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Covert listening device with SC audio masking

SRT-153 is a covert listening device (bug), developed around 1979 by the Dutch Radar Laboratory (NRP) for the US Central Intelligence Agency (CIA) as part of a long-term research contract under the codename Easy Chair. The device is a 'chinese copy' of an existing bug that was discovered in the desk of a US Ambassador in the mid-1970s. The transmitter features a subcarrier-modulated audio masking scheme, and was commonly used as part of the SRS-153 surveillance system.

The transmitter consists of three modules: a modulator, a noise generator and an RF-unit, each of which is housed in a separate metal enclosure. Each module is hermetically sealed and has its connections at the short sides.

The modules could be mounted separately inside a concealment, but the complete transmitter was also available in a single pre-wired enclosure for quick deployment. The transmitter produces a continuous wave (CW) radio signal. Its carrier is frequency modulated (FM) with a 22 kHz sub­carrier that is frequency modulated by the audio.
SRT-153 interior (cleaned)

In addition, a strong noise signal is injected into the transmitter's baseband signal. This noise, which resembles the natural noise in an empty radio channel, is added to hide, or mask, the presence of the subcarrier which carries the actual audio signal. The drawback of a CW system is that it consumes quite a bit of power, as a result of which its battery life is rather limited.

Furthermore, subcarrier bugs (SC) are easily discovered with a professional bug tracer of the era, such as the ScanLock Mark VB. For these reasons, the transmitter is remote controlled by means of the separate QRR-153 switch-receiver. It is housed in a similar enclosure and is activated by the QRT-153 activation-transmitter. The SRT-153 operates on a spot frequency in the 260-320 MHz band, whilst delivering 5 mW of power. The remote activation signal is in the 70 MHz band.

SRT-153 transmitter SRT-153 interior (cleaned) SRT-153 compared to the size of a hand Opening the SRT-153 SRT-153 interior - top view Transmitter mounted in a single enclosure Test board with 3-piece variant of QRR-153  Packaged and bare transmitter, both mounted on a test panel
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SRT-153 transmitter
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SRT-153 interior (cleaned)
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SRT-153 compared to the size of a hand
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Opening the SRT-153
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SRT-153 interior - top view
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Transmitter mounted in a single enclosure
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Test board with 3-piece variant of QRR-153
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 Packaged and bare transmitter, both mounted on a test panel

The SRT-153 was available in two case-variants:

  • Three-piece version
    The basic version of the SRT-153 came as a three-piece solution; one for each individual circuit. It is a copy of an alien bug that was found inside a wooden drawer divider in the desk of a US Ambassador in the mid-1970s. The complete bug, including the enclosures and the concealment, was meticulously copied by the CIA.

  • Single-piece version
    In this version, the three individual pre-fabricated modules are mounted inside a single metal enclosure that is filled with silicone paste. It is based on a later version of the alien bug described above. Power, antenna and microphone are connected to the five contacts at one of the short sides. The unit measures 106 x 18 x 6 mm.
The diagram below shows the contents of the single-piece variant in which we see the bottom side of the three individual modules, each of which is housed in its own metal enclosure. At the left is the noise generator, which is the smallest of the three modules. The largest one is the subcarrier-modulator at the centre. The rightmost module is the adjustable 290 MHz RF-unit.

At the far right are five wire terminals for connection of a microphone, an antenna and the power supply. The latter is provided via the QRR-153 switch-receiver. The modules are interconnected via thin PVC wires and two factory-selected resistors to determine the Noise/SC signal ratio.

Complete setup
The diagram below shows a complete setup of the SRS-153 system. The SRT-153 transmitter is installed at the target area (TA) at the bottom right (shown in red). It is powered by two strings of five Mallory mercury cells each, under control of the QRR-153 switch-receiver at the top right.

At the listening post (LP), which is generally located across the street from the target area, is the QRT-153 activation transmitter, which can send two carriers (one for the ON command and one for OFF) via a frequency in the 70 MHz band. It has presets for controlling up to four QRR/SRT-153 sets simultaneously. Once activated, the signal from the SRT-153 transmitter can be picked up by the SRR-153 surveillance receiver at the bottom left. The latter can also be replaced by an SRR-90 receiver which has been modified for the reception of subcarrier-modulated transmitters.

