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Sinitsa (USSR)   Синица
Body-worn intercept receiver

Sinitsa (Russian: Синица) is a Russian body-wearable intercept receiver that was used during the Cold War to track down, locate and intercept enemy communication in close proximity of the transmitter. It was developed around 1980 and is probably the most advanced portable intercept receiver developed in the USSR, covering all frequencies between 30 MHz and 1 GHz. Like most secret Russian equipment, it was named after a bird. Sinitsa (Синица) is the Russian word for tit.
Sinitsa is a non-selective (aperiodic) receiver, which receives an entire frequency band at once. As a result, the receiver is far less sensitive than a tuned receiver like the earlier Filin, but since it is used for reception of transmitters in close proximity (i.e. strong signals), this was not considered a problem. It has the nice side-effect that broadcast stations are less of a problem.

The image on the right shows a typical Sinitisa receiver with its fixed remote control unit. The unit shown here was used by the Stasi (Secret Police) of the former DDR (East-Germany).
Sinitsa with remote control unit

Sinitsa was usually supplied with many accessories, packed inside a domestic suitcase (see below). Like most other Russian intercept receivers its case is curved, so that it can be worn on the body of the operator. In order to cope with the enormous frequency range of the receiver, Sinitsa uses a system of plug-in frequency modules, 13 of which are supplied in the suitcase.

For inconspicious surveillance, Sinitsa could be hidden under the operator's clothing. A special cloth 'harness' allowed the receiver, the battery and a vibrator to be carried close to the chest. It was used in combination with a special body-worn antenna that was also concealed in cloth.
Sinitsa with remote control unit Sinitsa with remote control unit Sinitsa receiver Sinitsa top view Sinitsa bottom view Sinitsa bottom view with remote control unit on top Open suitcase with Sinitsa 'Agent' wearing Sinitsa at the exhibition 'Secret Messages' in 2008

Sinitsa was usually delivered in an unobtrusive cheap-looking suitcase, together with 13 plug-in units, battery packs, a battery charger, a telephone interface, a vibrator, an attenuator, a speaker, a harness (for carrying the receiver on the chest) and various antennas and spare parts.
The image on the right shows the contents of the suitcase. All items are neatly packed inside a pre-shaped polystyrene mold. The polystyrene mold was clearly made for a smaller suitcase and four polystyrene blocks are used to keep it in place. Inside the top lid is a checklist with a map showing the location of each item.

The cloth harness and antenna are usually packed on top of the other items. Two leather straps with buckles are used to keep the contents of the case from moving during transport. The case can be locked if necessary.
Sinitsa with plug-ins and accessories inside the suitcase

Sinitsa itself is stored at the left, with one plug-in already installed. 10 plug-ins are stored to its right; at the center of the case. Two more plug-ins are at the rear. To the right of the plug-ins are the accessories: the speaker, the vibrator and the batteries. At the far right are the battery charger and the telephone interface. The latter was used for unmanned surveillance. Under the receiver is another compartment where the remote control unit and the spare parts are located.
Sinitsa suitcase Sinitsa with plug-ins and accessories inside the suitcase Contents of the suitcase Top lid of the suitcase Checklist Releasing the leather straps Taking Sinitsa out of the suitcase Storage space below Sinitsa

When trying to locate a clandestine transmitter, the operator would select the appropriate frequency plug-in, turn on the device and select maximum sensitivity (i.e. maximum gain). When he picked up the transmitter's signal he had two options for detecting the field strength:
  1. Acoustic indicator
    A switch on top of the plug-in allows the operator to select between listening to the conversation (СИГН., signal) or a continuous 2 kHz tone (ПРОСЛ., trace). The stronger the signal, the louder the tone.

