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SRR-90   A/B
Surveillance receiver

SRR-90 was a surveillance receiver for the reception of a several types of covert listening device (bug), developed around 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 SRR-90 1 was the enhanced version of the SRR-91. It had a modular design and was available in two models.

In its basic configuration, the SRR-90 is suitable for the reception of pulse-modulated (PPM) bugs that feature the Rejected Pulse (RP) or Dirty Pulse (DP) audio masking schemes, such as the SRT-56, SRT-107 2 and SRT-90/91 transmitters.

In addition, it can be modified for the reception of 22 kHz or 40 kHz subcarrier (SC) modulated bugs, by swapping three of the modules. The image on the right shows the SRR-90B, which was intended for horizontal use and was usually installed inside a regular Samsonite briefcase. The modules are installed below the beige panel.
SRR-90B receiver

The SRR-90 was developed during the course of 1973 and 1974, following a request of the CIA to study possible improvements of the SRR-91, and started life as the SRR-91 Mark II in November 1974 [3]. After further modifications and improvements, it finally went into production during the course of 1975 as the SRR-90. 1 During its lifetime it was upgraded and improved several times.

In 1978 it was discovered that the receiver was susceptible to interference from the ignition of cars and motor cycles [J]. Motorola, another CIA contractor, had already successfully modified the circuits of the earlier SRR-52 receiver, but this appeared not to be suitable for the SRR-90, so the NRP had to devise a different solution [2].

The SRR-90 was one of the most successful and flexible products developed by the NRP. It had an extremely long lifespan, not least because of its expandability, and was used by the CIA well into the 1990s, long after the demise of the NRP.
SRR-90A receiver (upright model)

In 1991, a batch of SRR-90 receivers was returned to the manufacturer for reburbishment and modification. The receivers were re-aligned, repaired and optimised, as a result of which the modules were no longer interchangable with those of another SRR-90. At the same time, the pulse demodulator module (SLR-5) was modified for the reception of the SRT-99 transmitter. As part of the refurbishment, the updated receivers were given a new serial number (1001 onwards).

In April 1993, just before the NRP closed its doors, a special Test Generator for the SRR-90 receiver was developed and supplied to the CIA [Y]. It simulated the signals from pulse-based transmitters such as the SRT-91, but also from transmitters built by other CIA contractors.

  1. Although its model number suggests otherwise, the SRR-90 was developed and produced about one year later than the SRR-91. 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.
  2. 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-56.

SRR-90A (right) and SRR-90B (left) SRR-90B receiver SRR-90B receiver SRR-90A receiver (upright model)
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SRR-90A (right) and SRR-90B (left)
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SRR-90B receiver
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SRR-90B receiver
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SRR-90A receiver (upright model)

Improvements over SRR-91
The SRR-90 has all characteristics and features of the earlier SRR-91, plus the following:

  • Two case variants: upright (A) and flat (B)
  • Automatic frequency correction (AFC)
  • 6 preset channels
  • Automatic selection between RP (56) and DP (91) masked signals
  • Relay output, activated when locked onto a proper signal (COR)
  • Headphones socket at control panel (rather than at the side)
  • Remote control of the preset channels (via AUX socket)

The SRR-90 was available in the following models, making it suitable for a wide variety of operations. Each model is described in more detail below.

  • SRR-90 A
    Upright version with all controls at the front. Connections at the left and right bottom. Intended for desktop use. Modular construction, not easily accessible.

  • SRR-90 B
    Flat version with all controls at the top. All connections at the left side. Total height 60 mm. Suitable for operation inside a standard briefcase. Modular construction, easily accessible by lifting the flap that covers the rear half.

  • SRR-145
    With the addition of the SRR-145 down-converter, both models can be adapted for the reception of transmitters in the 1300-1600 MHz range, such as the SRT-107 and the high-band version of the SRT-56.
SRR-90 A   vertical model
The basic SRR-90A model is intended for desktop use. The unit measures just 26 x 15 x 11 cm, and has all controls at the front panel. The connections are at the bottom of the left and right sides, with the exception of the headphones bus, which is at the bottom right of the front panel.

This model is intended for use in narrow spaces, such as on a shelf or in a mobile installation. The unit is powered from the 90-240V AC mains, or from internal standard 9V block batteries. The radio consists of a so-called main frame into which a series of plug-in modules are installed.

The receiver covers a frequency range of 260 to 400 MHz, divided over 2 bands, selectable with a toggle switch and marked LO and HI. The image on the right shows a typical SRR-90A. At the left is the tuning knob with its two scales in 10 MHz units: 260-330 MHz (LO) and 330-400 MHz (HI).
SRR-90A receiver (upright model)

Getting access to the interior of the SRR-90A is rather cumbersome, and involves the removal of six screws from the dark brown cover. This means that replacing the internal batteries is not very convenient. In practice, this model was usually powered directly from the 90 - 240 V AC mains.

