8003 Precision Scalar Analyzer
2-78
Manual 20791, Rev. C, June 2001
In a two-sensor measurement, the power ratio of the two sensors is displayed as the measurement. After
pressing the
[DEFINE]
key, the [
RATIO
] softkey allows you to choose the proper ratio. The
numerator, or first sensor selected, should be the sensor monitoring the power out of the DUT. The
denominator, or second sensor selected, should be the sensor measuring incident power (power into the
DUT). As with single-sensor measurements, the two-sensor measurement compares the measured power
ratio through the DUT with a path calibration power ratio without the DUT. The [
ABS PWR/PTH
CAL
] softkey determines whether the analyzer just ratio’s the absolute power measured by the sensors
(ABS PWR), or also ratio’s with a stored path calibration (PTH CAL).
The actual path calibration is a little different from single-sensor measurements. You must do two path
calibrations, one for each sensor. The reason is that the analyzer memorizes each sensor’s path cal in
separate memories. This saves time when a sensor is used on more than one channel. For instance,
suppose channel 1 displays the ratio A/B, channel 2 displays sensor A power, and channel 3 displays
sensor B power. A path cal of sensor A and sensor B is then enough data to calibrate all three channels.
To path cal the two sensors, press the
[CAL]
key to bring up the softkey menu on the screen. Press the
[
THRU
] softkey. Now press the [
STORE THRU X
] softkey for the two sensors you are using for this
measurement.
Single-Sensor vs Two-Sensor Measurements
Single-sensor insertion loss measurements have the advantage of being very simple to configure with
little or no extra equipment required. As long as the source output level is repeatable from sweep to
sweep, and path cal is used, the measurement can be very accurate. However, it does require that the
power from the source does not change between path cal and measurement — something which may be
a problem if the source power changes with load match, or if the measurement requires swept power
levels (such as amplifier 1 dB compression measurements).
A two-sensor ratio can improve the accuracy of an insertion loss measurement by reducing the error due
to source mismatch. In any practical measurement, some power from the source is reflected back to the
source by the device under test. Any mismatch at the source causes some of this signal to be reflected
back to the device under test. The reflected signal looks like a variation in the incident power, which
would lead to errors in a single-sensor measurement. With two sensors, the incident sensor monitors the
same variations (due to reflected signals) from the source, that the output sensor measures. The ratio of
output to incident power therefore cancels the effect of source reflections. In practice, this cancellation
is not perfect but leads to a much improved effective source match.
Two-channel insertion loss measurements are also useful for measurements requiring sweeper power
changes such as amplifier or mixer 1 dB compression point. In a typical gain compression measurement,
the sweeper power level is ramped from the small signal region of the DUT to its saturation level. With
two-sensor measurements, both the incident and output sensor monitor the changing sweeper levels.
The ratio of output to incident power is then always the true gain of the amplifier. At some point, the
amplifier or mixer goes into compression and the gain rapidly decreases. The output level where the gain
decreases by exactly 1 dB is the 1 dB compression point of the amplifier (a second analyzer channel can
display just the power from the sensor connected to the amplifier output; the 1 dB compression power).
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