Block diagram
The SRT-153 transmitter consists of three separate circuits, each of which is built as a separate module, as shown in the diagram below. At the left is the microphone. It is connected to the sub­carrier modulator (SC), which consists of a pre-amplifier with AGC 1 and a subcarrier oscillator. It produces a frequency modulated subcarrier (FM/SC), which is fed to the RF-unit at the right.

At the bottom is a random noise generator, of which the output is also fed to the RF-unit. Both signals are frequency modulated onto the RF carrier. The noise is injected into the signal's base­band, first of all to mask its presence, but also to hide the presence of the modulated subcarrier.

  1. AGC = Automatic Gain Control.

Alien bug
The SRT-153 is actually an accurate copy of an alien bug that was discovered by the CIA in the mid 1970s in the desk of one of its Ambassadors. It was hidden inside a wooden divider of one of the drawers of his desk. The image below shows the layout of the divider, with the three modules of the transmitter highlighted in red. In this case, the three separate modules are used, but later variants have been found in which the transmitter was housed in a single metal enclosure.

It is currently unknown why the CIA copied the design of an adversary, whilst they had access far better technology from a variety of sources. A possible explanation is that they wanted to put the blame on the adversary if the bug was discovered, or that it was used as a bait to satisfy an alien sweep team, and that other bugs, of a different nature, were also present at the target area.

 Read the full story

Audio masking
To hide the RF carrier and its modulation from regular surveillance receivers, professional bugs often use a special technique that is known as audio masking. The SRT-153 uses a sophisticated masking scheme, in which a 22 kHz frequency modulated subcarrier, is frequency modulated onto the 290 MHz RF carrier, whilst at the same time injecting noise in the channel's baseband.

FM subcarrier, frequency modulated onto an RF carrier

This technique is known as subcarrier audio masking and defeats any non-compatible receiver. It is safe enough to hide the signal from an unexperienced eavesdropper and to prevent accidental demodulation in a standard receiver. A similar technique is used in the CIA's SRT-105 and in the OPEC bug, although the latter injects a strong 50 Hz hum into the baseband rather than noise.

 More about subcarrier audio masking

Receiver   SRR-153
In order to obtain a clear reproduction of the original audio, the signal from the SRT-153 should be demodulated with a compatible professional receiver that can handle subcarrier-modulated and noise masked signals, such as the SRR-153 that was designed especially for this purpose.

The image on the right shows a typical SRR-153. It was developed in 1981 and released in 1982. The SRR-153 was commonly used at the heart of a Listening Post in combination with an SRN-9 antenna. It can also demodulate other types of subarrier bugs that work in the same frequency range, such as the SRT-93 and the SRT-105.

Before the SRR-153 became available in 1982, existing SRR-90 receivers were upgraded for the reception of subcarrier bugs, by replacing three of its plug-in modules. This introduced a 10.7 MHz IF-stage and a 22 kHz SC demodulator.
SRR-153 surveillance receiver - click for more information

The first generation of the SRR-153 receiver was only suitable for the reception of the SRT-153 transmitter. A later version was also able to demodulate the 40 kHz subcarrier signal from the SRT-93 and SRT-105 bugs, but for this the position of a jumper on the audio board had to be changed. For the older SRR-90 receiver, a separate 40 kHz demodulator plug-in was produced.

 More about the SRR-153

Compatible receivers
SRR-153 surveillance receiver Improved modular surveillance receiver (vertical model) Improved modular surveillance receiver (horizontal model)

  1. Only when the appropriate subcarrier-compatible plug-in modules are installed.

Remote control
As the SRT-153 produces a continuous wave signal, it is easier to find than pulse-based bugs like the SRT-56. Furthermore, it consumes more energy as a result of which it has a reduced battery life. For these reasons it is commonly used in combination with the QRR-153 switch-receiver.

The switch-receiver controls the power supply to the transmitter, and is powered itself by a single mercury battery cell. In order to save power, it is only switched on for 23 ms every 1.5 seconds.

In the short time that it is active, it responds to two command signals: one to turn the SRT-153 ON, and one to turn it OFF again. It is estimated that the QRR-153 can run this way for one full year on a single mercury cell. The image on the right shows the QRR-153 receiver aside the some­what longer encapsulated SRT-153 trans­mitter. It is also available in a 2-piece variant.
SRT-153 and QRR-153

The SRT-153 transmitter is powered by two parallel strings of five cells each. By switching it off between sessions, e.g. during the night, precious battery power is saved. It is estimated that the transmitter can also run for one full year, when it is used for an average of eight hours each day.