  2. Vibrator
    An external vibrator could be connected to the main unit. It was fitted in a pocket of the harness (waist belt) in such a way that it pressed against the operator's side. The stronger the signal, the harder the vibrator would vibrate. The vibrator could be disabled with a switch on the remote control unit.
Next, he would turn down the gain until the point where the signal is barely heard. He would then start walking in the direction where the signal becomes stronger. Once the signal was too strong, he would turn down the gain and repeat the above procedure. Eventually, with the gain set to its minimum, he would literally be on the transmitter's doorstep. In case we has dealing with a very strong transmitter, an attenuator can be connected between the antenna and the receiver.
Location of the items on the operator's body

Although Sinitsa was primarily intended for tracking down clandestine (enemy) radio stations, the receiver was conceived in such a way, that it was suitable for other applications as well.
  • Tracking and tracing
    The main application was tracking and tracing of clandestine signals, such as the 2-way radio communication between foreign (enemy) agents under surveillance. The trace-tone and/or the vibrator where then used as signal strength indicators.

  • Intercepting agent communication
    Sinitsa could also be used for intercepting (tapping) conversations between agents under surveillance. In such situations, the vibrator would not be used and the receiver would be set to maximum gain.

  • Unmanned interception
    In cases were human surveillance was not possible, the intercept operator could set up a fixed reception point (e.g. in a hotel room) and replay the intercepted conversation via a telephone line (bugging), using the supplied telephone interface.

Sinitsa basically consists of three parts. At the center is the plug-in unit which is the actual receiver. The static part at the right contains the power supply unit and the LF amplifier. The part at the left contains the vibrator driver. The plug-in unit is slotted-in from the top.

The topmost drawing above shows the controls of the main unit and one of the plug-in units. The connections of the main unit are explained below. The controls of the plug-in unit are also explained below. The plug-in unit is kept in place by two locking bolts at the bottom of the unit.
At the bottom is a fixed cable that leads to the remote control unit. All other connections are at the top, together with the controls. The following connections are available:
  • Battery
    The cable on the left is fixed and cannot be removed. At the end of the cable is a connector that fits to the battery pack. Note: as you can see in the photographs, our receiver has the wrong type of connector at the end of the battery cable. In this case it is a BNC-type connector, rather than the required Russian battery connector. This is clearly a DDR-modification. It might have been used in combination with a converter cable, but it is also possible that the Germans used an alternative battery pack instead.

  • Vibrator (БШВ)
    Next to the battery lead is the connector for the vibrator. Although the receiver can be used without the vibrator, it's a quite useful accessory as it allows the receiver to be used for inconspicuous surveillance. The vibrator has only a short lead and should be carried in a special pocket of the carrying belt, pressing firmly against the body of the operator.

  • Speaker (ТЛФ)
    This circular connector is used to connect the small loudspeaker. Surprisingly, this is the same connector as for the battery pack, which might be confusing. Be careful not to connect the speaker to the battery pack, or the battery charger to the speaker socket, as this may result in permanent damage.

  • Recorder (МАГН.)
    A special 8-way extension connector is available for connecting an (optional) recorder. Pins 4 and 8 of this connector are unused. The audio output is available on pins 2 (signal) and 1 (ground). The purpose of the remaining 4 contacts is currently unknown. A special break-out cable with 3 flying leads is available for this connection. It might have been used with a small tape or wire recorder.

Battery pack Vibrator Expansion cable Expansion cable Expansion cable Using the recorder connection Expansion cable connected to the recorder connector

Remote Control Unit
The Remote Control Unit (RCU) is permanently attached to the Main Unit (receiver) and cannot be unplugged. When the receiver is worn on the body, the RCU should be carried in the right hand, with the cable running throught the right sleeve of the operator's coat.

The volume control is at the top of the RCU. It is a rotary switch with 8 pre-set volume levels. Volume should be set as low as possible, so that the operator can just hear the intercepted signal, or the tone. The volume knob also controls the level of the recording output.

The topmost control on the front of the RCU is a thumbwheel switch with three possible settings. According to the russian text is might be related to recording. To date, the exact function of this switch is unknown. The device only operates in the two leftmost settings.

