counter of adequate resolution, an alternative but effective method of "locking on" to the transmit frequency of the other station is to look at the Frequency Analysis display in the top right hand corner of the WSJT window.
The strongest 'ping' from the transmitting station generates a spectrum that is subsequently modified by the receiver's passband (see the section on 'Receiving FSK441' in the manual). The extremes of the peaks of the spectrum should (if the receiver is accurately tuned to the other transmitter) match with the outer tick marks at the top of the frequency analysis window denoting the 882 and 2205 Hz tones used by FSK441. If these conditions are met the 'ping' will be decoded to appear in the text box in the middle of the WSJT window.
In a recent QSO with ZR1L the author found that he was receiving bursts of FSK441 but the program was not decoding the information. Clearly a difference in transmitted and received frequency existed. It was then noticed that the received spectrum shape was displaced to the low frequency end of the frequency analysis window and that the extremes of the spectrum display did not match with the 882 and 2205 tick marks. Retuning the receiver by -200 Hz using the RIT shifted the spectrum shape of the received signal to within the tick marks. The received signal, once this was done, was properly decoded and the QSO completed without difficulty. The waterfall spectrogram on the top left and middle of the display can also help as the tone marks are shown as four yellow markers at each end of the waterfall. An accurately tuned receiver will receive a burst from the transmitting station exactly between these four marks. Changing the receive frequency will move the received signal up or down in the display. Try this on a local beacon or some birdie in the band. (making sure the birdie is NOT your radio).
Note that this RIT change could be a slow process as it relies on 'pings from your QSO partner. However, if you use a bit of masking tape to mark 100 Hz divisions on the x-axis of the frequency analysis window, the frequency change required can quickly be determined from one good 'ping'. The frequency difference may then be noted and the RIT adjusted accordingly (preferably before the next 15 or 30 second decode). The frequency tolerance built into the WSJT program should compensate for any slight remaining mismatch. Using the 800x600 size monitor window helps in making resolution easier
As the manual says, do not touch or change your transmit frequency. Your QSO partner may also be adjusting his RIT to compensate for inaccuracies in your transmit frequency! Maybe it should be the practice that the station calling CQ does the RIT changes to avoid both stations doing the same thing and nobody making the contact. If you have a planned sked, maybe the Northern station does the RIT changes
The foregoing is covered in the manual but has to be dug out by very careful reading. This may help.
73
Rad
ZS6RAD
14 June 2002
As an addition and the reason for distributing the above is the following from Pieter ZR1AEE who says
"…look at your DF if it says -119 for instance it means that you must change your frequency tx and rx at the same time by -119. It means anti clockwise with the dial. If it says +150 for example it means clockwise 150. Just that small change on the one side makes qso's very fast. Because everybody is not spot on frequency I normally do this small adjustment on my side. The important factor is that only one person must do that otherwise it can be chaotic."
Ian, ZS6BTE adds the following:
Hola Rad, All,
With respect to your attachment re frequency tuning the rig: It is even more
simple than this:
Warm up rig, at least 1 hr, preferably 2 hrs.
Tune to WWV/H in USB mode, offset 1 kHz, i.e. 14.999 MHz. Display audio tone
on audio spec analyser such as hamview, spectran (free on I-net). Tone should
be 1000 Hz. Otherwise twiddle tuning pot on rear panel of IC-746, etc, or reference
oscillator module itself to get 1000 Hz. finis and klaar.
MS has several Hz of random Doppler shift both positive and negative, so if everyone chases this the QSO frequency could separate too much, specially if the rigs are already off freq.
Cheers.
Ian Roberts ZS6BTE
p.s. while listening to WWV (now AM mode) set the PCs clock!