HP 3562A Service Manual page 261

MO D E L 3562A
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 s ignals
which are synchron ized with the sample rate. The LO out puts d igital sine and cosine signals
to the AS Digital Fi lter. The d igital f i l ter u ses these signals for the m ixing operation when
the instrument is in the 'zoom mode' (freq uency span <1 00 k H z). The LO a l so outputs
a cosine signal (N DAT) to the A1 D i g i tal Sou rce. T�e d igital sou rce uses the d igital data
from the LO to create band� l i m ited no ise and i nterfaces the LO output with the analog
s o u rce. The analog s o u rce is bas i c a l ly a d igital-to-an alog converter for the LO d igital sine
wave. The LO output to the d igital sou rce is determ ined by the operati on mode of the
i nstru ment as fol lows:
Mode
Baseband meas u rement
Zoom
Swept s i ne
Period i c C h i rp
Bu rst c h i rp
Fixed S i ne
Theory of Operation
T he A2 System CPU starts the s i ne wave generation p rocess 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 i nterface. T h i s d ata conta i ns a comm and byte to put the LO in the
desired mode of operation (the mode is determ i ned by the i nstru ment setu p). O n ce the
LO is i n iti alized with the mode of operation and desired frequ ency val u es, the system
C PU is no longer needed for the s i n e wave generation process.
U p the phase accu m u l ator with the freq uenc ies of the s i ne wave to be generated. The
frequency is set by 32 bits of data from the C P U . The add ress determ ines w h i c h phase
accu m u l ator registers are loaded and thus what the f req u e n cy represents (sou rce, c h i rp
start, chirp increment, d i gital fi lter frequency, etc.) The phase accu m u l ator sends an ad d ress
that represents the phase value to the s ine and interpol ator ROMs. For exam ple, an address
of 00000000 co rresponds t6 a phase value of 0 degrees, and an address of F F F F FFFF
corres ponds to a phase value of j ust under 360 degrees. The s i ne ROM is a look -u p tab le
contain ing the pos itive half of a s i ne wave. The interpolator ROM conta i n s a set of data
poi nts u sed for l i near inte rpolation between the s i ne ROM data poi nts.
Outputs from the s i ne (1 6 b its) and i nterpol ator ROMs (4 b its) are added fo u r b i ts at a
time to determ ine the s i ne, cosine, or N DA T data point. The 1 6-bit s u m is sent to the LO
output buffers in 4-b i t n i bbl es which a re converted to 16 b i t words. The word i s l atched
into the proper output buffer to be clocked out in series at the next SYNC2 pu lse.
LO output to A 1 (N DA T)
Co nstant output
C o s i ne to d igital fi lter
Cos ine to d igital fi lter
L i near swept s i ne starting at trigger point
Linear swept s i ne 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.
. . .
C I RC U I T D E SC R I PT I O N S
6-39