Below the thumbwheel is the ON/OFF switch for the vibrator. When set to the left, the vibrator is ON (БШВ), whilst in the rightmost position it is OFF (ВЫКЛ.). The lowest switch is the main ON/OFF switch. When set to the left, the receiver is turned ON (ВКЛ). In the rightmost position (shown in the drawing) it is OFF (ВЫКЛ.).
Using the remote control Remote control unit

Sinita comes with 13 frequency modules or plug-in units, each with its own frequency range. All modules have an antenna socket at the left and two switches at the right. The leftmost switch is a rotary switch that is used to select the required sensitivity. This rotary switch is missing from plug-in units 1 and 2. The rightmost switch is used to select between the audible signal (СИГН.) and the vibrator (ПРОСЛ.). A full list of plug-ins is given below:
  1. 30-800 MHz
  2. 100-1000 MHz
  3. 100-198 MHz
  4. 205-300 MHz
  5. 300-400 Mhz
  6. 400-500 MHz
  7. 100-130 MHz
  8. 130-160 MHz
  9. 160-185 MHz
  10. 205-255 MHz
  11. 240-270 MHz
  12. 270-300 MHz
  13. 300-330 MHz
Plug-in top view
The advantage of using wide-band reception with a non-selective receiver is that all frequencies within the selected range are monitored at once. This means that two-way communication with split-frequency or even frequency-hopping can easily be intercepted without advanced (computerized) methods.

A disadvantage of this method is that all radio signals in the selected range are heard, including unwanted signals, such as broadcast stations. The latter is less of a problem here, as non-selective receivers are rather insensitive. That said, when testing our Sinitsa with the 100-1000 MHz plug-in (#2), the many GSM base stations around us, caused the vibrator to be permanently activated. In fact non-selective receivers are hardly usable today, because of the many radio signals, interferences and electro-smog surrounding us.
Plug-in top view Plug-in bottom view Plug-in top view Plug-in unit, bottom view Sinitsa front view Setting the gain on the plug-in Listening to the signal Frequency modules (plug-ins)

A small loudspeaker is used with Sinitsa as headphones. Rather than using an earpiece, this speaker should be attached to the operator's clothing by means of a domestic safety pin that is fitted at the back. These speakers are commonly used with Russian equipment of the era.
The speaker has a diameter of approx. 3 cm and should be attached close to the operator's ear (e.g. under the collar of a coat). Volume should be adjusted so that the sound is audible by the operator, but not by any bystander.

The cable is about 70 cm long and has a rather strange (rare) plug at the end. It connects to the socket marked ТЛФ (telephone) on the right side of the top panel. A separate connector, marked МАГН. (recorder), is available for connecting a recording device.

The speaker, and hence the connector, are often missing from surplus equipment found today. As they are extremely hard to find, the drawing below might help when finding an alternative. The same connector was used with the Filin intercept receiver and the UFT-421 covert radio.

The rather strange loudspeaker plug

The speaker is relatively high-impedant (approx. 2000 ohm) and produces a loud signal. If the speaker appears to be dead, it can easily be checked with a multi-meter. The (DC) resistance should be approx. 200 ohm. In many cases, the wires inside the connector are broken.
Speaker Speaker Safety pin at the back of the speaker Speaker Connecting the speaker to Sinitsa Expansion cable connected to the recorder connector

Sinitsa has two methods for determining the strength of a signal. It can use (1) a continuous 2 kHz tone, which becomes louder when the signal gets stronger, or (2) use an external vibrator that is connected to the 4-pin socket on the top left of the main unit.
The vibrator can be used instead of a visual or acoustic signal strength indicator and is ideal for inconspicious surveillance, as it can not be heared by any bystanders. When the received signal is strong enough, the vibrator is activated.

The level of vibration is a measure for the signal strength. The stronger the signal, the harder the vibrator will vibrate. The image on the right shows a typical vibrator. It is an unmarked unit of approx 31 x 44 mm that is supposed to be carried in a special pocket on the inside of the chest belt, pressing firmly against the body.

The vibrator can be used under all circumstances, regardless the setting of the mode-switch on top of the plug-in. When the switch is set to ПРОСЛ (trace) a 2 kHz audible signal is injected directly at the antenna input. After detection, this tone is heared through the speaker, regardless of whether the signal is silent or not (i.e. when there is no modulation). This signal is also used to drive the vibrator. It will vibrate harder when the tone gets louder.

