HP 8530A, Руководство по эксплуатации и программированию, Страница: 326 / 439

Service Functions
IF Gain
The IF Gain menus allow you to select automatic or manual IF gain control for the HP 8530A
input signal paths . Remember , the IF section of the receiver:
Down converts the 20 MHz input signals to a lower frequency , so the signal is easier to
manipulate and sample .
Changes the amount of IF gain so the detectors and A/D converter operate with optimum
accuracy (refer to Figure 17-12).
Selects the input ratio (b1/a1 for example).
Normally the receiver uses automatic gain control. In this mode , the receiver automatically
adjusts IF gain for a high signal level (around 015 dBm) at the detectors and A/D converter .
This allows the detectors and A/D converter to operate with optimum accuracy . The default
(automatic) mode is appropriate for almost all measurement setups .
Why Use Manual control
Y ou should use manual IF gain control if any of the following applies to your measurement:
There are large power changes between adjacent data points (greater than 24 dB between
points).
The \IF OVERLOAD" message keeps appearing, even though RF P ower to the receiver inputs
is less than 010 dBm.
This situation can occur when measuring the copolar and crosspolar output of an antenna,
when using F ASC or F ASD F ast CW modes . The F ASAD (autoranging F ast CW) and F ast IF
Multiplexing modes are immune to this problem.
Why the problem occurs . When the receiver measures a data point, automatic IF gain control
detects the power at the input. Assume (for example) that this is a very low power level. The
gain control increases IF gain to amplify the signal. This allows the synchronous detectors and
A/D converter (shown in Figure 17-12) to operate in their most accurate range . Now assume
the receiver measures the next data point, and the power level is much higher . The IF gain
stages are still set for the previously-measured low power level. When the large signal arrives ,
it is amplied greatly and will often overdrive the detectors . The error message \IF OVERLOAD"
appears when this occurs .
Figure 17-13 shows a block diagram of the IF gain stage .
Using System Functions 17-19