Click for homepage
← Easy Chair
Surveillance receiver

SRR-52 and SRR-52-M were surveillance receivers for the reception of a special type of covert listening device (bug), developed around 1968 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 SRR-52 was developed as an essensial component of the CIA's SRS-52 surveillance system.

The receiver covers a frequency range of 240 MHz to 330 MHz, and is powered by the 50/60 Hz 90-270V AC mains. It is compatible with the SRT-52 covert transmitter (bug) and supports the Triple Pulse (TP) audio masking scheme, also known as type 52 modulation or the 52-system.

In 1972, some of the existing SRR-52 receivers were retrofitted with a new audio decoder that made it compatible with the Rejected Pulse (RP) and Dirty Pulse (DP) audio masking schemes. Modified receivers were given the designator SRR-52-M. An example is shown on the right.
SRR-52 surveillance receiver

The SRR-52 has a built-in alarm function that produces a tone when the receiver has accidently been tuned to a regular (non-TP) RF signal, or when the audio-masking feature of the SRT-52 fails and the bug loses its security. It also has a built-in test facility, that allows the video encoder of the SRT-52 to be connected directly to the video input of the SRR-52, thereby bypassing all RF components. And with the supplied video detector is is possible to connect the RF output of the SRT-52 to the video input of the receiver, thereby bypassing the RF and IF stages of the SRR-52.

Development of the SRR-52 was started around 1966, with the first prototypes being available in 1967. After a period of testing and improving, it was in production from 1969 - 1970, after which TP masking was abandoned in favour of the RP and DP schemes, that were easier to implement.

In 1972, the SRR-145 converter was added to expand the range to 1300-1600 MHz. In 1975, the Automatic Gain Control (AGC) of the SRR-52 was modified by Motorola to make it immune to peak-pulse interference in urban areas, caused by the ignition of cars and motorcycles [2].
SRR-52 receiver with SRR-145 down-converter on top

The SRR-52 is very similar to the SRR-56, which was introduced around the same time (1968) for use with bugs that supported the Rejected Pulse (RP) audio masking scheme, such as the SRT-56. In 1972, more than half of the existing SRR-52 receivers were retrofitted with a modification that made it compatible with the audio masking schemes of the SRT-56 and SRT-91. The SRR-52 was eventually succeeded in 1973 by the SRR-91 and in 1975 by the more versatile modular SRR-90.

SRR-52 surveillance receiver Front panel Video detector Dipole antenna with boom and reflector
1 / 4
SRR-52 surveillance receiver
2 / 4
Front panel
3 / 4
Video detector
4 / 4
Dipole antenna with boom and reflector

  • SRR-52
    Original version of the SRR-52, developed in 1967 and in production from 1969 to 1970. Covers the lower UHF band from 240 to 330 MHz, and can demodulate TP-masked audio signals (the so-called type 52 modulation). Compatible with the SRT-52 transmitter.

  • SRR-52-M
    SRR-52, retrofitted 1 with a modification to allow demodulation of RP-masked and DP-masked audio signals. This makes the receiver compatible with the SRT-56 and partly 2 with the SRT-91. With the SRR-145 converter it was also compatible with the SRT-107.
  1. More than half of the existing SRR-52 receivers were converted into an SRR-52-M in 1972.
  2. Only part of the frequency band of the SRT-91 is covered.

The SRR-52 is housed in a sturdy blue metal enclosure. It has all controls and connections at its front panel, so that it can be used both horizontally (as shown below) and vertically (with its control panel facing upwards). The diagram below shows the layout of the modified SRR-52-M. The modification was retrofitted in 1972 and involves the brighter part of the front panel.

The full 240-330 MHz frequency range is covered by one full turn of the large knob at the right. It is marked in tens of MHz, with '0' corresponding to 300 MHz. The MODE selector controls the function of the meter and also selects the appropriate audio masking decoder (SRR-52-M only).

When tuned-in on an SRT transmitter, the tuning knobs above the MODE selector should be used to adjust the timing of the pulsed signal. This means getting a maximum reading on the meter. An alarm will be heard when the timing is mis-adjusted or when audio masking facility fails.