Setting the switch to СИГН (signal) selects a 28 kHz tone to be injected. As this signal is not human-audible, it does not interfere with the conversion, but still allows the vibrator to be driven. The principle of tone-injection is further described below. The vibrator can be disabled with a switch on the remote control unit.
Vibrator Vibrator Vibrator connected to Sinitsa Vibrator connected to Sinitsa Using the trace-tone Vibrator connected to Sinitsa Installing the vibrator in the special pocket of the belt The vibrator in its pocket on the inside of the belt

HF Attenuator
A small external HF attenuator is supplied, which can be used in close proximity of strong HF signals. The unit measures only 60 x 35 mm and is connected to the antenna input of a plug-in unit (ВХ) by means of a short coaxial cable (supplied) of approx. 105 mm length.

The attenuator is shown in the image on the right. It can be fixed to the operator's clothes by means of two domestic safety pins that are fitted to its back.
Attenuator with cable

Attenuator with cable Rear view of attenuator. Note the safety pins. Attenuator with cable

Battery pack
Two or four battery packs are usually supplied with Sinitsa. The battery pack has a rather strange shape and contains 10 NiCd cells of 1.2V each, giving a total of 12V. At one of the corners is an extension with a fuse holder and the output socket.

The image on the right shows a typical Sinitsa battery pack. As the cells are rather old, there is a serious danger that they will start leaking. In such cases they should be replaced immediately.
Battery pack

Battery pack Battery pack Battery pack Close-up of the connector on the battery pack Interior of the battery pack

Battery charger
In order to charge the NiCd batteries, a battery charger is supplied. The battery charger is a rather simple device that connects directly to the 220V mains. A fixed wire with a circular connector at the end can be connected directly to the battery.

Note that the battery connector is identical to the connector used for the speaker. This is rather confusing and care should be taken to avoid the battery charger being connected to the audio output of the receiver. Doing so, might cause damage to the main unit.
Battery charger

Battery charger Battery charger

Telephone interface
Although Sinitsa was intended for portable use, it could also be used in a stationary unmanned surveillance operation. In such situations, the receiver could be left alone in a certain location (e.g. a hotel room), close to the clandestine transmitter, relaying the conversation by telephone.
For this purpose, the special telephone line interface shown on the right was developed. It is a small box with an extremely simple but clever circuit inside. The interface serves two purposes: (1) it feeds the receiver by drawing power from the telephone line and (2) it links the audio output from the receiver to the telephone line.

The telephone line is connected on the left (ЛИНИЯ). The fixed wire (top left) is connected to the speaker socket on the receiver (ТЛФ). The power cable from the receiver is connected to the socket at the bottom left (ПИТ).
Telephone line interface

Once the operator had setup Sinitsa this way, he would dial the number of the intercept station and leave the line off-hook. It is also possible that special (fixed) telephone lines were used for the intercept operation. In such cases the line would be connected directly to the intercept station and no number had to be dialled first. The circuit diagram of the interface is shown below:

Telephone line interface Telephone line interface Telefone interface interior Close-up of the interior of the telephone line interface

Spare parts
Sinitsa was delivered with a set of spares, so that a skilled technician could repair it 'in the field'. The spare parts kit contained switches, fuses, transistors and connectors. Difficult circuits, like the detection generator and the signal injector were supplied as pre-assembled units.

The spare parts are stored in an empty space under the main unit, together with the remote control unit.
Spare tone generator

Spare tone generator Spare tone generator Spare mixer circuit (injector) Spare mixer circuit (injector) Spare transistor Spare parts and additional antennas Spare parts and additional antennas

A white cloth 'harness' was used to conceal the receiver and the battery under the operator's clothing. The image on the right shows the harness. It has pockets for the main unit (receiver), the battery and the vibrator. An extra pocket is available for an alternative plug-in, the attenuator or a spare battery.

The harness is fitted to the body by means of elastic band that can be adjusted with buckles to fit tightly around the operator's waist.
Body harness with Sinitsa and accessories

Body harness Body harness Contents of the suitcase Body harness with Sinitsa and accessories 'Agent' wearing Sinitsa at the exhibition 'Secret Messages' in 2008

A special flat antenna was developed for Sinitsa. Like the harness, it was worn on the body. It consisted of a metal grid, packed in a large padded cloth bag, that was carried on the chest, just above the receiver-belt. A short coaxial cable, that was fixed to the antenna, connected the antenna to the receiver below.

