Keysight Technologies N5244/5B Service Manual page 210

Theory of Operations
Synthesized Source Group Operation
On the A20 IF MUX board, the configuration between the Main IF signals and
LFE IF signals are very different.
First, the connectors are different. For the main IF signals, on a 4-Port PNA,
there are 8 IF input connectors. The switches to select the R1.4 signals are on
the A20 IF MUX board. For the LFE IF signals, there are only 5 IF input
connectors. So, the switches to select the R signal are on the LFE board.
Second, the amplifier/attenuator/switch chains are different for the Main and
LFE IF signals.
The A14 motherboard bus supplies, among other things, the power supplies
to the LFE board. Many boards are already connected to these power supplies
in the PNA. So, different power supplies are close or very close to their limit
in terms of available current/power. This fact has an influence on which
power line can used to generate the voltages needed.
The A70 LFE board (4-port) and A75 (2-port) works with the A70-A74 bias
combiners. The design of this combiner has an impact on the design of the
board. Performances desired at the PNA test port are impacted significantly
by the bias combiner.
LFE frequency band
The LFE extension is changing the way we measure with the PNA at the low
end.
Version 6 synthesizers and below: Below 100MHz, the LFE option is enabled
in the firmware. Depending on the frequency we measure, the LFE board
receiver will be in 'mixer mode' or in 'direct conversion mode'. See also,
"Version 7 synthesizers" on page 5-42.
Version 7 synthesizers: Below 100MHz, the LFE option is enabled in the
firmware. With the direct digital synthesizer (DDS) assembly, a new LFE band
break is introduced at 4 MHz. This 4 MHz band break, corresponds to a switch
that directs the DDS output either through a balun (above 4 MHz) or
bypasses the balun (below 4 MHz). See also, "Version 6 synthesizers and
below" on page 5-42.
The Figure 5-13 on page 5-43 shows an example of current frequency bands
in a PNA. In an N5244/5B PNA, the IF frequency will change around 53 MHz.
Figure 5-13 on page 5-43 shows the new structure of the frequency bands
with the LFE option. The two receiver modes, mixer and direct conversion, are
used in different frequency bands.
5-42 Keysight N5244B/45B Service Guide