The frequency range of the SRR-52 could be expanded by using an SRR-145 down-converter. This makes the existing receivers suitable for the reception of bugs in the 1300-1600 MHz range, such as the SRT-107 and the high-band versions of the SRT-52 and the SRT-56 (see below).
SRR-52 receiver with SRR-145 down-converter on top

The SRR-145 has the same width as the SRR-52, so that it can be placed conveniently on top of it. Both receivers have a small circular notch at either side of the case, to which an optional carrying grip can be attached, making the device 'portable'. In practice however, the receivers were usually installed and operated from inside a common Samsonite executive style briefcase.

Compatible bugs
The SRR-52-M is suitable for receiving and decoding the following transmitters:

290 MHz bug with TP audio masking 350 MHz bug with RP audio masking 350 MHz bug with RP audio masking Low-power version of the SRT-91 Miniature 350 MHz transmitter (bug) with Dirty Pulse (DP) audio masking SRT-107 transmitter

  1. The SRT-107 can only be received when the SRR-145 down-converter is used as well. The SRR-145 is also needed when the SRT-52 or the SRT-56 is fitted with an SRK-145 RF-module (1300-1600 MHz).

Low-band setup
The diagram below shows a complete SRS-52 1 setup. At the left is the SRT-52 transmitter which consists of a microphone, an SWE-52 video encoder (the TP audio masking unit), and an SRK-29 RF-module with a center frequency of 290 MHz. A sleevex antenna transmits the signal that is picked up by the SRN-9 antenna at the listening post (LP), where it is fed to the SRR-52 receiver.

  1. SRS = Surveillance Radio Set.

High-band setup
In 1971, the CIA started using the newly allocated 1500 MHz band for the use of covert listening devices. The diagram below shows how the existing parts were used in the new setup. At the left is the SRT-52 transmitter, in which the SRK-29 RF-module has been replaced by an SRK-145.

A small SRN-58 plexiglass antenna transmits the 1500 MHz signal to the listening post (LP) where it is picked up by an SRN-55 antenna, and passed on to the SRR-145 down-converter. The latter converts the 1500 MHz signal into 300 MHz, so that it can be decoded with the SRR-52.

In the high-band setup, the SRR-52-M is also suitable for the reception of the SRT-107, which is in fact an integrated version of the SWE-56 video encoder, the SRK-145 RF-module and the SRN-58 antenna. The diagram above shows how the SRT-107 is used in this setup.

A complete SRR-52 listening post consists of one or more of the following items:

SRR-52 or SRR-52-M receiver Listening Post (LP) antenna for 300 MHz Headphones External detector (test head) Optional down-converter for the 1500 MHz band Listening Post (LP) antenna for 1500 MHz
Receiver   SRR-52
At the heart of a listening post (LP) of the 52-system, was the SRR-52 or SRR-52-M receiver featured on this page. It covers 240 to 330 MHz, and is capable of receiving TP-masked pulse transmitters like the SRT-52.

The modified SRR-52-M is also suitable for the reception of RP-masked pulse transmitters like the SRT-56 and (with the use of the SRR-145 down-converter) the SRT-107.

 Look inside the SRR-52
SRR-52 surveillance receiver

LP antenna   SRN-9
A suitable directional antenna for the SRR-52 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-52 receiver and its accessories.

 More information
Sen from the rear

The SRR-52 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.

Detector   test head
The SRR-52 has a direct video input on its front panel, which can be used for testing the video encoder of an SRT transmitter. By connecting the video output of the encoder directly to the video socket of the SRR-52, all RF components (in the transmitter and the receiver) are bypassed.

By connecting the video detector, shown in the image on the right, to the video socket of the SRR-52, the RF output of an SRT transmitter can be converted directly into pulses (video), thereby bypassing the receiver only.
Video detector

Down-converter   SRR-145
The frequency range of the SRR-52 (240 - 330 MHz) 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-52 and SRT-56 units that were equipped with a high-band SRK-145 RF module. It was also needed for the reception of the later 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

Block diagram
Below is the block diagram of the Triple Pulse (TP) decoder of the SRR-52. At the far left is the tuner, which consists of an RF and an IF stage. It delivers a video signal that is fed to a slicer of which the output contains clean groups of three pulses each. Each group is then delayed, after which the image is added to its original, in such a way that groups of five pulses are obtained. The third pulse (which has become twice as high as a result of the addition), is used to set a flip-flop. Likewise, the fourth pulse (with is position modulated) is used to reset the flip-flop.

