Agilent Technologies 8510C, Руководство по эксплуатации и программированию

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Netw ork Analyzer System Description
Ho w the 8510 Mak es Measurements
In a t ypical measuremen t, the signal source is sw ept from the low er measuremen t frequency
to the higher measuremen t frequency using a linear ramp con trolled b y the 8510. The sw eep
is called a ramp sw eep. Ramp sw eep oers the fastest up date of the measuremen t display .
In step-sw eep mo de, the source is phase-lo c k ed at eac h discrete measuremen t frequency
con trolled b y the 8510.
A t the rst frequency con v ersion stage, signal separation comp onen ts in the test set apply
a p ortion of the inciden t signal and the resp onses from the device under test to the rst
stage. Digital comm unication b et w een the receiv er and the test set pre-tunes the 65 MHz to
300 MHz v oltage-tuned lo cal oscillator (VTO) so that one of its harmonics mixes with the
stimulus to pro duce a 1
st
IF frequency close to 20 MHz.
Fine tuning is accomplished b y comparing the IF frequency with the in ternal 20 MHz crystal
reference and sw eeping the lo cal oscillator to track the stimulus frequency .
When the lo cal oscillator reac hes its upp er frequency limit, the sw eep is stopp ed, the lo cal
oscillator is tuned again, phase lo c k is reestablished, and the sw eep is con tinued. Since the
rst lo cal oscillator frequency is selected algorithmically from a kno wn stimulus frequency , the
measuremen t is free of harmonic skip.
The second frequency con v ersion pro duces an IF frequency of 100 kHz for application to the
detection and data pro cessing elemen ts of the receiv er. Because the frequency con v ersions
are phase-coheren t and the IF signal paths are carefully matched, magnitude and phase
relationships b et w een the input signals are maintained throughout the frequency con v ersion
and detection stages. Automatic, fully calibrated, autoranging IF gain steps maintain the IF
signal at optimum lev els for detection o v er a wide dynamic range.
T est and Measurement Input Channels
Eac h measuremen t c hannel can use either input a
1
or a
2
as the reference signal; b
1
and b
2
are
alwa ys used for the test signal.
The un used reference c hannel can b e used to measure a third test signal. F or example,
assume y ou c ho ose a
1
as the phase-lo c king reference c hannel. Y ou can no w use a
2
as a third
measuremen t c hannel.
Ratio Measurements and Sampling Details
Dep ending up on the test set conguration, the appropriate reference input and test input are
ratio ed to obtain the measuremen t.
During a ramp sw eep, the selected inputs are sampled up to 801 times, with sample timing
accomplished b y sensing the 0 to 10 V sw eep output from the source. F or example, with 401
p oints selected, at eac h p ositiv e 0.025 V c hange in the sw eep v oltage, all selected inputs are
sampled and applied to the reference and test sync hronous detectors.
In step sw eep, the selected inputs are sampled a certain in terv al after phase lo c k is
established.
Eac h sync hronous detector dev elops the real (X) and imaginary (Y) v alues of the reference,
or test signal, b y comparing the input with an in ternally generated 100 kHz sine w a v e. This
metho d practically eliminates measuremen t uncertain t y errors resulting from drift, osets,
and circularity . Eac h X,Y data pair is sequen tially con v erted to digital v alues and read b y the
cen tral pro cessing unit (CPU).
3-4 Principles of Operation