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When the PRF of the signal is greater than the
ADC sampling rate, signals with a certain PRF
must be avoided since image frequencies may mix
with the receiver's LO to produce the IF frequency.
For example, the 8753's first LO is tuned to 1 MHz
below the RF test signal. For a 200-MHz RF test
signal the LO is 199 MHz. Under pulsed conditions,
say for a PRF equal to 2 MHz, the first lower side-
band will fall at 198 MHz. This 198-MHz signal
will mix with the 199-MHz LO to also produce a
1-MHz IF. Therefore, it is important to avoid signals
with a PRF of 2 MHz/N, where N = 1,2,3, ...
(see Figure 9).
199 MHz
200 MHz
(1st LO)
(RF in)
1 MHz
198 MHz
1 MHz
(1st lower sideband
from signal
with PRF = 2 MHz)
Figure 9. Any PRF which generates a sideband at the ADC
sampling rate will be downconverted and sampled by the
receiver.
22
996 kHz
(2nd LO)
4 kHz
To IF
Filter
4 kHz
The hardware needed to make measurements
under pulse conditions includes two splitters, a
pulse generator, and a modulator. Connect a power
splitter to the output of the analyzer's RF source
output. One arm of the splitter connects to the
reference (phaselock) channel since the phase
reference channel cannot be pulsed. The other
arm of the splitter goes to a pulse modulator.
The pulsed RF is sent to a second splitter to make
ratioed transmission measurements. Ratioing is
necessary to remove the pulse transient response.
The transmission measurement of interest is B/A
(Figure 10).
Pulse generator
Figure 10. Using a pulse generator and a modulator, pulse
measurements can be made with a standard network
analyzer.
8753E with Option 011
RF out
R
A
B
Modulator
DUT
DUT
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