The two buttons at the top, give access to the actual antenna inside the bag. The antenna is further explained below.
Body-worn antenna

Body-worn antenna Body-worn antenna Body-worn antenna Contents of the suitcase 'Agent' wearing Sinitsa at the exhibition 'Secret Messages' in 2008

The antenna consists of a rectangular metal grid (like a mesh) with a long narrow cut-out at the center (see below). The shield of the coaxial cable is connected to the lower half, whilst the core (hot side) is connected to the upper half. This way, the antenna acts like a circular dipole.

The drawing below shows the construction of the antenna. At the heart is the metal mesh grid with the cut-out at the center. The cable is connected at the center, where it is strengthened with a small pertinax panel. The contruction around the horizonal gap is further strengthened by a piece of sturdy cloth that is glued to the back and a thick cloth rig at the front.

In order to make the antenna a bit more comfortable to the body, a thin layer of foam (polyether) was added at the back. The entire assembly was then packed into a large cloth bag, with sholder straps and elastic bands around the waist. As the polyether foam had completely desintegrated after all these years, we had to remove the protective foam from the antenna bag.

The image above gives an impression of how the antenna was worn on the chest of the operator. The bag at the front contains the actual antenna. It is closed by means of two small butonned straps. The shoulder straps keep it in place vertically, whilst the buckles at the rear are adjusted to make it fit tightly around the chest.
Technical description
The principle of the receiver is quite simple. The block diagram below shows the various functional blocks. The leftmost part shows a typical plug-in unit. The signal of a 2kHz/28kHz square wave generator is mixed directly with the incoming antenna signal. Next it is passed through a band-pass filter, so that only frequencies in the desired range are allowed.

At the output of the filter is a three stage HF amplifier with adjustable gain, followed by a diode detector. The gain of the HF amplifier can be set in 5 different steps by a rotary knob on the front panel of the plug-in unit (except for plug-in 1 and 2 which have no amplifier).

The rightmost part of the block diagram shows the components of the main unit. The LF output from the diode detector is fed into a high-gain pre-amplifier with Automatich Gain Control (AGC) and is finally amplified to speaker level. The audio level of the speaker and the recording output, is controlled by a 8-stage rotatable selector on the remote control unit. The output is also fed into a level detector that eventually drives an external vibrator.

The entire unit is powered by a single 12V battery pack, consisting of 10 NiCd cells of 1.2V each. Most of the electronics in the Sinitsa are directly driven from the raw 12V, but a stabilized voltage of 6V is provided for the HF amplifier.
Sinitsa is built inside a sturdy die-cast aluminium frame that is closed with aluminium panels at the rear. The panels of the main unit and the plug-in are removed by releasing the small bolts at the edges. This exposes the solder side of the PCBs in the left and right 'wings'.
The image on the right shows the interor of both the main unit and plug-in number 6 (400-500 MHz). The brown PCB on the left is the power supply and the LF amplifier. The power supply converts the 12V input into a stabilized 6V for the HF amplifier, plus various other voltages (P).

The PCB at the far right contains the vibrator driver. It takes the detected LF signal and uses it to drive a sine-wave generator that produces a rather low frequency that is fed directly to the vibrator. The stronger the intercepted signal, the harder the vibrator will vibrate (V).
Sinitsa interior

The unit at the center is the plug-in unit that contains the actual receiver. The one shown in the image above is the plug-in for the frequency range 400-500 MHz (#6). It consists of three functional parts: (1) a band-pass filter (the square block at the top), (2) a signal injector that produces the detection signal (top left) and (3) a 3-stage pre-amplifier with detector.

Internal diagram of the 218GG1 integrated circuit

The detection generator produces a square wave signal of 2 kHz (audible) or 28 kHz (non-audible). The circuit is based on a Russian 218GG1 (218ГГ1) integrated circuit (IC), which is mounted at the bottom of the smallest PCB. The drawing above shows the contents of the IC. The frequency is determined by two external capacitors (selectable by a switch on the top panel).

