Service manual

8510XF Network Analyzer Systems 5-15
System Maintenance
Theory of Operation
Advantages of IF leveling
Because the IF output of the mixer is fixed at 20 MHz, IF leveling has
important advantages over RF leveling:
• There is no need for expensive broadband detectors.
• The IF can be narrowly filtered, to keep noise to a minimum (typical RF
leveling schemes, which use broadband coupler/detectors, are inherently
noisy).
• The detector is always reading a 20 MHz signal, so the frequency
response of the detector itself is not an issue.
The Leveling Loop The 8510XF leveling loop is described in more detail below (see Figure 5-5
on page 5-16 for an illustration of the loop).
Feedback Path The IF feedback path from the mixer to the level-control amplifier passes
through the millimeter-wave controller, and includes the following: a
programmable gain circuit, a detector, a multiplying DAC, and a blanking
circuit. These are described individually below.
NOTE There is only one feedback path, but the IF input to that path can come from
any of the reference mixers (that is, the mixers for the “a1” and “a2”
channels, in the active frequency range). The active test port, and the current
test frequency, determine which mixer is supplying the IF feedback at any
one time.
Programmable gain
This circuit includes, in series, three fixed amplifiers and three
step-attenuators. Each attenuator includes three pads which can be switched
into the IF path (the smallest of these is a 0.5 dB pad).
By programming the attenuators, so that some or all of the pads are switched
into the IF path, it is possible to offset some of the gain from the amplifiers.
The result is that the overall gain of the circuit is programmable, in steps of
0.5 dB, over a 66.5 dB range.
Detector
The amplified IF is applied to a detector, and converted to a dc voltage which
is proportional to the level of the amplified IF input.