HP 8517B - Manual (Page 63)

HP 8517B S-Parameter Test Set Manual 5-3
Troubleshooting the Test Set
Theory of Operation
The Reference Signal
Path •
The reference signal is routed from the switch/splitter through the
20-7 dB tapered pad (AT1) to attenuate any VTO feed-through signals.
•
The signal is then applied to A10, the a1 sampler assembly, where it is
down-converted to a 20 MHz IF (intermediate frequency).
•
Because the RF power is being routed to Port 1, the a1 IF signal is used
to phase lock the network analyzer. Had the RF power been routed to
Port 2, the a2 IF signal would have been used to phase lock the network
analyzer.
The Test Signal Path •
The test signal is routed from the switch/splitter (A18) to the 60 dB
programmable attenuator (A16) for Port 1.
•
The signal may then be attenuated in 10 dB steps or passed through
without attenuation to Port 1 bias tee (A7).
•
A bias supply to provide dc voltage to Port 1 bias tee can be connected
to J8 on the rear panel of the test set, also.
•
The RF signal rides this dc level which is used to turn active devices on
(bias ON) so their S-parameters can be measured.
•
Beyond the bias tee, the test signal travels to the directional coupler
(A6).
•
At the coupler, the incident RF power is applied to the device under test.
The S
11
and S
21
Measurements
•
The signal reflected from the DUT is separated from the signal incident
to the DUT by the Port 1 coupler (A6).
•
The reflected signal exits the coupler and is applied to b1 sampler (A12),
then is down-converted to a 20 MHz IF signal.
•
The S
11
measurement is the ratio of the two IF signals from a1 and b1
samplers.
•
The incident signal is transmitted through the DUT, through Port 2
coupler, then into b2 sampler (A13).
•
The incident signal is down-converted to a 20 MHz IF so S
21
can be
measured. S
21
is the ratio of the two IF signals from samplers a1 and b2.
The Sampler Control
Switching •
The samplers may be turned on and off, depending upon the parameter
being measured.
•
Sampler switching is controlled by a microprocessor on the HP-IB board
assembly (A4). The default condition (test set power ON and no HP-IB
commands sent by the network analyzer) is for the test set to be active
with the RF signal applied to Port 1.