Circuit Description

 Applications  System Design


The Microwave Solutions Ltd Motion Detector Unit contains a dielectric resonator stabilised microwave FET oscillator, providing a frequency and amplitude stable signal at the operating frequency of the unit. The power from this oscillator is filtered to remove harmonic and spurious signals and is then split into two approximately equal amplitude signals.

One of these signals is further filtered and feeds the transmit antennas of the unit, illuminating the volume to be protected. The other signal is routed to the local oscillator input of a balanced mixer providing the reference signal against which the Doppler return signal is compared.

The Doppler return signal, reflected from the target is collected by the receive antennas and coupled to the RF input of the balanced mixer, where it is compared with the transmitted signal. The Doppler frequency is extracted and is available at the IF output of the unit for signal processing.


The oscillator requires 5V ± 0.25 V applied to the +5 V terminal of the device. If the oscillator is powered continuously (CW mode) the current consumption is typically 50 mA. For low power consumption it is usual to operate the unit in pulsed mode,supplying the oscillator with 5 V pulses with a typical pulse width of 30µ seconds and repetition rate of 1 to 3 KHz. The duty cycle of 3 to 10% reduces the average current consumption to 1.5 to 5 ma

The peak value of the pulse voltage must lie between 4.75 and 5.25 V and the flatter the pulse top the better the detection capability of the MDU will be. Under these conditions pulse chirp will be less than 1 MHz.

Application of a peak voltage in excess of 5.25 V will degrade the reliability of the unit and may cause it to transmit RF power at frequencies outside the authorised bands.

RF Power Levels

The RF power levels radiated by the MDU are extremely low under all conditions, and many orders of magnitude below the maximum recommended levels in normal operating modes.

The maximum transmitted power is less than 15mW.  This power is distributed within the coverage pattern of the MDU, and the maximum power density is 1mW/cm² at a distance of 5mm from the front face of the unit, reducing to 0.72µW/cm² at a distance of 1 metre.

Any equipment containing an MDU as the sole emitter of electromagnetic fields is therefore exempt from the testing requirements for human exposure to electromagnetic fields under the safety aspects of the R&TTE directive per EN 50371:2002  

The emissions from the front face of the MDU are also below the recommended maximum permissable exposure levels specified in IEEE standard C95.1-1991.  In fact under normal pulsed operating conditions, measured at a distance of 1 metre in front of the MDU, the emissions are a factor of 194,000 below the recommended maximum levels.

Balanced Mixer

The mixer in the MDU compares the frequency of the transmitted signal with that reflected back from targets in the coverage area. A balanced mixer configuration is used which provides superior matching and conversion loss compared with a single-ended mixer. This improves the sensitivity of the MDU, enhancing capture and reducing false alarms.

This configuration also means that the mixer diodes are protected to a large degree from static damage since each diode protects the other from excess reverse voltages. In addition it is relatively simple to design self-test circuitry which will verify that the mixer is operating correctly.

The mixer does not require an external DC return, however if it is desired to use a DC return (for self-test or other purposes) a value of between 1KW and 12KW is recommended. The IF output impedance of the mixer is approximately 400W.

A portion of the oscillator signal is fed to the LO (local oscillator) port of the mixer, and the return signal intercepted by the receive antenna is fed to the RF input. The magnitude of the IF output signal is proportional to the magnitude of the signal received at the RF input, and the frequency is proportional to the relative velocity of the target reflecting the received signal.

In a real life situation there are many signals received from many different targets moving at different velocities, so the total IF output is a spectrum of signals of varying frequency and amplitude.

In addition there is a DC component at the IF output, which is the vector sum of all signals reflected off static targets in the coverage area of the unit.