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The SR850 is a digital lock-in amplifier based on an innovative
DSP (Digital Signal Processing) architecture. The SR850 boasts a
number of significant performance advantages over traditional lock-in
amplifiers—higher dynamic reserve, lower drift, lower distortion,
and dramatically higher phase resolution. In addition, the CRT display
and 65,536 point memory make it possible to display and process
data in a variety of formats unavailable with conventional lock-ins.
Digital Precision
At the input of the SR850 is a precision 18-bit A/D converter which
digitizes the input signal at 256 kHz. The A/D converter, together
with a high-speed DSP chip, replace the analog demodulator (mixer),
low-pass filters and DC amplifiers found in conventional lock-ins.
Instead of using analog components, the SR850 is implemented by
a series of precise mathematical calculations which eliminate the
drift, offset, nonlinearity and aging inherent in analog components.
The same DSP chip digitally synthesizes the reference oscillator,
providing a source with less than -80 dBc distortion, 100 mHz frequency
resolution, and 2 mV of amplitude resolution.
Digital Flexibility
The SR850 has a 7" CRT display which supports a large selection
of display options. Data can be viewed numerically or graphically
in bar graph, polar plot and strip chart formats. With 65,536 points
of memory and data acquisition rates up to 512 Hz, you are able
to see exactly how your data changes in time—not just what
the current output value is. After the data has been acquired, the
SR850 offers a variety of data reduction options, such as Savitsky-Golay
smoothing, curve-fitting and statistical analysis. A built-in 3.5"
disk drive, along with standard RS-232 and GPIB interfaces, makes
it easy to transfer data to your computer.
Input Channel
The SR850 has a differential input with 6 nV/√Hz input noise.
The input impedance is 10 MΩ, and minimum full-scale input
voltage sensitivity is 2 nV. The input can also be configured for
current measurements with selectable current gains of 106
and 108 V/A. A line filter (50
Hz or 60 Hz) and a 2× line filter (100 Hz or 120 Hz) are provided
to eliminate line related interference. However, unlike conventional
lock-in amplifiers, no tracking band-pass filter is needed at the
input of the SR850. This filter is used by conventional lock-ins
to increase dynamic reserve. Unfortunately, band-pass filters also
introduce noise, amplitude and phase error, and drift. The DSP based
design of the SR850 has such inherently large dynamic reserve that
no tracking band-pass filter is needed.
Reference Channel
The reference source for the SR850 can be an externally applied
sine wave or square wave, or its own digitally synthesized reference
source. Because the internal reference source is synthesized from
the same digital signal that is used to multiply the input, there
is virtually no reference phase noise when using the internal reference.
The internal reference can operate at a fixed frequency or can be
swept linearly or logarithmically over the entire operating range
of 1 mHz to 102.4 kHz. Harmonic detection can be performed at any
integer harmonic of the reference frequency—not just the first
few harmonics.
The DSP approach also offers considerable advantages when working
with an external reference. The time to acquire an external reference
is only 2 cycles + 5 ms (or 40 ms, whichever is greater)—about
ten times faster than conventional lock-ins.
Because the SR850 uses a digital phase-shifting technique rather
than analog phase-shifters, the reference phase can be adjusted
with one millidegree resolution. In addition, the X and Y outputs
are orthogonal to within one millidegree.
Outputs and Time Constants
The output time constants on the SR850 are implemented digitally.
Low-pass-filter rolloffs of 6, 12, 18 and 24 dB/octave are available
with time constants ranging from 10 µs to 30 ks. Below 200
Hz, the SR850 can perform synchronous filtering. Synchronous filters
notch out multiples of the reference frequency—an especially
useful feature at low frequencies where the proximity of the 2f
component would otherwise require a long time constant for effective
filtering. The SR850 makes working at low frequencies a far less
time consuming task.
High Dynamic Reserve
The dynamic reserve of a lock-in amplifier at a given full-scale
input voltage is the ratio (in dB) of the largest interfering signal
to the full-scale input voltage. The largest interfering signal
is defined as the amplitude of the largest signal at any frequency
that can be applied to the input before the lock-in cannot measure
a signal with its specified accuracy.
The SR850 has the highest dynamic reserve (100 dB) of any lock-in
available. In conventional lock-in amplifiers, dynamic reserve is
increased at the expense of stability. Because of the digital nature
of the filtering and gain process in the SR850, the ultra-high dynamic
reserve is obtained without any sacrifice in stability or accuracy.
In addition, the SR850's high dynamic reserve is obtained without
the use of analog band pass filters, eliminating the noise and error
that such filters introduce.
Traces and Displays
Data acquired by the SR850 is stored in up to four user-defined
traces. Each trace can be configured as (A×B)/C, where A,
B and C are selected from X, Y, R, Θ, noise, frequency or
any of the four rear-panel auxiliary inputs. Common operations,
such as ratioing, can be performed in real time by defining an appropriate
trace. Trace values can be displayed as a bar graph with an associated
large numerical display, or as a strip chart showing the trace values
as a function of time. Additionally, you can display polar plots
showing the phasor formed by the in-phase and quadrature components
of the signal. All displays can be easily scaled from the front
panel or over the computer interfaces, and an autoscale feature
is available to quickly optimize the display. The screen can be
configured as a single large display, or as two horizontally-split
displays.
Convenient Auto Measurements
Common measurement parameters are available as single-key "auto"
functions. The gain, phase, dynamic reserve and display scaling
can all be set with a single key press. For many measurements, the
instrument can be completely configured simply by using the auto
functions.
Auxiliary A/Ds and D/As
Four rear-panel A/D inputs allow you to measure external signals
with millivolt resolution. The measured values can be incorporated
into one of the SR850's trace definitions, or can be displayed on
the front panel, or read via either computer interface. Four D/A
outputs can provide either fixed output voltages or a voltage level
which scans synchronously with the SR850's frequency scans. Both
the A/D inputs and the D/A outputs have a ±10 V range.
Analysis Features
The SR850's performance doesn't stop once data has been acquired—a
full set of data processing features is also included. Multiple-range
Savitsky-Golay smoothing can be applied to any of the trace arrays,
and statistical information (mean, variance, sum) can be calculated
for a selected trace region. A curve fitting routine calculates
best fits to lines, exponential curves, and Gaussian curves for
any portion of your data. And a trace "calculator" lets
you perform a variety of simple arithmetic and trigonometric operations
on trace data.
Interfaces and Hardcopies
The SR850 comes standard with RS-232 and GPIB interfaces. All instrument
functions can be queried and controlled via the interfaces. For
convenient debugging, characters received and sent via the interfaces
can be viewed on the front panel. Several hardcopy options are available
on the SR850. Screens can be dumped to a dot-matrix or LaserJet
compatible printer through the standard Centronics printer interface.
Displays can also be plotted on any HP-GL compatible plotter via
GPIB or RS-232.
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