SRR-90A receiver (upright model) Front panel SRR-90A without the cover Modules inside the SRR-90A Bare mainframe of the SRR-90A Expansion connector Input and output sockets Frequency dial
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SRR-90A receiver (upright model)
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Front panel
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SRR-90A without the cover
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Modules inside the SRR-90A
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Bare mainframe of the SRR-90A
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Expansion connector
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Input and output sockets
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Frequency dial

SRR-90 B   horizontal model
The SRR-90B is functionally identical to the SRR-90A, but is constructed differently so that its hight never exceeds 6 cm, allowing it to be installed in a regular Samsonite briefcase of the era. It has a rectangular shape that measures 26 x 24 cm, making it slightly wider than the SRR-91.

The image on the right shows a typical SRR-90B as seen from the front right. The case consists of two halfs, of which the front one is occuplied by the mainframe and the rear half holds the plug-in modules. The mainframe holds the recessed control panel, which is nearly identical to that of the SRR-90A, but has a different arrangment.

The rear half is covered by a beige hinged lid that is held in place by two screws. This provides easy access to the battery and the modules, making this model more suitable for portable use in areas where mains power is not available.
SRR-90B receiver

In practice the two screws were often omitted, so that the rear compartment could be accessed more easily. This was particularly useful if the receiver was also used for the reception of sub­carrier (SC) modulated bugs like the SRT-153, in which case some modules had to be swapped.

SRR-90B receiver SRR-90B with open lid Control panel Plug-in sockets Mainframe rear view Left side Modules seen from the rear Close-up of the controls
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SRR-90B receiver
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SRR-90B with open lid
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Control panel
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Plug-in sockets
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Mainframe rear view
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Left side
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Modules seen from the rear
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Close-up of the controls

Compatible bugs
350 MHz bug with RP audio masking 350 MHz bug with RP audio masking SRT-90 transmitter SRT-91 transmitter Two-channel (stereo) variant of the SRT-91 Subcarrier modulated bug from another contractor
SRT-99 2
Subcarrier modulated bug from another contractor
SRT-105 1
SRT-107 transmitter SRT-153 transmitter (bug) Subminiature transmitter developed as part of the Super Pulse project

  1. Only when SC-compatible modules are installed.
  2. Only with the refurbished SRR-90 (serial number 1001 onwards).
  3. This transmitter requires the use of the SRR-145 down-converter.

SRR-90A or SRR-90B receiver SRN-9 listening post antenna Headphones Optional down-converter for the 1500 MHz band Listening Post (LP) antenna for 1500 MHz Plug-in modules Alternative plug-in modules for the reception of subcarrier bugs
Receiver   SRR-90
A complete listening post (LP) always has an SRR-90 receiver at its heart. Depending on the circumstances and the available space at the LP, either the SRR-90A or the SRR-90B is selected, whichever is more suitable for the current job.

The SRR-90A fits in a narrow space and is easily connected and operated, but its interior is more difficult to access. The SRR-90B offers the same features but can be concealed and transported in a common executive style Samonite briefcase.
SRR-90A (right) and SRR-90B (left)

LP antenna   SRN-9
A suitable directional antenna for the SRR-90 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-90 receiver and its accessories.

 More information
Sen from the rear

The SRR-90 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-90 (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 transmitter.

 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

SRR-90A mainframe with all modules installed, seen from the rear.
Plug-in modules
A complete SRR-90 consists of a mainframe with 9 modules, each of which has a label with the model and serial number. For optimum performance, the serial numbers should match. 1 For additional functionality, alternatives were for some of the modules were made available.

Basic functionality
  • SLR-1
    RF converter 260-400 MHz (low: 260-330 MHz, high: 330-400 MHz)
  • SLR-2
    IF Filter
  • SLR-3
    IF Amplifier 60 MHz
  • SLR-4
    AFC-60 MHz
  • SLR-5
    Pulse Demodulator
  • SLR-6
  • SLR-7
    Audio Amplifier
  • SLR-8
  • SLR-9
    Battery Pack
  1. This is only the case for SRR-90 receivers that were refurbished in 1991, and that have a serial number of 1001 or higher. In the original series (serial number below 1000) the modules were interchangable.

Improved modules
Around 1978, the following modules were modified for immunity against pulse-type radio interference, such as the suprious signals caused by the ignition of cars and motor cycles. The modified modules were given the suffix 'M'.

