Stanford Research Systems SR844, User Manual

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1-6 The Basic Lock-In
SR844 RF Lock-In Amplifier
Remember, the signal is a 0.5 Vpk (1.0 Vpp) square
wave. A square wave is composed of signals at all
odd harmonics. The SR844 is a square wave
detecting lock-in and detects all of the odd
harmonics of the fundamental. The amplitude of the
fundamental (at 1.00 MHz) is 4/
π
x
Vpk. The
contribution from all odd harmonics is
1 + (1/3)
2
+ (1/5)
2
+ (1/7)
2
+ ...
≈
1.19
The detected amplitude is 4/
π
x 0.5 Vpk x 1.19 or
0.759 Vpk. The SR844 reads the signal in units of
Vrms (0.707
x Vpk) or 0.537 Vrms.
The CH1 display may not read exactly 0.54 V for a
number of reasons:
•
The REF OUT amplitude is only a nominal
specification.
•
The basic accuracy of the unit is
±
0.25 dB
(3%).
•
Since the phase may not be exactly 0
°
, X=Rcos
θ
is slightly less than R (amplitude).
4 Press
Shift
then
Phase
to perform
AutoPhase.
This adjusts the reference phase inside the
instrument. (The phase at which the signal is
detected changes, but Ref Out remains unchanged.)
This should set the value of Y (on the CH2 display)
to zero.
5 Press
Phase
. Display the reference phase. It should be close to
zero.
6
Press the
+90
°
key.
This adds 90
°
to the reference phase. The value of
X should drop to near zero, while Y changes to
about –0.54 V (negative of the previous X reading).
7 Use the knob to adjust the reference phase
until Y is zero and X is equal to the positive
amplitude.
While the reference phase is being displayed, the
knob can be used to change it. The adjustment
described should result in the phase returning to
nearly zero again.
In general, the knob is used to adjust the quantity
displayed in the Reference display (if it can be
changed). The keys below the display are used to
select the desired quantity.