HP 3336A Operating Manual page 111

Performance Tests
Model 3336A/B/C
4-24. On Frequency Return Loss Measurement Technique.
Since this performance test is new, a brief conceptual discussion of the measure follows:
4-26
A small signal is launched, in the forward direction, into the active
-hp- 3336 output. The addition of the 3336's output signal (f¢) and
the small signal (fo) at the 3336's output could be considered as the
3336 output signal with one sideband. Furthermore, this signal
(the carrier and one sideband) is mathematically equivalent to the
3336 output signal (the carrier) with a pair of AM sidebands and a
pair of PM sidebands, cancelling on one side of the carrier and
reinforcing on the other side. The 3336's amplitude leveling cir-
cuits will compensate for any amplitude changes, and effectively
cancel them at the Leveled Node (located on the other side of the
output termination, inside the 3336). Since no AM components
can exist at this Leveled Node, there will be no AM components
reflected, either. Any reflected AM components seen at the output
of the 3336 are solely due to the output termination impedance
and, hence, are a measure of the ''ON'' Frequency Return Loss. A
directional coupler introduces the small signal to the 3336's output
and isolates the reflected signals. A diode detector recovers the
AM components (recall that there are also PM sidebands) by
demodulating the reflected signals and a wave analyzer, tuned to
the modulating frequency, measures the relative magnitude of this
signal and a signal from a known, gross impedance mismatch, us-
ed as a reference. To insure that the measurement is ''ON'' Fre-
quency Return Loss, the frequency difference between fc and fg
must be well within the bandwidth of the 3336's Amplitude Level-
ing circuits. When Fast Leveling is ''ON'', this bandwidth is from
dc to 1 kHz. When Fast Levling is OFF, this bandwidth is from dc
to 1 Hz. In this particular procedure, the frequency difference is 10
Hz, therefore, Fast Leveling must be ON. Since the only difference
between Fast Leveling ON and OFF is the bandwidth of the level-
ing circuits, results obtained with Fast Leveling ON apply to the
instrument when Fast Leveling is OFF.
Return loss = 20log|(Zi - Zo)/(Zi + Zo)|
where: Zi is the ideal output impedance
Zo is the actual output impedance
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