HP 3562A, Service Manual

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MO D E L 3562A
C I RC U I T D ESCRI PT I O N S
6-6 A4 LOCAL OSCILLATOR
Introduction
(Refer to figure 6-A4) The A4 Local Osc i l l ator (LO) prod uces d igital sine and cosine signals
which are synchron ized with the sample rate. The LO out puts digital sine and cosine signals
to the AS Digital Fi lter. The d igital filter uses these signals for the mixing operation when
the instrument is in the 'zoom mode' (frequency span <1 00 k H z). The LO also outputs
a cosine signal (N DAT) to the A1 Digital Sou rce. T�e digital sou rce uses the d igital data
from the LO to create band� l i m ited noise and interfaces the LO output with the analog
source. The analog source is basically a d igital-to-analog converter for the LO d igital sine
wave. The LO output to the digital sou rce is determ ined by the operation mode of the
i nstru ment as fol lows:
Mode
Baseband measurement
Zoom
Swept sine
Periodic Chirp
Bu rst chirp
Fixed Sine
Theory of Operation
LO output to A 1 (N DA T)
Constant output
Cosine to d igital fi lter
Cosine to digital fi lter
L inear swept sine starting at trigger point
Linear swept sine starting at trigger point and end ing
in accordance with the percent of span setting.
S i n u soidal wave of spec ified freq uency.
The A2 System CPU starts the sine wave generation process by setting up the LO. When
the LO recogn izes it has been addressed, data from the system CPU is transferred into
the LO's system bus interface. This data contains a comm and byte to put the LO in the
desired mode of operation (the mode is determ i ned by the instru ment setu p). O n ce the
LO is initi alized with the mode of operation and desired frequency values, the system
CPU is no longer needed for the sine wave generation process.
...
...
U p the phase accum u l ator with the freq uencies of the sine wave to be generated. The
frequency is set by 32 bits of data from the CPU. The add ress determ ines which phase
accumulator registers are loaded and thus what the freq uency represents (sou rce, chirp
start, chirp increment, digital filter frequency, etc.) The phase accumulator sends an address
that represents the phase value to the sine and interpolator ROMs. For example, an address
of 00000000 corresponds t6 a phase value of 0 degrees, and an address of FFFFFFFF
corresponds to a phase value of j ust under 360 degrees. The s ine ROM is a look-u p table
contain ing the positive half of a sine wave. The interpolator ROM contains a set of data
poi nts used for l i near interpolation between the sine ROM data points.
Outputs from the s ine (1 6 bits) and interpolator ROMs (4 bits) are added fou r bits at a
time to determ ine the sine, cosine, or N DA T data point. The 1 6-bit sum is sent to the LO
output buffers in 4-bit nibbles which are converted to 16 bit words. The word is latched
into the proper output buffer to be clocked out in series at the next SYNC2 pu lse.
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