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← URS-1
Covert receiver - under construction

URR-1 was a crystal-operated monitoring and measurement receiver for the 315 MHz band, developed 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 receiver was commonly supplied as part of the URS-1 path loss survey system, along with the URT-1 covert transmitter.

URR-1 receiver

URR-1 receiver Front panel
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URR-1 receiver
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Front panel

All controls and connections of the receiver are located at the front panel.

URR-1 front panel

The receiver have been cleverly designed. The circuit works as follows: The calibrated output of a local oscillator (LO), is directly mixed with the input signal from the antenna by means of a directional coupler. The resulting mixed signal in then fed to a mixer/detector diode.

The frequency difference between the input signal and the local oscillator is just 16 MHz. If the received signal is at 314.500 MHz, the local oscillator is at 314.516 MHz. This is done so that (pre)amplification, filtering and logarithmic detection all take place in the low frequency domain: LC resonant circuits can be created with a rather high Q-factor to ensure small bandwidth in the IF-stage of the receiver. This way, a 65dB sensitivity span has been obtained.

Using three external attenuators of 20dB each, in combination with the 65db sensitivity span, results in a usable path loss range from 0dB to -125dB. Signal leakage at the antenna terminal is less than -23dB. For further details, check the circuit description in the technical manual.

  1. Technical Manual for URS-1
    February 1977.
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Crypto Museum. Created: Saturday 07 January 2017. Last changed: Tuesday, 13 June 2017 - 06:27 CET.
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