10 TEGAM WAY • GENEVA, OHIO 44041
440-466-
6100 • FAX 440
-466-
3-1
Section III
–
Principles of Operation
Understanding the 1830A Measurement Process
The Model 1830A RF Power Meter combines a DC substitution balancer with a digital voltmeter
system. The use of a A/D converter converts our analog measurement to a digital output on the
1830A front panel. This eliminates the need for any type of analog output that an instrument like
the Agilent 432A may require.
Because of the circuit construction, a “four
-
wire” circuit is used to sense bolometer resistance”,
allowing the user to use nearly any practical cable length without recalibrating the 1830A.
NOTE Limited by the resistive drop in the source leads. The small currents involved make
circuits of 100 meters or more possible, although electromagnetic coupling issues could
compromise measurements in some cases.
The balancer is composed of a controllable source that allows current to flow through R
ref
and
the bolometer. Identical precision instrument amplifiers feed the bolometer voltage and the
voltage across a reference resistor to a servo-amplifier that has been adjusted for low offset.
The servo amplifier controls the main supply transistor and therefore regulates power to the
combined reference/bolometer circuit. The 1830A is connected to support NTC devices. When
the bolometer voltage is higher than the reference resistor voltage, indicating that the
bolometer’s resistance is higher than desired, power is increased to heat the bolometer and
decrease resistance. When the bolometer voltage is lower than the reference resistor voltage,
power is decreased. At balance, the reference resistor voltage and the bolometer voltage match
closely. Since their currents are the same, the resistances also match closely.
NOTE The reference resistor is either 100 or 200 Ohms. Additionally, in some modes, the
reference resistor voltage is multiplied by a variable to produce the variable resistance feature.
At resistances other than 100 or 200 Ohms, the multiplication adds some uncertainty.
The 1830A computes power from changes in the voltage across the reference resistor as follows:
When ZERO is pressed, the voltage across the Reference resistor is stored in a combination of
DAC settings and software variables. This is done
on both the “RF” channel and the
“Compensating” channel. For any subsequent readings, the actual values of the voltages are
used.
When an uncompensated mode is selected, change in substituted DC power is computed as
R
V
V
P
On
Off
2
2
−
=
Equation 1 - Substituted Power Calculation
When a compensated mode is used, change in DC substituted power is computed using the
complicated methodology described in the H-P 432 Manual. This gives the identical answer to
the uncompensated equation in the case that the compensating thermistors do not have a
change in voltage (caused by a change in environment).
