SRS Labs SR510 - Manual (Page 33)

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The Signal Channel
The instrument has both current and voltage
inputs. The current input is a virtual ground, and
the 100 M
Ω
voltage inputs can be used as single-
ended or true differential inputs.
There are three signal filters. Each of these filters
may be switched 'in' or 'out' by the user. The first
filter is a line notch filter. Set to either 50 or 60 Hz,
this filter provides 50 dB of rejection at the line
frequency. The second filter provides 50 dB of
rejection at the first harmonic of the line frequency.
The third filter is an auto-tracking bandpass filter
with a center frequency tuned by the micro-
processor to the frequency of the signal. These
three filters eliminate most of the noise from the
signal input before the signal is amplified.
A high-gain ac amplifier is used to amplify the
signal before entering the phase sensitive
detector. The high gain which is available from this
programmable amplifier allows the lock-in to
operate with a lower gain in its dc amplifier. This
arrangement allows high stability operation even
when used on the most sensitive ranges.
Reference Channel
The processor controlled reference input
discriminator can lock the instrument's PLL to a
variety of reference signals. The PLL can lock to
sine waves or to logic pulses with virtually no
phase error. The PLL output is phase shifted and
shaped to provide a precision sine wave to the
phase sensitive detector.
Phase Sensitive Detector
The Phase Sensitive Detector is a linear multiplier
which mixes the amplified and filtered signal with
the reference sine wave. The difference
frequency component of the multiplier's output is a
dc signal that is proportional to the amplitude of
the signal. The low-pass filter which follows can
reject any frequency components which are more
than a fraction of a Hertz away from the signal
frequency.
DC Amplifier and System Gain
A dc amplifier amplifies the output of the low pass
filters. The total system gain is the product of the
ac and dc amplifier gains. The partitioning of the
system gain between these two amplifiers will
affect the stability and dynamic reserve of the
instrument. The output is most stable when most
of the gain is in the ac amplifier, however, high ac
gain reduces the dynamic reserve.
For the most demanding applications, the user
may specify how the system gain is partitioned.
However, with prefilters that are able to provide up
to 100 dB of dynamic reserve, and with chopper
stabilized dc amplifiers, most users will not be
concerned with just how the system gain is
allocated.
A Microprocessor Based Design
The instrument was designed to take full
advantage of its microprocessor controller. This
approach provides several key advantages...
The instrument may be interfaced to a laboratory
computer over the RS-232 and IEEE-488
interfaces. In addition to simply reading data from
the lock-in, the computer can control all of the
instrument settings with simple ASCII commands.
A key feature of the instrument is its four A/D
inputs and two D/A outputs. These analog I/O
ports may be used to read and supply analog
voltages to an experiment or measurement. All of
the input and output ports have a full scale range
of ±10.24VDC with 2.5 mV resolution and 0.05%
accuracy.
Computer control can eliminate set-up errors,
reduce tedium, and allow more complete data
recording and post measurement analysis. Also,
the computer can play an active role in the data
acquisition by adjusting gains, etc., in response to
changing measurement conditions.
The microprocessor based design eliminates
many analog components to improve
performance, reliability, and reduce cost. Each
unit is computer calibrated at the factory, and
calibration constants are placed in the instrument's
read-only memory. The SR510 has only one-fifth
of the analog trimming components that are found
in older designs.
Creative programming on the user's part can
extend the instrument's capabilities. For example,
the lab computer can instruct the lock-in to
measure the signal at zero and ninety degrees of
phase. Doing so allows both the amplitude and
phase of the signal of interest to be measured.