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Surveillance receiver - under construction

SRR-91 is a surveillance receiver for the reception of a several types of covert listening device (bug), developed around 1974 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 has a modular design and can easily be adapted to new and emerging audio masking schemes.
The SRR-91 had a flat rectangular design and was no higher than 60 mm, so that it would fit nicely inside a common briefcase of the era.

All controls are at the front half of the top panel, whilst all connections are located at the left side. The rear half of the receiver contains the various modules, and can be accessed by lifting the hinged lid that is held in place by two screws. The device is suitable for the reception of bugs that use the sophisticated Dirty Pulse (DP) audio masking scheme, also known as the 91 scheme, that was used by the SRT-90 and SRT-91 bugs.
SRR-91 receiver

It is also suitable for the reception of transmitters that use the Rejected Pulse (RP) audio masking scheme, such as the SRT-56, SRT-56-F and SRT-107 1 , although for these bugs, the DEMOD B plug-inhad to be installed the other way around. For this reason the DEMOD B plug-in has a connector at either end. The SRR-91 superceeded the SRR-56 receiver, but not the SRR-52, as it is not suitable for transmitters with Triple Pulse (TP) audio masking scheme, such as the SRT-52.

The SRR-91 was developed during the course of 1973 and 1974, with the first production devices being delivered in November 1974. It was in production until at least 1976. About a year after its introduction, in 1975, the SRR-91 was succeeded 2 by the more advanced SRR-90 A/B, that was available in two variants: a straight-up tabletop model, and a flat one for use inside a briefcase.
  1. For the reception of the SRT-107, the SRR-145 down-converter is required as well. This is also the case for the high-band version of the SRT-52 and the SRT-56.
  2. Although its model number suggests otherwise, the SRR-91 was developed and produced about one year earlier than the SRR-90. The reason for this is the fact the SRR-91 was initially known as the SRR-91 Mk I and the enhanced version, which would be produced later, as the SRR-91 Mk II. However, when the SRR-91 Mk II was ready for release, the CIA decided to rename it SRR-90.

SRR-91 receiver

Compatible bugs
The SRR-91 is suitable for receiving and decoding the following transmitters:
350 MHz bug with RP audio masking 350 MHz bug with RP audio masking SRT-91 transmitter
SRT-107 transmitter

  1. This requires the use of the SRR-145 down-converter.
Listening Post (LP) antenna for 300 MHz Headphones Optional down-converter for the 1500 MHz band Listening Post (LP) antenna for 1500 MHz

LP antenna   SRN-9
A suitable directional antenna for the SRR-91 listening post (LP) is the SRN-9-L, or the later SRN-9. It offers a gain of 7 dB and is in fact an adjustable dipole on a horizontal boom (which acts as a balun), mounted in front of a reflector.

The antenna can be disassembled completely, and the reflector plane can be folded at the centre, so that the entire unit can be stored inside a regular briefcase, along with the SRR-56 receiver and its accessories.

 More information
Sen from the rear

The SRR-91 has two audio outputs: a fixed one for connection of a recording device, and an adjustable one for connection of a pair of headphones. Virtually any type of headphones with an impedance of 600Ω can be used.

It was typically used with American military headphones of the era, such as the one shown in the image on the right.

Down-converter   SRR-145
The frequency range of the SRR-91 (260 - 400 MHz) 1 could optionally be enhanced with the 1300 - 1600 MHz band, simply by inserting the SRR-145 down-converter shown on the right, between the antenna and the receiver's input.

This was necessary for receiving SRT-56 units that were equipped with a high-band SRK-145 RF module. It was also needed for the reception of the SRT-107 transmitters.

 More information
SRR-145 down-converter

1500 MHz antenna   SRN-55
When using the SRR-145 down-coverter shown above, the existing SRN-9 listening post antenna has to be replaced by one that is suitable for the 1300 to 1600 MHz frequency range.

The SRN-55 is a flat stacked-dipole antenna that covers the entire range and offers a gain of approx. 17.5 dB.

 More information
SRN-55 directional antenna

Plug-in modules
Each module should be installed in a dedicated slot, as indicated by the name that is printed above each connector. The following modules were available:
  • RF Converter (tuner)
  • Filter
  • IF Amplifier
  • Demodulator A
  • Demodulator B ← Interchangable
  • Audio
  • Regulator
  • Battery
  1. tba

  1. NRP/CIA, Collection of documents related to SRR-91
    Crypto Museum Archive, CM302500 (see above).

  2. NRP/CIA, Collection of documents related to AGC ignition interference
    Crypto Museum Archive, CM302626.

Further information

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Crypto Museum. Created: Friday 10 March 2017. Last changed: Sunday, 23 April 2017 - 15:06 CET.
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