HP 3562A Service Manual page 242

C I RC U IT D ESCRI PTIONS
(Refer to figure 6-A1 e) The pu rpose of the LO input receiver is to synchron ize the local
LO Input Receiver
oscil lator input data (NDA T) to the sam ple rate (SAMP). This is necessary because in external
sample mode the LO data does not come at a regular rate. To synchron ize the data to
the sam ple rate, the LO input receiver sh ifts the serial data in and converts it to paral lel
d ata. The LO i nput receiver then clocks this data using the SAMPLE signal from the tim­
i n g control circu it. After the data is synchron ized, it is converted back to serial data and
sent to the m u ltip l ier.
The mu ltiplier is used to multiply the LO data by the noise and send it to the analog sou rce.
M ultiplier
This is done with a parallel noise word and a serial LO word. This provides the means
for frequency shifting the noise. When in zoom mode, the frequency shifting process shifts
the center of the noise power freq uency band to the frequency of the sine data from the
local osci llator. When i n chirp or sine mode, the output of the noise generator if forced
to a constant so that LO data is passed through the m u ltiplier with only an inversion.
T he noise generator produces a b inary random sequence that is used for band-l i m ited
Noise Generator
random noise and bu rst noise signals. The noise generato r is a 32-b it sh ift register with
feedback to generate a pseudorandom sequence. This sequence is mu ltipl ied by a squ ared
a nalog random signal. This randomizes the feedback of the shift register. The analog ran­
d o m noise is a zener diode biased so that it is barely on and is thus noisy. The output
of the zener d iode is ampl ified unti l it is at a high enough level to squ are up. The noise
f i lter is used to prod uce noise with necessary bandwidth. The bandwidth is determ ined
by the rate that the noise is sh ifted into the noise filter. The rate the noise is shifted i nto
the noise fi lter is determ ined by the effective sample rate.
The output of the noise generator is paral lel loaded i nto the multipl ier and m ixed with
the data from the local osci llator. This process is control led by the ti m i ng state machine.
Internal Signal Descriptions
..
ARML signal It goes h igh after the trigger point.
B R ST
6-20
S ignal from the phase resol ution circuit to the status register and the burst
.
. .
contro l circu it. ARM E D L goes low on the first sam ple point fol lowing the
BU RST RESET
U sed to abort a bu rst i n the burst control circu it. When low BRST causes
the A30 Analog Sou rce to output zero if the i nstru ment is in the burst mode.
This signal should be high for normal operation.
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MOD E L 3562A
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