INTRODUCTION
The Model
128A Lock-In
Amplifier enables the accurate
measurement of signals contaminated by broad band noise,
power line pickup, frequency drift, or other sources of
interference. It does this by means of an extremely narrow
band detector which has the center of its passband locked to
the frequency of the signal to be measured. Because of the
frequency lock and narrow bandwidth, large improvements in
signal-to-noise ratio can be achieved, allowing the signal
of interest to be accurately measured, even in situations
where it is completely masked by noise.
Signals applied to the input (single-ended or differential)
i routed through a series of amplifiers which allow
.ull-scale sensitivity ranges down to one microvolt. Switch
selectable low-pass and high-pass filters allow considerable
noise reduction ahead of the phase-sensitive detector. This
pre-detector noise reduction can be further enhanced by
making use of the optional
Plug-In (internal) selective
amplifier. At the phase sensitive detector, the signal is
compared with the reference signal derived from the
experiment. Only those signal components which are
synchronous with the reference yield a net dc detector
output. Noise and other non-synchronous signals do not
contribute a net dc output, but only ac fluctuations which
can be reduced to any arbitrary value according to the
amount of filtering selected with the Time Constant switch.
This switch allows time constants as large as 100 seconds to
be selected, with provision for achieving larger externally
determined time constants if necessary. Post-detector dc
amplifiers drive the panel meter and signal output
connectors. Other features include provision for calibrated
zero suppression of up to 10 x full scale, a two-position dc
prefilter, and the capability of driving the reference input
the detector at double the frequency of the signal applied
to the Reference Input connector to facilitate second
harmonic measurements. An optional
Plug-In Oscillator
(internal) is available for use in applications where the
experiment does not produce a reference signal itself, but
is capable of being driven by a signal furnished by the
Lock-In
Amplifier.
With its wide range of capabilities and ease of operation,
the Model
128A Lock-In
Amplifier should find extensive
application in situations where the accurate measurement of
signals is complicated by the presence of noise and
interference.
2.2 SPECIFICATIONS
SIGNAL CHANNEL
(1) INPUT TYPE
Single-ended or differential as selected by front-panel switch.
(2) INPUT IMPEDANCE
100 Mi2 shunted by no more than 20 pF.
(3) SENSITIVITY
12 full-scale ranges in 1-2.5-10 sequence from 1 jjV to 250 mV.
(4) FREQUENCY RANGE 0.5 Hz to 100 kHz.
(5) COMMON MODE REJECTION At least 100 dB at 1 kHz.
(6) MAXIMUM COMMON MODE VOLTAGE
3 V pk-pk to 20 kHz; then —6 dB/octave above 20 kHz.
(7) DETECTOR BIAS
Internal network allows dc bias current of either polarity
to be provided at the "A" Input to facilitate operation with
diode detectors which require biasing. (See page VII-3 and
Parts Location Diagram on page VII-2.)
(8) NOISE
At 1 kHz the signal channel noise will not exceed 10 nV/Hz1/a.
(9) LOW PASS
Filter
Switch selectable 6 dB/octave low-pass
Filter which can be
set to 3 dB down frequencies of 100 Hz, 10 kHz, or MAX
(greater than 100 kHz).
(10) HIGH PASS
Filter
Switch selectable 6 dB/octave high-pass
Filter which can be
set to 3 dB down frequencies of 50 Hz, 5 Hz, or MIN (below
0.5 Hz).
(11) OVERLOAD DETECT
Front-panel indicator lights if applied signal plus noise is
large enough to cause overload at any of several critical
overload monitor points.
(12) GAIN STABILITY 0.1%/°C.
(13) GAIN LINEARITY 0.05%.
(14) OVERALL GAIN ACCURACY ±2%.
REFERENCE CHANNEL
The Model
128A reference channel automatically locks onto
and tracks an applied reference signal over the entire
operating frequency range of the instrument. As a result,
the instrument is immune to frequency and phase shifts as
long as the reference and signal to be recovered change
together.
