ETS-Lindgren HI-3520 Microwave Monitor (Archived) - Manual (Page 15)

HI-3520 Manual Page 11
4.0 THEORY OF OPERATION
4.1 RF Circuit
The Model HI-3520 RF circuit consists of two orthogonal
tft 1 (thin-film thermoelectric) arrays, each containing a
number of series connecter thermoelectric junctions.
These are mounted between a pair of thermally
conductive dielectric wafers to enhance the detector
sensitivity. When placed in an RF field, the dipoles
absorb power which gives rise to thermal gradients
across the thermocouple junctions. By keeping the
temperature differential small, the detector acts as a true
rms device producing a dc output voltage directly
proportional to the absorbed power.
The tft elements, acting as resistive screens, operate over
the frequency range 1 to 18 GHZ.
1 tft registered trademark of General Microwave
Corporation.
4.2 DC Circuit
The DC output produced by the RF circuit is amplifies by
a differential chopper-stabilized amplifier. This circuit has
high common mode rejection and low DC drift. The high
level output of this circuit is connected to a
microcontroller which has a built in A/D converter. The
amplifier has two gain ranges that are controlled by the
microcontroller.
A self-contained test signal is constantly applied to the
RF detector. Should a fault occur in this circuit, a fault
signal is generated through the DC amplifier which then
generated a fault bit to the microcontroller.
Archived 4/2/10