HP 87114B, Service Manual

Page: 138 / 174

Receiver Theory
The A5 receiver assembly is responsible for separating and measuring the RF signals and
converting these signals into digital data for further processing by the A2 CPU . Figure 6-3
shows a simplied block diagram of the HP 8711B/12B receiver assembly .
Note
The following paragraphs describe an HP 8711B/12B . The HP 8713B/14B are
very similar . See the section \Dierences Between the HP 8711B/12B and
HP 8713B/14B (1300 MHz vs 3000 MHz)" at the end of this chapter .
Figure 6-3. Simplied Receiver Block Diagram
The source RF output from the A4 source assembly is input into the receiver . The signal passes
through an optional step attenuator and/or AM delay modulator , then through two directional
couplers . The rst coupler measures the source output signal and uses this as a reference
(R input). The source output signal travels the through arm of the second coupler , which is
used in reection measurements to sample the signal reected from the REFLECTION test port.
This reected signal is referred to as the A input. The source output signal then goes out the
REFLECTION port, propagates through the device under test, and enters the receiver again at
the TRANSMISSION or RF IN port. The signal from this port is the B input.
The R, A, and B inputs can be processed in one of two ways . F or narrowband measurements ,
the RF input signal is downconverted to a lower IF frequency of 27.778 kHz using mixers .
These mixers are driven by the receiver LO signal from the A4 source assembly . After the
signal is downconverted, it is amplied, ltered, and sent to the analog to digital converters
(ADCs).
6-6 Theory of Operation