The output of the flip-flop is a pulse-width modulated signal, that is fed to a quasi integrator followed by a peak detector and a sample-and-hold circuit to fill the gaps between two samples. The result is a staircase signal that resembles the shape of the original audio signal. A low-pass filter is used to smoothen the signal before it is fed to the audio amplifier at the bottom right.

The SRR-52 is extremely well built and is housed in a bright blue metal enclosure. All parts are mounted to a strong metal chassis that itself is mounted to the front panel. After removing four screws around the edges of the front panel, and one at the rear, the case shell can be removed.

The image on the right shows the SRR-52 after the case shell has been removed. When looking at the device from the rear, it is obvious that the case is divided into four sections, separated by metal panels. The leftmost section contains the power supply unit (PSU), whilst the RF tuner is contained in a separate enclosure at the right.

The large centre part of the receiver is divided in a upper and a lower section. The lower section holds the IF/AF stages, whilst the upper section holds the audio masking decoders, with their adjustable delay lines visible at the rear right.
SRR-52 interior

The SRR-52 was clearly developed with TEMPEST in mind. Unlike many contemporary receivers, the front-end was constructed in such a way that no signal from the local oscillator (LO) would leak out through the antenna connector, which might otherwise have revealed the LP's presence.

This also explains why the meter on the front panel is housed inside a circular purpose-made brass enclosure. It prevents any internal signal residue from leaking out through the fairly large hole in the front panel. This also explains the brass pane around the meter on the front panel.

When the SRR-52 was modified in 1972, it was retrofitted with an extra printed circuit board that was fitted below the existing decoder (C). The new board has the same size as the existing one, and supports bugs that use the Rejected Pulse (RP) and Dirty Pulse (DP) masking schemes.

On the modified SRR-52-M, switching between the two decoders is done with the MODE selector that was given an extra setting. In 52-mode it supports the old Triple Pulse (TP) masking scheme. This scheme is used by the SRT-52 transmitter. In the new 56-mode, the receiver is suitable for the reception of RP and DP masked transmitters, such as the SRT-56 1 , the SRT-90 and SRT-91.

  1. In combination with the SRR-145, the SRR-52 is also suitable for receiving and decoding the SRT-107.

SRR-52 interior SRR-52 interior - rear view Bottom view Bottom view PCB detail Delay lines TEMPEST meter Power Supply Unit (PSU)
1 / 8
SRR-52 interior
2 / 8
SRR-52 interior - rear view
3 / 8
Bottom view
4 / 8
Bottom view
5 / 8
PCB detail
6 / 8
Delay lines
7 / 8
8 / 8
Power Supply Unit (PSU)

  1. Manual for SRT-52 and UWP-52 prototype equipment
    CM302489/A, May 1967.

  2. Operating Manual for SRS-52 Equipment
    CM302489/B, October 1968.

  3. Technical Manual for SRS-52 Equipment
    CM302489/C, October 1968.

  4. Operating Manual for SRS-52 Equipment
    CM302489/D, December 1968.

  5. Technical Manual for SRS-52 Equipment
    CM302489/E, December 1968.

  6. Operating Manual for SRS-52 Equipment
    CM302489/F, January 1969.

  7. Technical Manual for SRS-52 Equipment
    CM302489/G, January 1969.

  8. Operating Manual for SRS-52 Equipment
    CM302489/H, October 1969.

  9. Technical Manual for SRS-52 Equipment
    CM302489/I, October 1969.
  1. NRP/CIA, Collection of documents related to SRS-52
    Crypto Museum Archive, CM302489 (see above).

  2. NRP/CIA, Collection of documents related to AGC ignition interference
    Crypto Museum Archive, CM302626.
Further information
Any links shown in red are currently unavailable. If you like the information on this website, why not make a donation?
Crypto Museum. Created: Sunday 16 April 2017. Last changed: Sunday, 13 August 2017 - 17:15 CET.
Click for homepage