Keysight N9038A, Руководство по обслуживанию

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Keysight N9038A MXE Service Guide 167
RF Preselector Section
RF Preselector Section Purpose
Radiated Filter Path Prefilter
This circuit consists of two filters, a lowpass filter and a highpass filter. The
lowpass filter has a 3 dB roll-off at 4.4GHz. The highpass filter has a 3 dB
roll-off at 2 MHz.
Besides limiting the frequency range of the input signals within operating
range of A22 Radiated Filter assembly, the secondary function of the circuit
is to act as a DC block to reject low frequency signals which are mostly
impulsive in electromagnetic emissions measurements. This will provide
further protection to the A22 Radiated Filter assembly.
Preamplifiers
There is an individual preamplifier for both the conducted filter band and
the radiated filter band on the A21 RF Preselector Input assembly. These
preamplifiers are in the signal path after all of the filtering has been done.
The gain of each of these preamplifiers is nominally 20 db. The user has the
ability to turn these preamplifiers on and off as their measurements
conditions require.
Overload Detection
Because of the frequency range differences between the two different filter
paths there are two different overload detector designs used. However,
both detectors serve the same purpose and operate in the same basic
manner. They detect input signal levels that will cause an overload
condition in the instrument Front End.
A23 Limiter Assembly Control
Since the A23 Limiter assembly is not able to communicate directly with the
instrument local bus (ILB) or PCI interface its control is implemented
through A21 RF Preselector Input assembly. There is not much to control on
the A23 Limiter assembly, but there are required bias voltages and control
lines, the most obvious of which is the control it provides for the front panel
Input selection LEDs.
SW1 & SW2 Control
Both the SW1 Cal Switch and the SW2 Transfer switch are controlled by the
A21 RF Preselector Input assembly.
Alignment Signals
There are two different alignment signal sources on the A21 RF Preselector
Input assembly, a Direct Digital Synthesizer (DDS) and a broadband noise
source.
The Direct Digital Synthesizer provides a CW signal up to 50 MHz which is
used by the system alignment routine to adjust the amplitude accuracy of
the conducted filter path.