  • SLR-3M
    IF Amplifier 60 MHz
  • SLR-5M
    Pulse Demodulator
Alternative frequency range
The frequency range of the SRR-90 can be changed by swapping the SLR-1 for one of the following modules:

  • SLR-15
    RF converter 390-520 MHz
  • SLR-16
    RF converter 235-360 MHz (low: 235-290 MHz, high: 290-360 MHz)
  • SLR-16*
    RF converter 235-360 MHz (low: 235-300 MHz, high: 300-360 MHz)
Alternative audio masking
In its basic configuration, the SRR-90 was only suitable for the reception of pulse-type signals. By swapping the following modules, it could be modified for the reception of bugs with subcarrier audio masking (SC). As SC-masked bugs use much less bandwidth than pulse-based systems, the entire IF section had to be replaced. The following modules were available as an upgrade kit:

  • SLR-2
    IF Converter 60 →10.7 MHz
  • SLR-3
    10.7 MHz Amplifier
  • SLR-5
    40 kHz Demodulator
  • SLR-17
    22 kHz Demodulator (alternative for SLR-12)

  • SLR-13
    IF PSK Demodulator
  • SLR-14
    Digital Decoder
Standard modules for demodulation of pulse-based bugs Standard pulse demodulator (SLR-5M) Modules for the reception of subcarrier bugs (SC) Modules for the reception of subcarrier bugs (SC) Two different SC demodulators 10.7 MHz IF amplifier 10.7 MHz IF amplifier - detail
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Standard modules for demodulation of pulse-based bugs
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Standard pulse demodulator (SLR-5M)
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Modules for the reception of subcarrier bugs (SC)
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Modules for the reception of subcarrier bugs (SC)
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Two different SC demodulators
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10.7 MHz IF amplifier
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10.7 MHz IF amplifier - detail

Block diagram
The block diagram below shows the SRR-90 in its basic configuration in which it is suitable for the reception of pulse-based transmitters. At the bottom is the mainframe that acts as a carrier for the controls, the connections, the mains PSU and the plug-in modules SLR-1 thru SLR-9.

Wideband configuration for reception of Pulse Position Modulation

The antenna is connected to the mainframe, but is directly passed to the RF converter (SLR-1) via a modified BNC socket. The SLR-1 is tuned to the desired frequency and converts it to a 60 MHz wideband signal, which is filtered in SLR-2 and then amplified in SLR-3. The actual demodulation of the pulse-based signals is done by the SLR-5, which automatically selects between RP and DP audio masking. After that, the signal is amplified to headphones level in SLR-7. The COR unit (SLR-6) produces a delayed confidence signal that can be used to control an external recorder.

Narrowband configuration for reception of subcarrier masked signals

The block diagram above shows the same receiver, but modified for the reception of transmitters with subcarrier audio-masking (SC). This is done by swapping modules SLR-2, SLR-3 and SLR-5 for SLR-10, SLR-11 and SLR-12 respectively. Subcarrier bugs produce a continuous wave signal (CW), with a much narrower bandwidth than a pulse-transmitter. The SLR-10 module converts the 60 MHz signal from the tuner to the more common 10.7 MHz, which is then amplified in SLR-11.

The SLR-12 module contains a double FM demodulator. It first demodulates the carrier signal, and then demodulates the resulting 40 kHz subcarrier, resulting in a clear reproduction of the original audio. As in practice different frequencies were used for the subcarrier, alternatives for the SLR-12 may have to be used in order to obtain proper locking. One example is the SLR-17.

Like its predecessor, the SRR-91, the SRR-90 consists of a mainframe and a set of plug-ins, or modules, that are housed together in an aluminium enclosure with a removable cover. The main­frame occupies about half the available space, whilst the other half is taken by the modules.

The image on the right shows a bottom view of the interior of the mainframe of the SRR-90B, as seen from the rear left. It shows the internal wiring and the sockets, or slots, for the plug-in modules. Apart from the mains PSU, the main­frame does not contain any active components.

At the top is the mains power lead that is guided to the transformer in the opposite corner. To the left of the mains cable, the wiring of the AUX bus is clearly visible. It allows remote control of the preset channels. The mainframe of the SRR-90A has a different layout but is otherwise identical.
SRR-90B mainframe bottom view

Each module is assigned a dedicated slot with a 14-pin socket. Although all sockets are identical, a module should only be installed in the slot that carries its name. The two horizontally placed sockets are for the RF converter and the battery. Note that the RF unit is also connected to a BNC socket that carries the antenna signal. The use of plug-in modules, made the SRR-90 extremely service friendly and allowed quick and easy adaption to alternative audio-masking schemes.

