HP 8340A, Руководство по обслуживанию

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Model 8340A '- Service
The 20-30 Loop generates an output between 20-30 MHz in CW mode and 15-30 MHz when the
20-30 Loop is swept. This loop has an output resolution of 1 Hz. Comparing and locking the
down-converted YO frequency to the 20-30 output produces a
1
Hz resolution in the YO
frequency.
YO LOOP
The YO Loop contains the YIG Oscillator (YO) that is the tunable local oscillator source for all
frequency bands. When the HP 8340A is set to a specific CW frequency, the processor sets the
frequency of the M/N and 20-30 Loops accordingly. It also sets the YO pretune voltage which
tunes the YO to the approximate frequency.
The output of the YO is fed through the directional couplerto the RF Section. The directional
coupler splits off part of the YO signal which goes to the Sampler. The M/N output frequency
and its harmonics are mixed with the YO frequency in the Sampler to produce an IF signal
between 20-30 MHz. The 20-30 MHz IF from the Sampler is compared to the 20-30 Loop output
in the Phase/Frequency Detector. The error-induced voltage from the Phase/Frequency Detec­
tor is fed through a sample/hold and is summed with the pretune voltage that drives the YO
tuning coils. The YO Frequency changes until the output voltage from the Phase/Frequency
Detector goes to zero and phase-lock is achieved. For sweep widths greater than 5 MHz, the YO
is phase-locked at the start of the sweep. The sample/hold is then set to hold, breaking the loop,
and allowing the YO to sweep. The Sweep Generator initiates a Voltage-Sweep (VSWP) ramp
that is sunimed with the YO pretune voltage. This voltage causes the YO to sweep to the STOP
frequency. This action is referred to as Lock and Roll. During multi-band sweeps, the YO will
again phase-lock at the start of each band before continuing on with the sweep, thus ensuring
frequency accuracy across the full range of the HP 8340A.
CONTROLLER SECTION
The Controller Section performs all of the data transfer and coordinates the control signals that
operate the HP 8340A. It contains a 16 bit microprocessor, a total of 34K x 16 ROM and SK x 16
RAM. This section also contains interface circuitry for communicating with the rest of the
instrument.
Digital information is exchanged between the microprocessor and other sections of the instru­
ment on a bidirectional bus. In the power-on condition or at Instrument Preset, the controller
runs through an instrument self-test. The microprocessor will also set the front panel controls to
preset conditions iflnstrument Preset was pressed.
The rear panel interface signals, such as the HP-IB bus, SWEEP OUTPUT, Z-AXIS, and others
are also routed through or generated in the Controller functional group. The sweep control signal
from the controller stops and starts the Sweep Generator. As previously mentioned, during
bandcrossings, the sweep must be stopped to phased lock the YO before the sweep can continue.
The controller monitors sweep events such as bandcrossings, end of sweep, and markers. It then
executes specific instructions according to each type of sweep event.
FRONT PANEL
The front panel contains both the displays and the keyboard as well as some interface connec­
tors. The display has a dedicated microprocessor that keeps the display refreshed and updated.
When the instrument processor places display data into the display interface latch, it signals the
display processor that some information is ready. The display processor immediately takes the
data from the latch and stores it in internal memory. The data will be handled when the display
processor has time. The keyboard communicates directly with the instrument processor. When
a key is pressed, the instrument processor will either execute that key (if it is a single key
operation), or wait fo:r completion of the key sequence. Different key sequences not only allow
the operator to set up the HP 8340A for normal operation but also allow access to internal
registers, latches, D to A converters, and calibration constants for troubleshooting.
8-11
Scans by HB9HCA and HB9FSX