HP 3562A, Service Manual

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MOD E L 3562A C I RCU IT DESC R I PT I O N S
6-1 3 A30, ANALOG SOURCE
The descriptions in this chapter are written to help you understand the operation of the
circu its of the analog source board . The circ u it functions are discussed beyond the block
leve l but not to the component level. Refer to the A30 schematic for part and pin nu mbers
while read ing this circ u it description.
The analog sou rce board performs two fu nctions: 1) converts digital information from
the d igital sou rce' board (for sinusoidal signals) into an analog signal avai lable at the front
panel, and 2) generates pseudo random noise (PRN) and squ are wave signals which are
used to cal ibrate the i nput and ADC boards. Refer to the block di agram and schematic
d rawings for the fol lowing d iscussion.
Sine Wave 'nterface
The d igital source board provides the sou rce signal i n d igital form to the analog sou rce
board which converts it into the selected analog signal. The d igital i nformation enters
the analog sou rce board i n a serial format as a signal cal led DACDA T at pin 39 and is
sh ifted into serial-to-paral lel sh ift registers U553 and U552. The clock used to shift in the
data is on pin 40 and the enable signal is on pin 27. The paral lel output is latched by
U551 and U550 into the D/A converter (DAC) U551 .
Source Signa' DAC
U551 changes the digital information originating on the digital sou rce board into an analog
signal. The output is a cu rrent signal which is converted to a voltage signal by U350 and
buffered by U352.
1 00 kHz LPF
The fi lter consists of passive components fol lowed by an am pl ifier stage (U50) which buf­
fers the fi lter and has enough gain to m ake up for losses in the filter and losses i n U251
(next stage).
Attenuator
i nput for the DAC. I n this configu ration the circuit performs as a step attenuator such
that its output is 0 V when al l contro l l ines are low and it increases 5 mV for consecutive­
ly larger binary nu mbers. The digital information which sets the output level of the at­
tenuator comes onto the board as a serial bit stream through the front end interface cir­
cuit as described for that block later in this section. The output of U251 is a cu rrent signal
which is converted to a voltage signal by U250.
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