ETS-Lindgren HI-1710A Microwave Oven Survey Meter - Manual (Page 47)

www.ets-lindgren.com Theory of Operation 47
6.0 Theory of Operation
The HI-1710A Microwave Measurement System uses a unique diode/dipole
antenna sensor that is coupled with digital filtering techniques developed for
Automatic Microwave Oven Scanner Systems. A radial array of eight antennas is
located perpendicular to the axis of the probe handle. The electric field is
detected by hot carrier diodes operating in the square law region. A negative
voltage is applied to the cathode connection of the diodes to bias them in an
optimum operating region. The bias is adjusted individually for each
probe/preamplifier assembly. The diode signals are summed and input to
two stages of amplification. The amplifying stages perform analog filtering to
minimize external interference at 60 Hz.
The analog output of the preamplifier is connected to one input of a
seven channel analog-to-digital converter. The bias input is connected to another
channel. Scaled values of the various power supply voltages are connected to
the remaining inputs. The digital equivalent of the RF signal is input to the
microprocessor. A software-implemented filter is used to condition the RF signal
for most effective RF measurement. The filtering in the HI-1710A is
accomplished by an integrating software filter. Sets of fixed filter parameters can
be selected from the front panel of the HI-1710A.
Selecting filter 1 (F1 / SLOW) results in a digital filter characteristic with a rise
time 90% of less than three seconds. This response is equivalent to the SLOW
response of the common analog microwave survey meter.
Filter 2 (F2 / FAST) results in a digital filter characteristic with a rise time 90% of
less than one second. The response of this filter is much faster and is equivalent
to the FAST response of an analog survey meter.
After filtering, the final leakage value is displayed as a 3- or 4-digit value. The
microprocessor also performs the peak hold and alarm functions. In the
Peak Hold mode, the bar graph display is driven without any digital filtering to
display the real time variations in the RF level. In the Peak Hold mode, the
highest reading sent to the digital display by the processor is held and
continuously displayed until either a higher reading is transmitted or until the
probe is zeroed. The peak value displayed is the value obtained after all filtering
is accomplished.