|
The SR844 is the widest bandwidth lock-in amplifier available. It
provides uncompromised performance with a frequency range of 25
kHz to 200 MHz and up to 80 dB of drift-free dynamic reserve. The
SR844 includes the many features, ease of operation, and programmability
that you've come to expect from SRS DSP lock-in amplifiers.
Digital Technology
The SR844 uses the same advanced DSP technology found in the SR850,
SR830 and SR810 lock-in amplifiers. DSP offers many advantages over
analog instruments—high dynamic reserve, low zero-drift, accurate
RF phase shifts and orthogonality, and digital output filtering.
Signal Input
The SR844 has both 50 Ω and 1 MΩ inputs. The 1 MΩ
input is used with high source impedances at low frequencies, or
with a standard 10× scope probe. The 50 Ω input provides
the best RF signal matching. Full-scale sensitivities range from
1 Vrms (+13 dBm) down to 100 nVrms (-127 dBm). Up to 60 dB of RF
attenuation or 20 dB of RF gain can be selected in 20 dB increments.
Gain allocation can be optimized to provide up to 80 dB of dynamic
reserve.
Reference
The SR844 offers both external and internal reference operation.
In both cases, the entire 25 kHz to 200 MHz frequency range is covered
without any manual range selection. The external reference input
has an auto-threshold feature which locks to sine, square or pulsed
signals. The internal reference is digitally synthesized and is
adjustable with 3-digit frequency resolution.
Harmonic detection of the 2F component is available for both internal
and external reference modes.
A reference output (1.0 Vpp square wave into 50 Ω) which
is phase synchronous with the lock-in reference is available in
both external and internal mode.
Output Filters
Time constants from 100 µs to 30 ks can be selected with
a choice of 6, 12, 18 or 24 dB/oct rolloff. For high-bandwidth,
real-time outputs, the filtering can be by-passed entirely. In this
"No Filter" mode, the effective time constant is about
30 µs, with the analog outputs updating every 10 to 20 µs.
Ease of Operation
The SR844 is easy to use. All instrument functions are set from
the front-panel keypad, and the knob is used to quickly adjust parameters.
Up to nine different instrument configurations can be stored in
non-volatile memory for fast, reliable instrument setup. Standard
RS-232 and GPIB (IEEE-488.2) interfaces provide connections to your
data acquisition systems.
Useful Features
Auto-functions allow parameters that are frequently adjusted to
be set automatically. Sensitivity, dynamic reserve, phase and offset
are each quickly optimized with a simple key stroke.
The offset and expand features are useful for evaluating small
fluctuations in your signal. The input is nulled with the auto-offset
function, and output expand increases the resolution by up to 100×.
Ratio mode is used to normalize the signal to an externally applied
analog voltage. It is useful to eliminate the effect of source intensity
fluctuations.
Transfer function measurements can be easily made from the front
panel by a programmable scan of up to 11 frequencies. Setups and
offsets are recalled at each frequency in the scan.
Analog Inputs and Outputs
The two displays each have a user-defined output for measuring
X, Y, R, R(dBm), Θ, and X-noise or Y-noise. Two user-programmable
DACs provide -10.5 V to +10.5 V outputs with 1 mV resolution. These
outputs may be set from the front panel or via the computer interfaces.
In addition, there are two general purpose analog inputs. These
are 16-bit ADCs which can be displayed on the front panel, read
over the interface, or used to ratio the input signal.
Internal Memory
The SR844 has two 16,000 point memory buffers for recording the
time history of each measurement display at rates up to 512 samples/s.
Data may be transferred from the buffers using either interface.
A trigger input is also provided to synchronize data recording with
external events.
|
