Figure

Scanning

Optics 

F

ron

t

and

Bac

k

Views

circuit

app

ears

in

Fig 

The

circuit

is

relativ

ely

simple 

It

consists

of

t

w

o

Burr 

Bro

wn

MPY   

analog

m

ultipliers

and

a

pair

of

lo

w pass

 lters 

The

in phase

and

quadrature

reference

signals

from

the

clo

c

k

generator

are

terminated

with

resis 

tors

follo

wing

the

toroidal

transformers 

as

men

tioned

ab

o

v

e 

One

of

the

m

ultipliers

is

fed

with

I

and

the

other

with

Q 

The

signal

from

the

APD

camera

is

also

connected

to

b

oth

m

ultipliers 

This

will

mo

dulate

the

photo

dio

de

signal

to

baseband

and

t

wice

the

fundamen

tal 

A

lo

w noise 

lo

w pass

 lter

stage

follo

ws

the

m

ultipliers

to

select

the

baseband

signal 

An

Analog

Devices

OP    

lo

w drift

ampli er

is

con gured

as

the

lo

w pass 

with

a

capacitor

and

resistor

in

feedbac

k 

The

p

ositiv

e

terminals

of

the

opamps

are

set

b

y

t

w

o

p

oten

tiometers 

determining

the

DC

o set

of

the

signal 

The

cuto 

frequency

w

as

set

fairly

lo

w appro

ximately

   Hz 

to

pass

the

lo

w

est

amoun

t

of

noise 

but

high

enough

to

enable

a

  Hz

scan

rate 

In

running 

this

cuto 

could

probably

mo

v

ed

higher 

as

the

optimized

la

y

out

pro

vided

a

v

ery

clean

signal 

The

photo

dio

de

output

is

directly

demo

dulated

to

baseband

in

this

design 

This

allo

ws

a

simple

circuit 

but

has

the

added

problem

of

not

allo

wing

another

A

GC

to

b

e

run

at

an

in

termediate

frequency

An

impro

v

emen

t

of

the

design

w

ould

demo

dulated

the

signal

to

an

in

termediate

frequency

and

then

again

to

baseband 

The

Burr Bro

wn