Double channel bolometer bridge method

Microwave power measurement finds the average power delivered to a matched load. At microwave frequencies voltage and current are hard to define (especially in waveguides), so power is the basic quantity that is measured.

Power measurement – overview

Power ranges and usual sensors:

A bolometer is a small resistor whose resistance changes with temperature when it absorbs microwave power:

• Barretter – thin platinum wire, positive temperature coefficient (R rises with heat).
• Thermistor – semiconductor bead, negative temperature coefficient (R falls with heat).

The bolometer forms one arm of a bridge. In the DC substitution method, DC bias first balances the bridge; RF power then heats the bolometer, and the DC is reduced until the bridge balances again. The DC power removed equals the RF power absorbed:

P_RF = (I1² − I2²) · R

Double channel (dual) bolometer bridge

A single bridge drifts because the thermistor also responds to room temperature changes. The double channel bridge (e.g. self-balancing thermistor mount, HP 432 type) removes this error.

 RF in
   |                              
+--v---------+         +------------+
| RF thermis-|         | Compensat- |
| tor bridge |         | ing thermis|
| (channel 1)|         | bridge (ch2)|
+-----+------+         +-----+------+
      | V_rf                 | V_c
  self-balancing         self-balancing
  amplifier              amplifier
      |                      |
      +------> Meter <-------+
          P = (V_c² − V_rf²)/(4R)

Working principle:

1. Two matched thermistors are mounted in the same thermal block, so both see the same ambient temperature.
2. Channel 1 (RF bridge): its thermistor absorbs the microwave power. A feedback amplifier automatically adjusts the bias voltage V_rf to keep the thermistor at resistance R (e.g. 200 Ω), so the bridge stays balanced.
3. Channel 2 (compensating bridge): its thermistor gets no RF; its bias V_c also keeps it at R. V_c changes only with ambient temperature.
4. With no RF, V_rf = V_c. When RF is applied, the RF bridge needs less bias, so V_rf drops.
5. The metering circuit computes the difference of the DC powers:

P_RF = (V_c² − V_rf²) / (4R)

Since temperature drift changes both channels equally, it cancels.

Advantages: automatic balance, temperature compensation, accurate (about ±1 %), direct reading, good for 1 µW – 10 mW.