System Design

       
Circuit Description Mechanical       

 

If the oscillator is powered continuously the IF output needs only to be amplified with a narrow band low frequency amplifier covering the Doppler frequency range of interest. The Doppler frequency, which is linearly proportional to relative velocity between the MDU and the moving target, is 70 Hz per metre per second (31 Hz per m.p.h.) for a microwave frequency of 10.525 GHz.

If the MDU is operating in an environment where fluorescent lights could be operating at the same time as the MDU, narrow band notch filtering will be required in the amplifier or in the subsequent signal processing to reduce the sensitivity of the unit to the moving plasma in the fluorescent tube.

The frequency of this notch filter needs to be centred on twice the mains supply frequency in the area of operation (100Hz for 50Hz supply or 120Hz for 60Hz supply).

Typical amplifier characteristics would be 70dB gain with a -3dB bandwidth of 3Hz to 80Hz, with a 60dB notch filter at twice the mains supply frequency.

If the MDU is operated in a pulsed mode as suggested in (3) above its sensitivity will be reduced in proportion to the duty cycle of pulsing.

This loss of sensitivity can largely be recovered using a sample and hold circuit between the IF output of the unit and the amplifier. The sample and hold circuit would typically consist of a FET series switch turned on when the oscillator is turned on, and a shunt capacitor which is then charged from the IF output of the unit. In practice any switching transients generated during the turn-on or turn-off of the oscillator can be eliminated from the IF output by in-setting the sample and hold pulse by approximately 1µs within the oscillator pulse.


The video impedance of the IF output of the MDU is approximately 400W when the oscillator is running, and no DC return is required on the IF output.

A low DC level (<± 150mV) will be present on the IF output of the MDU whilst it is operating. Under pulsed operating conditions this will appear as a square wave on the IF output,the magnitude of which will vary with the mounting location of the unit and with static reflecting targets in its coverage area. This voltage is the vector sum of a large number of reflected signals from both within the MDU and the environment in which it is operating.

As long as this DC level is less than ± 150mV under all operating conditions the functional performance of the unit will be in specification. If external DC bias is applied to the IF output this should be such that the DC level does not fall outside these limits.

Coverage Pattern

The MDU uses separate transmit and receive antennas. As well as improving the sensitivity of the unit by providing isolation between transmit and receive paths this features also permits the shape of the coverage pattern to be optimised.

The coverage pattern of the standard unit is 72° horizontally and 36° vertically, with the connection tab facing downwards. This represents the angular coverage over which the sensitivity is at least 70% of the peak sensitivity directly in front of the MDU.

In practice, in an intruder alarm sensor, this equates to a horizontal coverage pattern of 90° so that a unit mounted in the corner of a rectangular room will give complete coverage along the walls without unprotected creep zones.

Other patterns, including a 360º coverage for use in a ceiling mounted application, are also available. Call or e-mail for further details.