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.
Features:
- 1 mHz to 102.4 kHz range
- >100 dB dynamic reserve
- 0.001 degree phase resolution
- Time constants from 10 µs to 30 ks
(up to 24 dB/oct rolloff) - Auto-gain, phase, reserve and offset
- Data logging (up to 65k samples)
- Smoothing, curve fitting & statistics
- GPIB & RS-232
SR850 100 kHz DSP Lock-In Amplifier
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 flat panel
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's display supports a large selection
of 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. Standard RS-232 and GPIB interfaces make it easy to transfer data to your computer.
Numeric Readout with Bar Graph |
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 12 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.