(1) TRACKING RANGE
5 Hz to 100 kHz (FAST) or 0.5 Hz to 100 kHz (SLOW) as
determined by the setting of two internal switches. Faster
lock-on time and slewing rate obtained with switches set to
FAST make this range preferable except when operating below
5 Hz.
(2) MODES
Either of two modes, f and 2f, can be selected by means of a
front-panel switch. In the "f" position, the phase-sensitive
detector is driven at the same frequency as the applied
reference signal. In the "2f" position, the phase-sensitive
detector is driven at twice the frequency of the applied
reference signal to facilitate second harmonic measurements.
(3) INPUT IMPEDANCE
10 Mf2 shunted by no more than 20 pF.
(4) MINIMUM REFERENCE SIGNAL REQUIREMENT
100 mV pk-pk, any waveshape crossing its mean only twice
each cycle. Minimum time required on either side of the mean
is 100 ns. Amplitude excursions must be at least 50 mV on
each side of the mean. Maximum input signal is 5 V
(pk-to-mean). Best phase accuracy is obtained with a 1 V rms
sinewave.
(5) LOCK-ON TIME
A function of internal switch setting as follows.
Selected Range Lock-On Time
SLOW (0.5 Hz to 100 kHz)............. 20 sec. per octave
FAST (5 Hz to 100 kHz)...................2 sec. per octave
(6) PHASE
Calibrated Phase controls allow the phase of the reference
drive to the Phase-Sensitive Detector to be set at any angle
relative to the input signal. The controls consist of a
Phase Dial with a range of 100° and a Phase Quadrant switch
which provides incremental phase shifts of 90°. The phase shift
accuracy of the dial is better than 0.2° over the entire
frequency range. The resolution of the dial is better than
0.1°. The incremental phase shifts provided by the Quadrant
switch are accurate to 0.2°. The overall phase accuracy of
the instrument, including shifts in both the reference and
signal channels, is typically better than 5°.
(7) DETECTOR BIAS
Internal network allows dc bias current of either polarity
to be provided at the REF. IN connector to facilitate
operation in situations where the reference signal is taken
from diodes requiring biasing. (See page VII-6 and Parts
Location Diagram on page VII-5.)
PHASE SENSITIVE DETECTOR, DC
Amplifier
(1) OUTPUT DRIFT
0.1%/ °C.
(2) OVERLOAD CAPABILITY
1000 times full scale up to a maximum at the input of 650 mV
rms. Overload capability is defined as the ratio, at the
input of the Model
128A
of the maximum pk-pk non coherent
signal which can be applied without overloading the Model
128A to the pk-pk coherent signal required to yield full
scale Model
128A output. Note that, expressed as the ratio
of the pk-pk non-coherent signal to the rms value of the
coherent signal required for full-scale output, this number
can be as great as 2800. Maximum acceptable signal is a 650
mV rms sinewave.
(3) NON COHERENT REJECTION
50 ppm maximum. Non-coherent rejection is defined as that
offset which results from applying a non-coherent signal
having a pk-pk amplitude 1000 times the pk-pk amplitude of
the coherent signal required to obtain full-scale output.
Example: With a non coherent signal applied having a pk-pk
amplitude 1000 times the pk-pk coherent signal required to
obtain full-scale output, there will occur an offset at the
output caused by the non coherent input signal. The
amplitude of this offset will be no greater than:
50 x 10“6 x 1000
= 50 x 10~3 of f.s. output = 50 mV (f.s. = 1 V)
(4) TIME CONSTANT
Front-panel switch allows selection of 6 dB/octave
Filter
time constants of 1 ms, 10 ms, and .1 s through 100 s in
1-3-10 sequence. Also MIN (time constant =0.7 ms) and EXT,
which allows time constants longer than 100 s to be achieved
by means of external
Comments