SRR-90B mainframe bottom view SRR-90B mainframe bottom view SRR-90B mainframe bottom view Bottom view PSU detail Auxiliary connector wiring Mainframe detail Presets
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SRR-90B mainframe bottom view
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SRR-90B mainframe bottom view
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SRR-90B mainframe bottom view
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Bottom view
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PSU detail
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Auxiliary connector wiring
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Mainframe detail
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The SRR-90A and SRR-90B featured on this page, were rediscovered in 2016 in parts, along with a range of unidentified plug-in modules. At Crypto Museum we have meanwhile been able to restore them both to fully operational condition. The SRR-90A, of which the interior is more difficult to access, has been configured for the reception of RP and DP masked bugs and can be demonstrated in combination with the RP-masked SRT-56 and the DP-masked SRT-91 bugs.

The SRR-90B is fitted with modules that provide the same basic functionality as the SRR-90A, but has an alternative set of modules on the side, allowing it to be converted for the reception of subcarrier (SC) bugs within a few seconds. The image on the right shows the three alternative modules that have to be fitted inside the unit, to make it suitable for the reception of SC bugs.

The receiver can now be demonstrated with the RP-masked SRT-56, the DP-masked SRT-91 and the SC-masked SRT-153.
Modules for the reception of subcarrier bugs (SC)

  1. Provisional data on SRR-91-MK II Receiver
    NRP, November 1974. CM302495/A

  2. Electrical Design Verification of the SRR-91 Mark II 1
    CIA, date unknown, but probably March 1975. CM302495/B.

  3. Receiver alignment sequence
    NRP, personal notes (Dutch). 8 October 1975. CM302495/C.

  4. Manual for SRR-90 A and B Receiver
    NRP, December 1975. CM302495/D.

  5. SRR-90 Manual Amendment (ignition protection)
    NRP, date unknown, but probably 1975. CM302495/E.

  6. Subcarrier Demodulation System (prototype)
    Manual Amendment for SRR-90 Receiver.
    NRP, January 1976. CM302495/F. Superceeded by [H].

  7. Manual for SRR-90 A and B Receiver
    NRP, March 1976. CM302495/G.

  8. Subcarrier Demodulation System
    Manual Amendment for SRR-90 Receiver.
    NRP, March 1977. CM302495/H.

  9. Manual for Spare Plug-in Modules for use with SRR-90A/B Receivers
    NRP, March 1977. CM302495/I.

  10. Report on Investigation of Ignition Interference Susceptibility
    In SRR-90 Receiver Systems. NRP, February 1978. CM302495/J.

  11. SRR-90 Manual Amendment for use with 390-510 MHz RF Module
    NRP, July 1978. CM302495/K. Superceeded by [N].

  12. Manual for ARR-90 A and B Receiver
    NRP, March 1980. CM302495/L.

  13. Subcarrier Demodulation System
    Manual Amendment for SRR-90 Receiver.
    NRP, March 1980. CM302495/M.

  14. SRR-90 Manual Amendment for use with RF-converter Module SLR-15
    NRP, September 1980. CM302495/N.

  15. SRR-90 Manual Amendment for use with RF-converter Module SLR-16
    NRP, September 1980. CM302495/O.

  16. Environmental Test Report on SLR-17 22 kHz Demodulator
    NRP, July 1981. CM302495/P.

  17. Manual for ARR-90 A and B Receiver
    NRP, March 1982. CM302495/Q.

  18. Subcarrier Demodulation System
    Manual Amendment for SRR-90 Receiver.
    NRP, March 1982. CM302495/R.

  19. Environmental Test Report on SRR-90 subcarrier demodulation devices
    NRP, April 1982. CM302495/S.

  20. Manual for SRR-90 A and B Receiver - subcarrier mode (draft)
    NRP, June 1983. CM302495/T.

  21. Manual for SRR-90 A and B Receiver - subcarrier mode
    NRP, July 1983. CM302495/U.

  22. Environmental Test Report on SRR-90 Receiver
    Receiver Modules SLR-10, SLR-11 and SLR-12. NRP, December 1983. CM302495/V.

  23. Technical Manual for the SRR-90 receiver
    NRP, July 1991. CM302495/W. 2

  24. Collection of test and acceptance sheets for SRR-90
    NRP/CIA, October 1991 - January 1992. CM302495/X. 2

  25. Technical Manual for the SRR-90 Test Generator
    NRP, April 1993. CM302495/Y.
  1. During the development phase, the SRR-90 was known as the SRR-91 Mark II.
  2. Issued with refurbished SRR-90 receivers that were given a serial number from 1001 onwards.

  1. NRP/CIA, Collection of documents related to SRS-90
    Crypto Museum Archive, CM302495 (see above).

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

  3. Study of Further SRS-91 System Developments
    NRP, October 1973. CM302629/F.
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Crypto Museum. Created: Saturday 06 May 2017. Last changed: Wednesday, 16 August 2017 - 06:08 CET.
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