26 Keysight 8478B Thermistor Mount
Operating Principles
General Operation
3-
General Operation
Two matched pairs of thermistor elements are used in the 84 78B. Each pair is
connected in series and the two pairs are mounted in a common thermal
conducting block. The thermistor pairs are used in the bridges of the power
meter. One pair, the detection thermistors, is used as an arm of the RF
detection bridge. The other pair, the compensation thermistors, is used as an
arm of the compensation and metering bridge.
Operation with 432 Power Meter
With the 8478B attached to the 432 Power Meter the detection thermistors are
part of the RF bridge and the compensation thermistors are part of the
compensation bridge. Since the two pairs of thermistors show the same
thermal environment, any change in temperature which affects the RF bridge
simultaneously affects the metering bridge; this allows the power meter circuit
to compensate for changes in temperature and thus minimize drift.
During operation sufficient amounts of dc current are supplied from 432 Power
Meter to heat the thermistors until their resistances are reduced to
approximately 2000 per series pair. Capacitor C1 is practically a short to RF.
This causes “D” to appear series connected to the dc bridge, parallel
connected to RF. In this manner, “D” appears to the dc bridge in the 432 Power
Meter as a 2000 resistance that terminates the coaxial cable into 50
Ω
.
Capacitor C1 blocks any dc and audio power that may be present in the
incoming signal and passes only RF power.
Operation with 431 Power Meter
The RF detection bridge balance is affected by RF power input to the 8478B,
and the bridge is balanced by the application of 10kHz audio bias power. The
compensation and metering bridge is indirectly, but equally affected, by the
application of RF power. Deviation from a near-balance condition is caused by
an equal change in the 10kHz audio bias power to the compensation
thermistor pair, as initiated by the RF detection bridge. The compensation and
metering bridge is returned to near-balance by the application of dc power.
During 8478B/431 operation, sufficient amounts of dc and 10kHz bias currents
are supplied from the 431 Power Meter to heat the thermistors until their
resistances are reduced to approximately 2000 per series pair. Capacitor C3
offers high impedance to 10kHz, but is practically a short to RF. This causes
“D” to appear series connected to 10kHz, but parallel connected to RF. In this
manner, “D” appears to the audio bridge of the 431 Power Meter as a 2000
resistance, but terminates the coaxial cable in 50
Ω
. Capacitor C1 blocks any dc
and audio power that may be present in the incoming signal, and passes only
RF power.
