HP 3562A, Service Manual

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C I RC U IT D ESCRI PTIONS
MOD E L 3562A
LO Input Receiver
(Refer to figure 6-A1 e) The pu rpose of the LO input receiver is to synchronize the local
oscil lator input data (NDA T) to the sample rate (SAMP). This is necessary because in external
sample mode the LO data does not come at a regular rate. To synchronize the data to
the sample rate, the LO input receiver shifts the serial data in and converts it to paral lel
data. The LO input receiver then clocks this data using the SAMPLE signal from the tim­
ing control circu it. After the data is synchronized, it is converted back to serial data and
sent to the multiplier.
Multiplier
The multiplier is used to multiply the LO data by the noise and send it to the analog source.
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 frequency band to the frequency of the sine data from the
local oscillator. When in chirp or sine mode, the output of the noise generator if forced
to a constant so that LO data is passed through the multiplier with only an inversion.
Noise Generator
The noise generator produces a binary random sequence that is used for band-limited
random noise and bu rst noise signals. The noise generator is a 32-bit shift register with
feedback to generate a pseudorandom sequence. This sequence is mu ltiplied by a squared
analog random signal. This randomizes the feedback of the shift register. The analog ran­
dom noise is a zener diode biased so that it is barely on and is thus noisy. The output
of the zener diode is ampl ified unti l it is at a high enough level to square up. The noise
fi lter is used to produce noise with necessary bandwidth. The bandwidth is determ ined
by the rate that the noise is shifted into the noise filter. The rate the noise is shifted into
the noise fi lter is determ ined by the effective sample rate.
The output of the noise generator is paral lel loaded into the multiplier and mixed with
the data from the local osci llator. This process is controlled by the timing state machine.
Internal Signal Descriptions
..
.. .
---- - - - - - - - -- - ._. - --� =. ...:.. :=. .;. - - - - -
BRST
6-20
Signal from the phase resolution circuit to the status register and the burst
control circu it. ARMEDL goes low on the first sample point fol lowing the
ARML signal It goes high after the trigger point.
BU RST RESET
Used to abort a bu rst in the burst control circu it. When low BRST causes
the A30 Analog Source to output zero if the instrument is in the burst mode.
This signal should be high for normal operation.