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Pulsed Cavity
Resonant cavity microphone

The EC Cavity was a resonant cavity microphone that was used as a covert listening device (bug), developed and manufactured between 1954 and 1965 by the Dutch Radar Laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of a long-term research contract under the name Easy Chair. The device was modelled after a similar Russian device that was known as The Thing.
The Russian device, that was also known as The Great Seal Bug, was discovered in the study of the US Ambassador in Moscow (Russia) in 1952, after it had been operational for nearly 7 years.

After its discovery, the CIA started a top secret research project under the name Easy Chair (EC), with the aim to develop similar devices based on its technology. The research was carried out in the Netherlands at the Dutch Radar Laboratory (NRP) in Noordwijk. Two prototypes are shown here: one for 1100 MHz and one for 360 MHz.

Initially, the NRP was unable to duplicate the Russian cavity microphone with a certain degree of reproducibility, but they were able to come up with some clever alternatives which were eagerly adopted by the CIA. The first spin-off from the secret research was the Easy Chair Mark I (EC I).
Like the Russian device, the EC I did not need local power but was instead powered by the energy from a strong radio frequency (RF) signal, beamed at it from a nearby listening post (LP).

Unlike the Russian device however, it contained electronic components. Between 1954 and 1964, no less than five generations of EC devices were produced by the NRP. They became known as Easy Chair Mark I (EC I) thru Easy Chair Mark V (EC V), and were successfully used by the CIA against a number of targets, including the the Russian Embassy in The Hague (Netherlands).
Three versions of the PE

In between the other developments, the NRP kept working on the cavity microphone and built several protypes, initially for use on 360 MHz and later also for 1100 MHz. Finally, in 1965, they had grasped the concept of The Thing and were able to operate it reliably, using radar pulses. 1
The image on the right shows one of the original prototypes for use on 360 MHz. Although this frequency was probably the most suitable one for a bugging device in terms of performance, range and path attenuation, it was less suitable for a device like this in terms of dimensions.

The cylinder has an outer diameter of 6 cm and is 6.5 cm long, and weights nearly 600 grams. It is shown here without the aluminium membrane, or diafragm. The screw at the bottom is used for fine tuning of the frequency. At the rear side is a BNC socket for connection of an antenna rod.
Resonant cavity microphone for 360 MHz

Although the NRP was able to demonstrate a working cavity microphone in 1965, which was the goal of the initial Easy Chair research contract, the CIA had meanwhile lost interest in Resonant Cavities and Passive Elements (PEs), mainly because both the Russians and the Americans had been complaining about excessively strong RF signals beamed at them by the other party.

As a result, the NRP's resonant cavity microphone was not developed further and apart from a few prototypes, no further devices of this kind were built. In order to satify the customer (CIA), they concentrated on the development of Active Target Elements (ATEs) with audio-masking facilities.
  1. Note that unlike the pulsed cavity resonator featured here, the original Russian device did probably not use radar pulses. Instead it worked by separating the small amount of energy reflected by the cavity, from the strong activation signal, in a similar way to the first Easy Chair devices.

Resonant cavity microphone for 360 MHz Frequency adjustment Close-up of the mushroom-shaped capacitor Rear view
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Resonant cavity microphone for 360 MHz
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Frequency adjustment
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Close-up of the mushroom-shaped capacitor
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Rear view

The diagram below shows the 360 MHz version of the NRP/CIA resonant cavity microphone, as seen from the front (left) and rear (right). The main tuning is done by adjusting the mushroom-shaped disc at the centre to be as close to the membrane as possible. It is then soldered in place. After that, the exact frequency can be fine-tuned with the smaller knob at the side. Once that is done, the frequency can be fixed with the larger knob, which effectively secures the smaller one.

At the front side centre is a circular opening through which the head of a mushroom-shaped tuning post is visible. Together with the membrane (not shown here), this disc forms a condenser microphone. The grooves in the disc are for the reduction of pneumatic damping, also known as the cushion-effect. For a proper understanding of the working principle behind resonant cavity microphones, we have to take a look at the original Russian device that was discovered in 1952:

Although there are some differences between the two designs, such as an inductive coupling of the antenna instead of a capacitive one, the operating principle is largely identical. It is important to realize that the frequency of the activiation beam is the same as that of the reflected signal.

 Full description of the Russian device

The diagram below shows a cross-section of the pulsed cavity. It consists of a cylindrical body with a heavy machined frame at the rear that holds the adjustable main stem. At the front end, the stem is held in place by a teflon ring. The front surface of the stem has machined grooves and forms a capacitor with the aluminium coated maylar membrane that is mounted in front of it.

Cross-section of the pulsed cavity

A perforated disc holds the thin mylar membrane in place an protects it against damage. The stem is adjusted from the rear, so that the air gap between the membrane and the stem is as small as possible. This gives the highest possible capacity, and the highest sensitivity to sound vibrations. Once the stem is adjusted, it is fixed in place by filling the centre hole with solder.

Equivalent circuit diagram of the pulsed cavity

A suitable antenna is connected to the BNC socket at the rear. Contrary to the Russian original, it is inductively coupled to the stem - which is also an inductor - with a rather high transformation ratio, in order to keep the quality factor (Q) of the entire system as high as possible.

Block diagram
The diagram below explains the basic operation of the Pulsed Cavity System as developed by the NRP. The transmitter at the top right transmits short pulses that active the cavity. As a result of its high Q-factor, the cavity will not immediately stop ringing once a pulse has disappeared. As the receiver is synchronised with the transmitter, it will only 'listen' during the gaps between two pulses. The demodulated audio pulses are stretched in a sample-and-hold circuit, in order to produce a proper analogue audio signal that is a copy of the sound picked up by the cavity [C].