Having the ability to control the bug remotely, is also very useful if the surveillance team at the listening post (LP) notices the arrival of a sweep team at the target area. In such situations, the transmitter can be switched off to reduce the chance of detection. As soon as the sweep team has left, the transmitter can be activated again.

The QRR-153 switch-receiver was controlled via a signal in the 70 MHz band, sent by the QRT-153 activation transmitter shown in the image on the right. It has a built-in telescopic antenna and has the ability control up to four targets.
QRT-153 activation transmitter

Each QRR-153 switch-receiver works on a factory-set spot frequency (channel), and responds to two factory-set command tones (one for the ON command and one for OFF). Channel frequency and command frequencies are set by means of the thumbwheels at the top left of the device.

 More about the QRR-153 switch-receiver
 More about the QRT-153 activation transmitter

Despite the fact that the SRT-153 features a professional-grade audio masking technique, bugs of this type can be discovered relatively easy with a bug detector, such as the Audiotel Delta V, or with a professional bug tracer, like the Audiotel Scanlock Mark VB shown in the image below.

When trying the Scanlock in the vicinity of an active SRT-153, it was able to lock onto its RF carrier, demodulate the subcarrier, and produce a clear audio signal, in less than a second.

Likewise, the Audiotel Delta V bug detector was able to detect the SRT-153 at a distance of 10+ metres. By using its tone indicator with signal-strength dependent pitch, we were able to find the bug in less than one minute. Nevertheless, it provides reasonable security against an average inter­cep­tor, especially because it can be turned OFF remotely whenever a sweep team is spotted.
Scanlock Mark VB in operation

But despite the fact that the bug can be switched OFF remotely, it is no match for a so-called non-linear junction detector (NLJD), which is able to find nearly any piece of electronic circuitry, even if it is switched off. As the two-stage RF-unit is coupled directly to the antenna, it is vulnerable to the NLJD. Other CIA bugs, like the SRT-107 had special anti-NLJD provisions.

 More about the Scanlock bug tracer
 Example of an NLJD

Because of the wide variety of subcarrier-modulated bugs used by the CIA, produced by different contractors, there were sometimes difficulties when trying to receive their signals on an SRR-153 or the earlier SRR-90 receivers. In many cases it was unclear whether this was caused by the bug or by the receiver.

Especially for this purpose, the NRP developed the UVK-153 transmitter tester, which is able to check every aspect of a bug.

 More information

UVK-153 transmitter tester

The SRT-153 transmitter consists of three different circuits, each of which is built on a separate printed circuit board (PCB) and housed in a separate enclosure. The three modules hold the RF-unit, the subcarrier modulator and the noise source, and are interconnected by thin white wires.

The image on the right shows the individual circuit boards mounted on a test panel. It was used at the laboratory as a reference model. The PCBs are a close copy of the alien original, both in the choice of the components and in choice of production methods. Even the engraving of the serial numbers, the numbering scheme and the markings of the terminals, was nearly identical.

All three PCBs are single-sided. As can be seen in the close-ups below, chip capacitors are used, as well as SMD transistors, but the resistors are conventional subminiature types with wires.
Transmitter test board

Again, this was done to copy every aspect of the discovered alien bug. Although some circuits were improved by the NRP, such as the operational temperature range, this was done in a nearly invisible manner, so that even the originating agency would not be able to identify it as a copy.

In the actual production version, each of the three units is housed in a separate metal case that is filled with KF-400 silicone paste [2]. When the units were finished and adjusted, they were soldered onto a temporary PCB, with wires for power, microphone and antenna. This was done to allow the CIA to do an acceptance test, and to soak test the units prior to deployment.

The image on the right shows a production board aside the laboratory model. Each of the modules has a number of long connection wires, that will later be shortened to the desired length.
 Packaged and bare transmitter, both mounted on a test panel

When built inside a wooden desk drawer divider, as in the original example, the three individual modules were glued to the bottom of the concealment by means of thin pieces of double-sided adhesive phenolic. The modules were then interconnected by means of thin white PVC wiring.

With later versions of the original desk drawer divider bug, the three modules were housed inside a single long metal enclosure, with five wire terminals at one of the short sides. In that case, the modules were fitted to the bottom of the enclosure, which was then filled with a thick white silicone paste, and soldered hermetically.

