7-4
Operating Concepts
System Operation
The RF output power is leveled by an internal ALC (automatic leveling control) circuit. To
achieve frequency accuracy and phase measuring capability, the analyzer is phase locked
to a highly stable crystal oscillator. For this purpose, a portion of the transmitted signal is
routed to the R channel input of the receiver, where it is sampled by the phase detection
loop and fed back to the source.
The Source Step Attenuator
The step attenuator contained in the source is used to adjust the power level to the test
device without changing the level of the incident power in the reference path.
The Built-In Test Set The ES model features a built-in test set that provides signal-separation devices and
connections to the device under test. The signal separation devices are needed to separate
the incident signal from the transmitted and reflected signals. The incident signal is
applied to the R channel input through a jumper cable on the front panel. Meanwhile, the
transmitted and reflected signals are internally routed from the test port couplers to the
inputs of the A and B sampler/mixers in the receiver. Port 1 i s connected to the A input and
port 2 is connected to the B input. A transfer switch routes the source output power to
either port 1 or 2, allowing measurements to be made in both the forward and reverse
directions.
The ET model features a built-in test set that provides signal-separation and connections
to the device under test. Part of the source output is coupled off and applied to the R input
as the incident signal. The reflected signal is routed from the port 1 coupler to the input of
the A sampler. The transmitted signal is routed from port 2 to the B sampler input.
The Receiver Block The receiver block contains three sampler/mixers for the R, A, and B inputs. Instruments
equipped with Option 400 contain an additional R sampler/mixer . The signal s are sampled,
and mixed to produce a 4 kHz IF (intermediate frequency). A multiplexer sequentially
directs each of the three IF signals (four IF signals in Option 400) to the ADC (analog to
digital converter) where it is converted from an analog to a digital signal. The signals are
then measured and processed for viewing on the display. Both amplitude and phase
information are measured simultaneously, regardless of what is displayed on the analyzer.
The Microprocessor A microprocessor takes the raw data and performs all the required error correction, trace
math, formatting, scaling, averaging, and marker operations , according to the instructions
from the front panel or over GPIB. The formatted data is then displayed. The data
processing sequence is described in “Processing” on page7-5.
Required Peripheral Equipment Measurements require calibration standards for vector accuracy enhancement
(error-correction), and cables for interconnections. Model numbers and details of
compatible power splitters, calibration kits, and cables are provided in “Options and
Accessories” chapter of the reference guide.