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← Easy Chair

SRR-145 is a so-called down-converter, developed around 1972 by the Dutch Radar Laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of a long-term research contract under the name Easy Chair (EC). It extended the frequency range of the existing surveillance receivers.
In 1971, following a series of studies into the usabilty of the newly allocated frequency bands 4, 5 and 6 (1000-1900 MHz) 1 for their covert listening devices (bugs), the CIA decided to move the operating frequency of some of their existing bugs to Band 5 (1300-1600 MHz). This band is also referred to as the 1500 MHz band.

The SRR-145 converts the new 1300-1600 MHz frequency range down to the old 240-470 MHz range, so that any covert listening devices (bugs) operating on 1500 MHz, can be received and decoded on existing CIA surveillance receivers.
SRR-145 down-converter

The SRR-145 was developed during the course of 1970/1971, with the first prototypes delivered in February and June 1971. After several improvements, it was ready for production in November 1972. The SRR-145 had a long life cycle, as it was used with all known NRP surveillance receivers.
  1. Not to be confused with the regular definition of UHF Band V. The band allocation numbers used here, are internal CIA designators.

SRR-145 down-converter Front panel Four outputs for connected receivers Mains power socket and loop socket Adjusting the local oscillator of the SRR-145 Adjusting the SRR-52-M receiver SRR-52 receiver with SRR-145 down-converter on top Interconnection cables

The SRR-145 was initially designed for use in combination with the existing SRR-52 and SRR-56 surveillance receivers, but it was later also used with the SRR-90 and SRR-91 models.

The image on the right shows the SRR-145 on top of an SRR-52 receiver, which has the same width. Mains power is looped via the SRR-145, so that only one wall socket is used. A short coaxial cable connects one of the IF outputs to the RF input of the SRR-52 follow-on receiver. Later receivers, like the SRR-90 and SRR-91, were commonly placed on top of the SRR-145.
SRR-52 receiver with SRR-145 down-converter on top

Compatible receivers
Surveilance receiver SRR-52 Surveilance receiver SRR-56 Surveilance receiver SRR-91 Surveilance receiver SRR-90-A Surveilance receiver SRR-90-B

Compatible bugs
The SRR-145 made the above receivers compatible with the following bugs:
High-band version of the SRT-52 bug with TP audio masking Hiigh-band version of the SRT-56 bug with RP audio masking SRT-107 transmitter

All controls and connections of the SRR-145 are located at its front panel, which allows the device to be used in horizontal as well as vertical position. A suitable Listening Post (LP) antenna, such as the SRN-55, should be connected to the N-type socket at the bottom right. It is powered from the 90-240V AC mains, which should be connected at the top left. An extra socket is available for looping the mains power to the follow-on receiver, so that only one wall socket is needed.

The Local Oscillator (LO) of the SRR-145 is adjustable with the frequency tuning knob, so that the coversion point can be chosen conveniently for least interference and optimum signal/noise ratio. Four independent IF outputs are available at the front panel, allowing the connection of up to four follow-on receivers, each of which can be used for the reception of a single 1300-1600 MHz bug.
Front panel Four outputs for connected receivers Mains power socket and loop socket Adjusting the local oscillator of the SRR-145 Four IF output Antenna socket with switchable phantom power for (optional) pre-amplifier Local oscillator adjustment Adjusting the SRR-52-M receiver

Block diagram
The block diagram below shows the basic operation of the SRR-145, which is pretty straight­forward. At the top left is the LP antenna, for which commonly an SRN-55 with a gain of 17.5 dB was used. After filtering the antenna signal in a very sharp bandpass filter, it is mixed with the signal of an adjustable local oscillator (LO), filtered again and then amplified in a pre-amplifier.

The signal from the amplifier is then passed via a 5 dB attenuator, to a power splitter with four outputs, each of which can be used to connect an independent follow-on receiver. In this case, the follow-on receiver acts as the IF-stage of the SRR-145. The output frequency is calculated as:

fout = fin ± fLO

The SRR-145 is housed in a strong beige metal enclosure that measures 306 x 225 x 74 mm and weights approx. 3.6 kg. It has the same width as the SRR-52 and SRR-56, so that it can be placed nicely on top of those receivers. The interior can be accessed by removing four screws around the edges of the front panel (two at the top and two at the bottom) plus one large bolt at the rear.

After sliding-off the case shell, the well-organised interior is exposed, as shown in the image above. The internal construction is very neat and robust, and the individual parts are easily recognised by following the block diagram above. Some of the parts, such as the mixer and the pre-amplifier were readily bought from other manufacturers, but some of the critical parts, such as the input filter, were purpose-built at the NRP.
SRR-145 interior Interior top view Power supply unit (PSU) Filter and pre-amplifier Input filter and IF output splitter Local oscillator Fuse (plus spare) Pre-amplifier

  1. Manual for Prototype Band 5 Conversion Equipment
    NRP, July 1971. CM302474.

  2. Manual for SRK-145 and SRN-58
    NRP, June 1972. CM302479.

  3. Technical Manual for SRR-145
    NRP, February 1973. CM302475/A.

  4. Abbreviated Operating Instructions and Technical Data on Modified SRR-145
    NRP, February 1984. CM302475/B.

  5. Operating Instructions for SRR-145-B down-convertor
    NRP, May 1984. CM302475/C.

  1. NRP/CIA, Collection of documents related to SRR-145
    Crypto Museum Archive, CM302475 (see above).

Further information

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Crypto Museum. Created: Thursday 09 March 2017. Last changed: Saturday, 13 May 2017 - 07:23 CET.
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