In the world of HF, spectral efficiency is usually the gold standard. Modern digital modes strive to pack as much data as possible into narrow bandwidths. However, when monitoring tactical networks originating from the CIS (Commonwealth of Independent States) region, we may encounter a signal that flips this paradigm entirely: the 20Bd/7000 Wide FSK waveform. Operating at a "glacial" 20 Baud but utilizing a massive 7000 Hz frequency shift, this waveform intentionally sacrifices spectral real estate.
On a standard SDR waterfall display, the CIS 20Bd/7000 is highly distinctive yet easily misidentified by automated classification algorithms. Instead of a single coherent data stream, it appears as two completely detached Continuous Wave (CW) carriers separated by a 7 kHz void (Figure 1).
I recently heard this waveform on 25th June at about 2235 UTC on 6945.0 kHz (CF) using the remote AirSpy SDR receiver managed by EA1FAQ in Spain, whom I wish to thank.
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| Fig. 1: CIS 20Bd/7000 Wide FSK |
Technical Specifications & RF Fingerprint (Figure 2):
Modulation: Frequency Shift Keying (FSK / F1B), Lower tone: 6941.5 kHz, Upper tone: 6948.5 kHz, Center freq: 6945.0 kHz
Symbol Rate: 20 Baud (Symbol duration: 50 ms)
Shift (Δf): 7000 Hz (±3500 Hz from center)
Modulation Index β = fm/Δf = 20/7000 = 350
Primary Users: Russian / CIS Military and Diplomatic Networks
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| Fig. 2: main FSK technical parameters (shift & speed) |
Apart from the center frequency noted here (6945.0 kHz), this same transmission can be observed on various other frequencies around 7 MHz, acting as an intruder and causing interference within the corresponding amateur radio band, as regularly documented by the IARU Monitoring System (IARUMS). Specifically, I observed a "twin" transmission at 9966.5 kHz. While it is difficult to establish whether both originate from the same transmitter site, their signal strengths on the waterfall display are nearly identical, suggesting they might be emitted from antennas with different take-off angles.
An intriguing aspect captured in Figure 3 is the simultaneous ±1500 Hz offset from the center frequency of both transmissions, with the signals returning to their baseline values within a few minutes.
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| Fig. 3 : simultaneous ±1500 Hz offset from the center frequencies |
Such a deterministic and simultaneous shift points to two likely scenarios. The first is that this offset is pre-programmed by design, requiring the receiving modem to have prior knowledge of the transition to maintain carrier lock and ensure uninterrupted demodulation. Alternatively, if no prior agreement exists, the receiving system must employ an aggressive Automatic Frequency Control (AFC) or a wide DSP tracking loop capable of instantly detecting the energy shift and re-synchronizing within milliseconds.
By appropriately filtering the signal, it can be easily demodulated using the Signals Analyzer (SA) FSK demodulator. The resulting bitstream—at least in this sample—features a continuously repeating 255-bit pattern (typical of an 8-bit LFSR, 2^8 −1), as shown in Figure 4. However, the pattern does not appear to be generated by a polynomial.
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| Fig. 4: 255-bit period demodulated bitstream |
To confirm this finding, I retrieved two additional recordings of this signal from separate YouTube channels. In both samples, the demodulated bitstreams exhibited the exact same characteristic (a 255-bit pattern length). However, analyzing further recordings could help to better characterize the payload.
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| Fig. 5: comparison of 3 demodulated bitstreams |
Why Spend 7 kHz on 20 Baud?
This signal is engineered for high-availability survivability in contested environments. Using a massive 7000 Hz shift in HF FSK modulation deliberately trades away channel space to achieve near-indestructible reliability. Here is the breakdown of why it is used:
- Beating Ionospheric Fading. The ionosphere often creates narrow "dead zones" in the spectrum. By separating the two signaling tones by 7 kHz, it is statistically impossible for a single ionospheric drop-out to wipe out both frequencies at the same time. If one tone dies, the other gets through.
- Crushing Electronic Jamming. Spreading a slow data rate across such a massive frequency gap creates a huge modulation index. To block the signal, an adversary has to dilute their jamming power across a wide 7 kHz swath, making the jamming highly ineffective.
- Immunity to Doppler Drift. In unstable conditions (like polar regions or solar storms), frequencies can drift by tens of Hertz. For narrow shifts, this blends the tones together; for a 7,000 Hz shift, a 50 Hz drift is completely negligible.
- Signal Obfuscation (Stealth). Because it spans a bandwidth wider than a normal 3 kHz HF intercept receiver's window, an operator or automated classifier looking at a standard waterfall display will simply see isolated, alternating CW (Continuous Wave) pulses separated by a massive gap. It effectively hides its identity as a unified, synchronized FSK data stream, making it harder to automatically classify, intercept, or parse unless the intercepting party knows exactly what to look for.
The CIS 20Bd/7000 is a textbook example of Soviet-legacy military engineering surviving into the modern era. It prioritizes absolute link reliability and Electronic Counter-Countermeasures (ECCM) over spectral efficiency, providing a robust, jam-resistant command link that remains a frequent sight on the HF bands for modern SIGINT monitors.
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