Operation & Service Manual
Section III
Model 1805B RF Control Unit
Theory & Operation
Figure 3-8 Simplified dc Substitution Circuit
With the RF power applied to the system thermistor (RF ON), the mount requires less bias
current to hold its operating point of 30 mW. In fact, by decreasing the dc bias by a known
amount, RF can be applied until the original operating point is again achieved. The amount of
RF power needed to fmd this operating point is proportional to the dc current removed. The dc
substitution board gets its name from this substitution ofdc power for RF power. Resistive
sections bias the bridge for eleven different calibrated bias currents which correspond to eleven
separate power substitutions. Switching of one of the eleven relays (K1-K11) selects the
corresponding resistive sections and shunts the sections across the bridge via relay K12.
The dc substitution board also contains the bridge error circuit which senses an error condition
directly across the thermistor. The error detection circuit is only active during the RF OFF
state. This circuit senses that the 30 mW operating bias point is within .7 mW by comparing
the sensed potential found across the thermistor to two reference voltages. These reference
voltages are equivalent to those required to produce 29.3 mW and 30.7 mW across the
thermistor. If the thermistor fails to bias within .7 mW of the 30 mW bias point, the front panel
indicator illuminates.
CALIBRATION
The following paragraphs provide theory used to perform calibration on the Model 1805B and
its associated thermistor mount.
DC SUBSTITUTION
A calibration system, such as the TEGAM System IIA, utilizes the principle ofdc substitution.
This method involves substituting a precise amount ofdc power for some amount of RF power
where the two powers are proportional. Thus, the calibration factor, K, is determined by the
formula:
Pdc
K=
PRF
K = the calibration factor of the mount,
Pdc = the substituted dc power that is precisely derived and measured, and
PRF = RF power into a matched load.
CALIBRATION FACTORS
For a terminating mount, K is denoted as the measure calibration factor (K1), and PRF is the
RF power from a match-terminated source incident upon the mount. If the mount is connected
to the side arm of a coupler or a port of a power splitter (for a feedthrough mount), K is
denoted as the monitor calibration factor (K2 ), and PRF is the RF power emerging from the
mount into a match-terminated load. PRF is measured by the terminating mount as:
3-13
