3.1
Sensitivity and gain
3 – 3
found in the detailed schematics, it is useful to group the gain stages
into several blocks:
Pre-gain : The AC gain preceding the tunable input filter.
Post-gain : The AC gain following the tunable input filter.
Mixer gain : The AC gain applied at the input to the phase sensitive detector.
DC gain : The DC gain applied following the mixer, and following the
output low-pass filter.
The front panel “Signal Monitor” BNC (located in the Reserve block)
provides a bu
ff
ered copy of the signal after it is amplified by the
Pre-gain and Post-gain. The total AC gain, for all sensitivities and
reserve settings, is given in Table 3.1.
The nominal overall gain, AC and DC combined, can be calculate
from the equation
Nominal Gain
=
10 V
V
FS
(3.1)
where
V
FS
is the full-scale sensitivity, in volts.
For example,
if the sensitivity is set to 200
µ
V, the nominal overall gain is
(10 V)
/
(200
µ
V)
=
50,000.
3.1.2
Scale normalization
The SR124 is calibrated for RMS units, but the square wave demod-
ulator actually measures an absolute value average. As a result,
there can be confusion about the precise values of gain used in the
instrument.
On a properly calibrated unit, the SR124 will output exactly 10.00 V
when the input is
V
FS
, a full-scale (RMS)
sine
wave, properly phased
with the oscillator.
The input function
V
(
t
) for a full-scale sine wave is
V
(
t
)
=
√
2
V
FS
sin(2
π
f t
+
φ
)
When properly phased for maximum signal, the PSD multiplies by
the square-wave function
PSD(
t
)
=
(
+
1
:
0
≤
(2
π
f t
+
φ
) mod 2
π < π
−
1
:
π
≤
(2
π
f t
+
φ
) mod 2
π <
2
π
The time-averaged value of the product
V
(
t
)
×
PSD(
t
) can be evaluated
by integrating the first half-cycle of the input sine wave
1
π
Z
π
0
√
2
V
FS
sin(
θ
)d
θ
=
2
√
2
π
V
FS
SR124
Analog Lock-In Amplifier
