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SRT-91 →
Covert listening device with DP audio masking

SRT-90 was a covert listening device (bug), developed in 1975 by the Dutch Radar laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of a long-term research contract code­named Easy Chair. The device features the Dirty Pulse (RP) 1 advanced audio masking scheme.

The SRT-90 is in fact a low-power variant of the SRT-91 that was released one year earlier. It is housed in a similar rectangular brass enclosure, that measures 13.5 x 22.5 x 100 mm; one 1cm shorter than the SRT-91. The reason for using a low-power version, is that it is more difficult to detect than a medium or high power version.

The SRT-90 was intended for deployment in operations where the Listening Post (LP) was close to the Target Area (TA). In such cases a Path Loss Survey System, such as the URS-1, was used first to determine the required power.
SRT-90 transmitter

The device consists of an RF module that operates between 270 and 400 MHz, and a video coder for the Dirty Pulse (DP) 1 audio masking scheme, housed in a single brass enclosure. It could be powered by a DC source between +2.25V and 3.25V, generally consisting of two military-grade mercury cells, and consumes typically 160µA, whilst delivering a peak-output-power of 1 mW.

The image on the right shows the RF-unit of the SRT-90 aside the RF-unit of the SRT-91, which delivers a peak output power of 20 mW. Because of the lower power, the last stage of the RF-unit (i.e. the PA) has been omitted in the SRT-90.

The rest of the circuits, which consists of a microphone amplifier, a noise generator and a video encoder, are identical to those of the SRT-91, which means that both transmitters can be received with the same receivers.

 For further details, check out the SRT-91
RF-unit of the SRT-91 (top) and SRT-90 (bottom)

  1. Dirty Pulse (DP) audio masking is also known as Type 91 modulation.

SRT-90 transmitter SRT-90 with wiring Size of the SRT-90 (front) compared to that of the SRT-91 (rear) Wiring detail RF-unit of the SRT-91 (top) and SRT-90 (bottom) RF-unit of the SRT-91 (top) and SRT-90 (bottom)
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SRT-90 transmitter
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SRT-90 with wiring
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Size of the SRT-90 (front) compared to that of the SRT-91 (rear)
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Wiring detail
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RF-unit of the SRT-91 (top) and SRT-90 (bottom)
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RF-unit of the SRT-91 (top) and SRT-90 (bottom)

The SRT-90 operates on a pre-determined spot frequency between 270 and 400 MHz, that is factory adjusted. The selected frequency is presented as a suffix to the transmitter's model number (e.g.: SRT-90-A). The following frequencies were used (± 5 MHz):

  1. 275 MHz
  2. 295 MHz
  3. 315 MHz
  4. 335 MHz
  5. 355 MHz
  6. 375 MHz
  7. 395 MHz
  1. Technical Manual for SRT-90
    NRP, June 1975. CM302507/A.

  2. Technical Manual for SRT-90 Transmitter (concept)
    NRP, September 1975. CM302507/B.

  3. Operation and Test Manual for SRT-90 Transmitter
    NRP, December 1975. CM302507/C.
  1. NRP/CIA, Collection of documents related to SRT-90
    Crypto Museum Archive, CM302507 (see above).

  2. Study of Further SRS-91 System Developments
    NRP, October 1973. CM302629/F.
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Crypto Museum. Created: Thursday 09 March 2017. Last changed: Tuesday, 13 June 2017 - 06:24 CET.
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