WSJT FSK441 Modem – Scicos Simulation


Joe Taylor K1JT came out with a ground breaking paper in the December 2001 issue of the ARRL magazine QST (Reference 1). He described a new digital modulation scheme “WSJT” (Weak Signal Communication by Joe Taylor) using a 4FSK modem that was designed to perform well over 6m & 2m amateur radio meteor scatter paths. The modem was designated as FSK441.

In this post I will show how we can simulate the FSK441 modem using ScicosLab/Scicos. The modem design parameters are:

-4 Level Phase Continuous FSK Modulator
-f0 = 882Hz, f1 = 1323Hz, f2 = 1764Hz, f3 = 2205Hz
-Modulator Centre Frequency = 1543.5Hz
-Shift = +/- 661.5Hz
-Baud Rate = 441bdps, BW = 4 x BR = 1764Hz
-Characters represented as 3 consecutive tones (4 x 4 x 4 = 64 chars)
-Character Rate = 441/3 = 147char/sec
-4 Characters formed by 3 equal tones reserved for R26, R27, RRR, 73
-15 Characters beginning with f3 not defined
-Message sync by “space” insertion at end & character never starts f3

Fig.1 FSK441 Character Codes Ref.1


Figure 1 shows the FSK441 character codes taken from Reference 1_Table 1. Character “C” = 103 which means 1bd of 1323Hz, then 1bd of 882Hz then 1bd of 2205Hz. Figure 2 shows the ScicosLab script file used to build the message “W8WN 27 K1JT 2727.”

Fig.2 FSK441 Code Data

Figure 3 shows the phase continuous VCO that generates the 4 tone FSK signal of f0 = 882Hz, f1 = 1323Hz, f2 = 1764Hz and f3 = 2205Hz. The VCO is driven by the four level amplitude signal generated by the ScicosLab script file. Figure 4 shows the multilevel data input signal on the bottom trace and the VCO output on the top trace. The most negative voltage corresponds to f0 and the most positive to f3. The first character of the message is “W” = 213 which is what we see on the bottom trace for the 1st three bauds f2 – f1 – f3. Figure 5 shows an interesting design feature of FSK441. For each baud of the waveform, we have 2 cycles of f0, 3 cycles of f1, 4 cycles of f2 and 5 cycles of f3. Figure 6 shows the output spectrum for this particular message (BW = 4 x BR = 4 x 441Hz = 1764Hz). Note that f0 & f3 occur more often so their peaks are more prominent.

Fig.3 FSK441 Modulator
Fig.4 FSK441 Modulator Output for “W8WN 27 K1JT 2727.”
Fig.5 FSK441 Modulator Output 1st 5 Bauds
Fig.6 FSK441 Modulator Output Spectrum

Figure 7 shows an FSK441 receiver using 4 matched filters. The FSK441 output signal is fed into the receiver and multiplied by the four tones f0, f1, f2 and f3 in four parallel branches. The results are integrated and fed into threshold comparators. When tone f0 multiplies itself, a DC voltage of 0.5 volts is generated + a tone at 2xf0. Figure 8 shows the multiplied f3 on the top trace, next f2, next f1, next f0, then the received FSK441 waveform and the original 4 level data signal. So for example the 1st baud is at level f2 so we see a burst of 2xf2 sitting on 0.5 volts on the second trace from the top. Figure 8 shows the output after the matched filter integrator and threshold comparator which is the original multilevel signal.

Fig.7 FSK441 Matched Filter Receiver
Fig. 8 FSK441 Matched Filter Multiplier Output
Fig.9 FSK441 Matched Filter Integrator/Threshold Detector Output

Figure 10 shows the 0.1293msec burst of data created by the modulator (18 characters plus trailing “space” = 19 characters). Approx 15 seconds of silence is added before and after this burst using Audacity. This creates a transmit waveform of 30 sec duration. This is decoded by WSJT in Figure 11. Right clicking on the pulse gives the proper decode in the second line.

Fig.10 Audacity For 30sec Tx Waveform
Fig.11 WSJT Received Waveform
Fig.12 WSJT FSK441 Modem – Scicos Simulation YouTube Video

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#1 – “WSJT: New Software for Meteor-Scatter VHF Communication”, Joe Taylor K1JT, ARRL QST Magazine Dec 2001

#2. ScicosLab with Modnum Toolbox

#3. “Learn Telecommunications by Simulation” EBook

#4. “HF High Frequency Radio Telecommunications Learn by Simulation” EBook