The image on the right shows the single-piece version of the SRT-153, both open (rear) and closed. Removing the paste is extremely difficult and is likely to damage the components, as the CIA found out when analysing the original bug.
SRT-153 open and closed

Like the 3-piece variant, the single-piece encapsulated version was mounted onto a temporary PCB prior to delivery to the CIA, to allow acceptance and burn-in testing. Mounting the devices inside a concealment was not the NRP's responsibility, but was done elsewhere, probably by the CIA, or by another contractor. Is is currently unknown how and where the bugs were deployed.

Transmitter test board RF unit (transmitter) Modulator Noise generator  Packaged and bare transmitter, both mounted on a test panel Test board with 3-piece variant of QRR-153 Noise generator Modulator (top) and RF unit (bottom)
Transmitter mounted in a single enclosure SRT-153 open and closed Close-up of the silicone paste inside the SRT-153 SRT-153 interior (cleaned) SRT-153 terminals Connection between modulator and RF-unit Connection between noise generator and modulator SRT-153 compared to the size of a hand
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Transmitter test board
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RF unit (transmitter)
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Noise generator
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 Packaged and bare transmitter, both mounted on a test panel
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Test board with 3-piece variant of QRR-153
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Noise generator
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Modulator (top) and RF unit (bottom)
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Transmitter mounted in a single enclosure
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SRT-153 open and closed
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Close-up of the silicone paste inside the SRT-153
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SRT-153 interior (cleaned)
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SRT-153 terminals
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Connection between modulator and RF-unit
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Connection between noise generator and modulator
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SRT-153 compared to the size of a hand

The diagram below specifies the wiring of the five contact terminals of the SRT-153 transmitter. Note that pins 2 and 4 are internally connected to ground and that the device is powered by a negative voltage (i.e. plus connected to ground), whilst the QRR-153 receiver is powered by a positive voltage and has the negative connected to ground. Power is between 2.7V and 8.1V.

  1. Microphone (signal)
  2. Microphone (ground)
  3. Power supply (-V)
  4. Ground (+V)
  5. Antenna
The diagram below shows how the SRT-153 is wired to the QRR-153 switch-receiver. The negative terminal of the battery array is directly connected to pin 3 of the transmitter, whilst the positive terminal is switched by the receiver. The receiver itself is powered by a separate 1.35V mercury battery. It has a built-in energy saver that switches it on for just 23 ms every 1.5 sec.

Note that transmitter and receiver each have their own antenna, which more or less resembles a dipole. One half of the dipole is formed by the antenna wires, whilst the ground wiring acts as the other half. In the orignal desk drawer divider (in which the bug was first found), the ground wires are clearly visible just above the battery array. Antennas of this kind are by no means ideal.

  • Frequency
    260 - 320 MHz (with a channel spacing of 10 MHz)
  • Subcarrier
    22 kHz ± 1.5 kHz (-10°C and +50°C)
  • Power
    2.2 - 4.5V (typically: 2.7V), max. 6V
  • Current
    10 mA at 2.7V
  • Output
    5 mW at 2.7V, 2.5 mW at 2.2V
  • Link budget
    > 104 dB (when using SRR-90 receiver)
  • Temperature
    3 MHz drift between -10°C and +50°C
  • Storage
    -20°C and +70°C
  1. XSRT/XQRR-53 Operating Notes
    NRP, October 1977. CM302627/A.

  2. Proposal for Prototype SRS/QRS-53
    NRP, November 1977. CM302627/B.

  3. Environmental Test Report on XSRT-53 Transmitter
    NRP, August 1979. CM302627/F.

  4. Environmental Test Report on XQRR-53 Receiver
    NRP, August 1979. CM302627/G.

  5. Operation and Test Manual for SRT-153 & QRR-153
    NRP, April 1980. CM302627/H.

  6. Operation and Test Manual for SRT-153 & QRR-153
    NRP, May 1980. CM302627/I.

  7. Operation and Test Manual for SRT-153 & QRR-153 (draft)
    NRP, September 1980. CM302627/J.

  8. Operation and Test Manual for SRT-153 & QRR-153
    NRP, September 1981. CM302627/N.
  1. NRP/CIA, Collection of documents related to SRS-153
    Crypto Museum Archive, CM302627 (see above).
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Crypto Museum. Created: Thursday 18 May 2017. Last changed: Tuesday, 16 January 2018 - 15:54 CET.
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