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Covert listening device with DSBSC audio masking - this page is a stub

SRT-57 is a covert listening device (bug), developed around 1967 by the Dutch Radar Laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of a long-term research contract under the codename Easy Chair. The device works at a spot frequency in the 230-250 MHz band and features audio-masking by means of double-sideband suppressed-carrier (DSBSC) modulation. It has a built-in switch receiver that operates in the same RF band, with separate on/off signals.

The device was developed during the course of 1967 and was presented as an evaluation kit to the CIA in October 1967. It consists of an RF unit (transmitter) with built-in switch receiver, an audio modulator with subcarrier modulation of which the 20 kHz subcarrier is suppressed (DSBSC), and an external demodulator for this signal type was supplied that could be retrofitted to an existing SRR-40 surveillance receiver.

The image on the right shows the complete kit as it was provided to the CIA. At the right is a spare 20 kHz crystal, for subcarrier re-insertion .
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The the switch receiver allows the bug to be remote controlled and is 'borrowed' from the earlier Pied Piper research project, that resulted in the evaluation version of the SRT-40 target element (bug) and SRR-40 listening post receiver.

A complete SRT-57 evaluation kit consists of the following:

  • SRT-57 RF unit with switch unit
  • SRT-57 modulator with 20 kHz DSBSC
  • Patch cable for testing above modules
  • Demodulator for existing SRR-40 receiver
  • Spare 20 kHz crystal (for subcarrier insertion)
  • Operating manual
Modulator   SRT-57
  • Supply
    +3.7 V DC
  • Current
    95-100 µA
  • Subcarrier
    20 lHz
  • Input
    500 µV RMS at 4000Ω
  • Output
    230 mW p-p, or 56 mV RMS into 25kΩ
  • Response
    300-5000 Hz ±3dB
  • Suppression
    > 30dB
  • Compression
Transmitter   SRT-57
  • Supply
    +4.05V (on)
  • Switched
    +3.7V (on), +0.15V (off)
  • Current
    1.96 mA (on), 24µA (off)
  • Frequency
    230-250 MHz
  • Output
    ≥ 1 mW (on)
  • Impedance
    50 Ω
  • Pulling
    0.3 MHz p-p (VSWR = 2)
  • Sensitivity
    0.27 kHz per mV
  • Index
  1. Manual for SRT-57 evaluation equipment
    NRP, October 1967.
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Crypto Museum. Created: Sunday 27 November 2022. Last changed: Tuesday, 13 December 2022 - 09:26 CET.
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