Mixer circuit (injector)

The signal from the LF square wave generator is fed directly to the input of the receiver by mixing it with the antenna input, by means of the very simple mixer shown above. The circuit is located between the antenna socket and the input of the band-pass filter. In most cases it is a small PCB that is mounted directly to the back of the antenna socket, but in some plug-in units it is part of the band-pass filter. To understand how it works, we need to look at the frequency spectrum:

Frequency spectrum when injecting an LF signal

When receiving a relatively strong signal (f), the injected LF signal is directly mixed with the HF signal, producing a number of sidebands, of which f+2kHz and f-2kHz are the strongest. As this is now an Amplitude Modulated signal (AM), it can be detected by the diode detector, regardless the original modulation type of the signal (AM, FM, PM, etc.).
Sinitsa interior Sinitsa interior Plug-in interior Detection signal generator HF pre-amplifier Remote control unit interior Power supply and LF amplifier Integrated circuit used in the detection generator

Inside the top lid of the suitcase is a checklist with all items and their location [2]. Please note that, although our Sinitsa is one of the most complete objects we've ever found, the list does not completely match with the items inside the suitcase. Over time, some parts may have been discarded and other items may have been added without altering the checklist.

Sinitsa suitcase layout

The drawing above shows the layout of the suitcase as it is drawn on the map inside the top lid of the suitcase. The list below is a free translation of the checklist that is printed alongside the map. The location of each item is given in brackets at the end of each line. This number corresponds with the drawing above.
  1. Прибор 1АА3-1: Flat antenna (12)
  2. Прибор 1АА3-2: 2 x Unknown (11)
  3. Кабель: Short coaxial cable for attenuator (11)
  4. Кабель: 2 x Unknown cable, probably telephone lead (11)
  5. Вибратор: Vibrator (8)
  6. Блок 1АБ4-1: Plug-in 30-800 MHz (1)
  7. Блок 1АБ4-2: Plug-in 100-1000 MHz (3)
  8. Блок 1АБ4-3: Plug-in 100-185 MHz (4)
  9. Блок 1АБ4-4: Plug-in 205-300 MHz (4)
  10. Блок 1АБ4-5: Plug-in 300-400 MHz (4)
  11. Блок 1АБ4-6: Plug-in 400-500 MHz (4)
  12. Блок 1АБ4-7: Plug-in 100-130 MHz (4)
  13. Блок 1АБ4-8: Plug-in 130-160 MHz (4)
  14. Блок 1АБ4-9: Plug-in 160-185 MHz (4)
  15. Блок 1АБ4-10: Plug-in 205-255 MHz (4)
  16. Блок 1АБ4-11: Plug-in 240-270 MHz (4)
  17. Блок 1АБ4-12: Plug-in 270-300 MHz (4)
  18. Блок 1АБ4-13: Plug-in 300-330 MHz (2)
  19. Блок 1АБ3: Main Unit (4)
  20. Прибор 1АБ1: Unknown device 1AB1 (5)
  21. Блок 1АБ2: Attenuator (9)
  22. Прибор 1АУ3: Telephone Line Interface (11)
  23. Прибор 1АУ2: Battery charger (10)
  24. Прибор 1АУ1: 4 x Battery Pack (6)(7)
  25. Телефон: Speaker (8)
  26. Ремень: Belt (2)
  27. Ремень: Unknown belt (2)
  28. Коробка ЗИП: Box with spare parts (2)
  29. Отвертка 160 х 0.5: Screwdriver (2)
  30. Ключ торцовый 8 х 200: Open-ended spanner (2)
  1. Paul Reuvers and Marc Simons, Sinitsa, Covert Intercept DF Receiver
    First published 2007. Preliminary release, 18 May 2008.

  2. Sinitsa Checklist, Located in the top lid of the suitcase
    Crypto Museum. Translated June 2007.

  3. Louis Meulstee, USSR Portable Intercept Receivers
    Wireless for the Warrier. Volume 4. September 2004. ISBN 0952063-36-0.
    The unit featured in this book is now part of the Crypto Museum collection.

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