|
|
|
|
|
|
The device measures 19.7 x 13.3 x 2.7 mm
– about the size of a penny – and should therefore be considered
subminiature for the era. It is cast in epoxy resin,
and weights just 1.4 grams. The image on the right was taken from the manual.
It operates by frequency modulating (FM) the intercepted audio
onto a 40 kHz subcarrier (SC), which is then frequency modulated onto
the 350 MHz carrier. As an extra secrecy measure, a strong (random)
noise signal,
is injected into the device's baseband audio channel, which defeats
the squelch of an ordinary surveillance receiver.
|
|
|
Compared to other bugs of the era, such as the SRT-91
— which was developed by the NRP
in the same year —
the SRT-105 is extremely small and much easier to conceal.
Nevertheless, it is a continuous wave (CW)
transmitter, which was easily detected with the available
TSCM equipment.
Although its noise-injected subcarrier modulation masking scheme was very
sophisticated, it could be detected and demodulated within seconds by
Audiotel's Scanlock Mark VB bug tracer.
|
 |
The diagram below is taken from the original instruction manual, and
shows the layout of the device [A]. The body of the transmitter, which is
cast in epoxy resin, measures just 19.7 x 13.3 mm, with the top of the
Knowles electret microphone
extending from the upper side. At the right is a cut-out with two
terminals, to which the 1.2 to 3V battery voltage should be supplied.
At the top-left is a recessed socket to which the antenna (wire) should
be connected. The wire should be approx. 20 cm long.
The frequency of the continuous wave (CW) FM-carrier can be
adjusted between 350 and 385 MHz with a small screw at the bottom left.
An adhesive aluminium label, fitted at the flat side of the device's body,
shows the model and serial number. This label was usually removed from the
device, prior to installation in the target area.
|
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-105 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.
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-153
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
|
The SRT-105 was supplied with a tiny instruction manual that is shown
in the image on the right. It measures approx. 105 x 85 mm (roughly DIN A7 format)
and the pages are held together with a plastic ring.
It was made in October 1974.
The manual contains operating and installation instructions, various measurement
charts, the technical specifications and the circuit diagram.
➤ Download manual
|
|
|
Despite the fact that the SRT-105 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 on the right.
➤ More information
|
|
|
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
|
|
|
Below is the circuit diagram of the SRT-105 as it is presented in figure 4-1
of the instruction manual [A]. At the top right is a single-stage oscillator,
which forms the actual transmitter. It is coupled directly to the antenna.
The circuit at the top left is the audio masking noise generator.
The signal from the noise generator is amplified and fed directly
to the oscillator/transmitter.
The bottom half of the circuit diagram comprises the
Knowles BL-1681
electret microphone (at the center), a two-stage audio amplifier (Q1 and Q2)
and the 40 kHz subcarrier oscillator (Q3 and Q4 at the right),
the output of which is also fed to the oscillator transmitter at the top right.
The circuit around Q7, Q8 and Q9 at the bottom left, is a voltage
stabilizer for the subcarrier stage.
|
Unfortunately, we have only found the instruction manual of the SRT-105,
but not the actual device itself. Crypto Museum are currently looking for an
original SRT-105 transmitter for its collection. It you have such a transmitter,
or if you can provide other information about this device,
please contact us.
|
Frequency 350 to 385 MHz (tunable) Subcarrier 40 kHz ± 3kHz (0 - 55°C) Modulation FM/FM Masking Random noise Voltage 1.2 to 3V DC Current 2.5 mA to 5.8 mA (1.35V to 2.7V) Power 0.4 mW to 2.9 mW (at 25°C) Dimensions 13.3 x 19.7 x 2.7 mm (0.525 x 0.775 x 0.105 inches) Weight 1.4 grams Temperature 0 to 55°C (-20 to +71°C with reduced performance)
|
- Anonymous source, SRT-105 instruction manual
Retrieved March 2017.
|
|
|
|
Any links shown in red are currently unavailable.
If you like the information on this website, why not make a donation?
© Crypto Museum. Created: Thursday 09 March 2017. Last changed: Tuesday, 12 July 2022 - 09:24 CET.
|
|
|
|
|
