Digital RF Memory (DRFM)
Understanding Digital RF Memory
Traditional noise jammers flood the radar receiver with broadband energy, raising the noise floor and obscuring targets. However, modern radar processing techniques (pulse compression, Doppler filtering, sidelobe blanking) are specifically designed to reject noise-like interference. DRFM overcomes these defenses by retransmitting a signal that is indistinguishable from a genuine target echo: it has the correct waveform, the correct modulation, and the correct phase relationship to the radar's transmitted pulse. The radar's own matched filter compresses the DRFM signal just like a real echo, and Doppler filters pass it as a valid target return.
The DRFM architecture consists of a wideband receiver (antenna, LNA, downconverter), a high-speed ADC (2 to 10+ GSPS, 8 to 14 bits), digital memory (storing microseconds to milliseconds of digitized signal), a signal processor (applying time delays, frequency shifts, and amplitude modulation), a DAC, and a transmit chain (upconverter, power amplifier, antenna). The entire capture-modify-retransmit cycle must complete within the radar's pulse repetition interval, typically microseconds. Latency through the DRFM determines the minimum range of the first false target.
DRFM Signal Processing
ΔR = c × Δτ / 2
where Δτ = additional time delay applied by DRFM
False Target Doppler Offset:
Δfd = Δφ / (2π × PRI)
where Δφ = phase increment per pulse, PRI = pulse repetition interval
Minimum ADC Requirements (Nyquist):
fs ≥ 2 × BWsignal
For 500 MHz chirp bandwidth: fs ≥ 1 GSPS (practical: 2-4 GSPS)
Phase Coherence Requirement:
Phase error < λ/16 across the digitize-store-retransmit cycle
DRFM Jamming Techniques
| Technique | DRFM Modification | Effect on Radar | Target Subsystem |
|---|---|---|---|
| False Target Generation | Multiple delayed copies | Multiple phantom targets appear | Detection / track initiation |
| Range Gate Pull-Off (RGPO) | Progressively increasing delay | Range tracker follows false target | Range tracking loop |
| Velocity Gate Pull-Off (VGPO) | Progressively increasing Doppler | Velocity tracker follows false target | Doppler tracking loop |
| Cross-Eye | Phase-reversed retransmission | Monopulse tracker points away | Angle tracking |
| Coherent Cover Pulse | Wideband replica at high power | Masks true echo in noise-like background | Matched filter / detection |
Frequently Asked Questions
How does a DRFM create coherent false targets?
The DRFM digitizes the incoming pulse, preserving carrier frequency, modulation, and phase. It stores the digital replica and retransmits it with controlled modifications: time delay for false range, frequency offset for false Doppler, amplitude scaling for false RCS. Because the signal is a phase-coherent copy, it passes through the radar's matched filter and pulse compression exactly like a real echo, making it indistinguishable from genuine returns in the radar's processing chain.
What sampling rate does a DRFM require?
Phase-coherent DRFM needs sampling rates sufficient to preserve carrier phase. Modern systems use 2 to 4 GSPS ADCs with 8 to 12 bits, providing 1 to 2 GHz instantaneous bandwidth. Higher-end systems targeting wideband LPI radars use 10+ GSPS ADCs. The bit depth determines dynamic range: 8 bits gives 48 dB spurious-free dynamic range, while 12 bits provides 72 dB. Memory depth determines maximum storage time: at 4 GSPS with 12 bits, one millisecond of signal requires 6 megabytes.
What is range gate pull-off (RGPO)?
RGPO gradually moves the radar's range tracking gate away from the true target. The DRFM first retransmits a coherent copy at the correct delay (matching true range). Over successive pulses, it progressively increases the delay, and the radar's tracking loop follows the stronger DRFM signal. Once pulled far enough, the DRFM shuts off, leaving the radar tracking empty space. The delay increase rate must be slow enough that the tracking loop follows without breaking lock, typically less than the loop bandwidth.