The overall timing of the system is provided by a Master Timing Unit. There are adjustments in many places, which makes it difficult for an untrained technician to obtain satisfying results. In order make the setup procedure easier, a performance check oscillator is added to the top. It can produce a 1350 or 4000 Hz sinewave audio signal for modulating the transmitted pulses. These audio tones can only be heard through the receiver, if the cavity is successfully activated and the signal is successfully demodulated by the receiver. Once adjusted, the system is extremely stable.

As a proof of concept, the first Pulsed Cavity system delivered to the CIA operated at approx. 375 MHz. The large cavity resonator shown in the images above was used for these experiments. The transmitter produced a peak power output in the order of 25 Watts, which was enough to cover a distance of 50 metres in free space and pass through several walls of the laboratory building. If necessary, peak power could be enhanced later to 1-5 kW by using magnetron radar oscillators.

Most of the development of the above 375 MHz system took place in 1963 and 1964, after which a transistion was made to 1100 MHz, using a much smaller resonant cavity, in combination with the described listening post, albeit in an adapted form in order to support the higher frequency.
Surviving parts
Although it was initially thought that all components of the Easy Chair pulsed cavity system had been lost over time, we have managed to retrieve the following bits and pieces:
Cavity resonator for 375 MHz (described above) Master Oscillator Transmitter power amplifier (PA)
Buffer amplifier Timing and Audio unit Receiver mixer and Local Oscillator IF section, with amplitude limiter and FM discriminator
Power Supply Unit
Master oscillator
This stripline oscillator is the master oscillator, that provides the basic signal for the transmitter and is controlled by the pulse generator.

Note that the signal from the pulse generator is first buffered in the external buffer amplifier.
Master oscillator - interior

Power amplifier   PA
This is the actual transmitter. It takes the (pulsed) signal from the master oscillator and amplifies it to an appropriate level. The output of the transmitter is supplied directly to one of the antennas.

The PA consists of three transistor-based stages, all of which are clearly visible in the image on the right. At the right is the fist stage to which the signal from the master oscillator is supplied. At the centre is the driver stage built around a 2N3375 transistor. The output stage at the left is built around a 2N3632 transistor.
Power Amplifier (PA) - interior

This small amplifier is used to buffer the signal from the master oscillator and provide it to the transmitter (PA) and to the master timing unit.   
Buffer - interior

Master timing unit
Apart from the PSU, this is the largest component of the system. It is driven by the master oscillator and provides the central timing for all other parts. Furthermore it drives the transmitter, the local oscillator and the audio.

Inside the timing unit are five PCBs. At the top left is the tuning control unit. At the bottom left is the audio circuit. The PCB at the center provides the timing and gating signals. The rightmost PCB contains the transmitter control circuits and the receiver's AFC gating control.
Timing unit - interior

The converter unit (marked CONV) contains the RF front-end and the mixer. It is connected to the receive antenna and the local oscillator (LO), and delivers its output to the IF section.   
Converter - interior

IF section
The IF section amplifies the output from the converter, limit its amplitude and demodulates the Frequency Modulated (FM) pulsed signal.

The output of the IF section is supplied to the timing unit, where the pulses are stretched and the resulting (audio) signal is amplified and filtered, ready for supplying it to a pair of headphones.
IF section - interior

Power supply unit   PSU
The image on the right shows the complete Power Supply Unit (PSU) of the listening post. It consists of a regular transformer, an electronic regulated voltage circuit and a switch panel.

The PSU provides the necessary voltages for the various parts of the transmitter and receiver.
Power Supply Unit (PSU)

Master oscillator Buffer amplifier Timing/Audio unit Power Amplifier (PA) Converter IF section Power Supply Unit (PSU) Various parts
Master oscillator - interior Buffer - interior Timing unit - interior Power Amplifier (PA) - interior Converter - interior IF section - interior PSU PA transistor
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Master oscillator
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Buffer amplifier
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Timing/Audio unit
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Power Amplifier (PA)
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IF section
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Power Supply Unit (PSU)
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Various parts
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Master oscillator - interior
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Buffer - interior
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Timing unit - interior
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Power Amplifier (PA) - interior
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Converter - interior
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IF section - interior
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PA transistor

  1. Easy Chair Progress Report 1963-1964 (I)
    June 1964. Chapter 3.4 Pulsed Interrogation of resonators.

  2. Easy Chair Progress Report 1963-1964 (II) Final
    December 1964. Chapter 3.1 Pulsed Interrogation of r.f. resonators.

  3. Easy Chair Progress Report 1964-1965 (I)
    June 1965. Chapter 2 Progress of pulsed cavity system.

  4. Easy Chair Progress Report 1964-1965 (II) Final
    December 1965. Chapter 2 Progress of pulsed cavity system.

  5. Easy Chair Progress Report 1965-1966 (6 months summary)
    October 1966. Chapter 2 Pulsed cavity system.

  6. Easy Chair Progress Report 1965-1966 (final)
    January 1967. Chapter 2 Pulsed cavity system.

  7. Easy Chair Progress Report 1966-1967 (6 months summary)
    August 1967. Chapter 2 Pulsed cavity system.

  1. NRP/CIA, Collection of Easy Chair progress reports
    Crypto Museum Archive, CM302467 (see above).

  2. Gerhard Prins, Letter to his heirs
    Date unknown, but probably written shortly before his death in April 1993.
    Vertrouwelijk (confidential). Published by [3].

  3. Maurits Martijn & Cees Wiebes, Operation Easy Chair
    De Correspondent. 24 September 2015.

  4. CIA Contracting Group, Report on Research on EASYCHAIR
    14 July 1955. Classification status unknown. Not marked as secret.

Further information

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Crypto Museum. Created: Tuesday 07 February 2017. Last changed: Tuesday, 13 June 2017 - 05:54 CET.
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