Local Oscillator Description (Service Group B); Sum Loop; Step Loop (A23 And A26) - HP 3585A Service Manual

Model 3585A
6· 1 9.

LOCAL OSCILLATOR DESCRIPTION (Service Group B)

The Local Oscillator section is the largest single operating section within the instrument. It
provides all fixed reference signals used for mixing in the IF conversion section and for
clocks in other portions of the instrument. It also provides sweeping signals for the Tracking
Generator and for mixing in the IF conversion section. This section is divided into five
subsections. They are:
Each of these subsections will be discussed in more detail. For the discussions that follow,
you should refer to Figure 6-1 2 . Local Oscillator Block Diagram in this manual section and
the schematic drawings found in Service Group B of Volume Two.
There are two modes of operation for the local oscillator. The actual mode of operation is
dependent on the selected resolution bandwidth (RBW). For resolution bandwidths of 1 0
kHz and 3 0 kHz, the local oscillator operates in the single loop (SL) mode. For all other
resolution bandwidths, the local oscillator operates in the multiple loop (ML) mode. The
reason for this is that there is more noise on the LO signal in the single loop mode and while
this is allowable for the large resolution bandwidths, it is too much noise for the smaller
res9lution bandwidths. The multiple loop is constructed such that the local oscillator noise is
minimal.
Before discussing each subsection of the local oscillator, let's examine the LO section as a
whole. Since the local oscillator is based on phase lock loop (PLL) operation, a short review
of PLL will be given first. Figure 6-4. Basic PLL shows the traditional PLL configuration.
The voltage controlled oscillator (VCO) output is compared with a reference frequency by
using a phase detector. The phase detector generates an output pulse proportional to the the
phase difference between the reference frequency and the VCO frequency. This output pulse
is low pass filtered and integrated to become the dc correction voltage for the VCO.
REFERENCE FRACTIONAL N LOOP

SUM LOOP

LO CONTROL
STEP LOOP
R E F E R E N C E
F R E Q U E N C Y
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Y C O
LOOP M O D E
D C C O R R E C T I O N V OL TA G E
Figure 6·4. Basic PLL
Scans by ART EK MEDIA =>
Circuit Functional Descriptions
L P F
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3 5 B 5/1-6-4
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