SR810 & SR830 DSP Lock-in Amplifiers
The SR810 and SR830 are part of the SRS family of DSP lock-in amplifiers. Using the same revolutionary DSP technology pioneered by SRS in the SR850 Lock-In Amplifier, the SR810 and SR830 deliver unmatched digital performance at a price below that of most conventional lock-in amplifiers. While sharing most features and specifications, the SR810 and SR830 differ in a few important respects. In particular, the SR830 has two data displays, allowing the simultaneous display of X and Y, or R and Θ, while the SR810 has only a single data display (although the SR810 allows access to all parameters, including X,Y, R and Θ over the computer interfaces.)
Like their predecessor, the SR850, the SR810 and SR830 offer performance unmatched by any analog lock-in. Both offer 100 dB of true dynamic reserve without the need for input prefiltering. Both offer time constants from 10 µs to 30 ks with 6, 12, 18 and 24 dB/octave filter rolloff. And both include a precision synthesized reference source with 25 ppm frequency accuracy and <-80 dBc distortion. Of course, RS-232 and GPIB interfaces capable of reading and setting all instrument functions are standard.
The SR810 and SR830 both have a fully differential input with only 6 nV/√Hz of input noise at 1 kHz and an input impedance of 10 MΩ. The minimum full scale input voltage is is 2 nV, and the lock-ins both include a current input amplifier with a switchable gain of 106 or 108 Volts/Amp. Line (50 Hz or 60 Hz) and 2x line filters are included to reduce line related interference, and the SR810's and SR830's digital architecture eliminate the need for input bandpass filters. Digital design makes it possible to obtain 100 dB of dynamic reserve without the noise and phase error introduced by input bandpass filters.
The heart of the SR810 and SR830, like any lock-in amplifier, is the demodulator, which multiplies the input signal by the reference signal and filters the result. The demodulator of the SR810 and SR830 is a DSP processor capable of performing 16 million 24-bit multiplications and additions every second. In the case of the SR810 and SR830 the input is digitized at 256 kHz by a precision 18-bit A/D convertor. It is this digital demodulation technique which is at the root of so many of the SR810 and SR830's performance advantages. The digital technique eliminates the drift, distortion, and aging that are inherent in all analog demodulation designs and allow the SR810 and SR830 to achieve up to 100 dB of dynamic reserve. Dynamic reserve, a key figure of merit for lock-in amplifiers, is a specification often mentioned by lock-in manufacturers without offering a precise definition. At Stanford Research Systems Systems, dynamic reserve is defined as follows:
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.
It is always important to keep in mind that many manufacturers do not use this conservative definition of dynamic reserve. Figures may be quoted for interfering signals that are outside the frequency range of the instrument and therefore easier to reject. Some manufacturers define the largest interfering signal as that necessary to overload the instrument, at which point the lock-ins output accuracy may be so degraded as to make the reading useless, The SR810 and SR830 both have up to 100 dB of true available dynamic reserve.
The digital signal processor also handles the task of output filtering, allowing time constants from 10 µs to 30,000 s to be selected, with a choice of 6, 12, 18 and even 24 dB/oct filter rolloff. For low frequency measurements (below 200 Hz), synchronous filters can be engaged to notch out multiples of the reference frequency. Since the harmonics of the reference have been eliminated (notably 2F), effective output filtering can be achieved with much shorter time constants. This is particularly useful at low frequencies, where the proximity of 2f components to the signal frequency often forces users of conventional analog lock-ins to employ very long time constants and consequently extend measurement times.
Digital Phase Shifting
The clumsy analog phase shifting circuits found in conventional lock-ins have been replaced in the SR810 and SR830 with a precise numerical calculation performed by the DSP processor. This allows phase to be measured with 0.01° resolution and the X and Y outputs to be orthogonal to 0.001°. This represents a significant improvement over analog instruments.
The built-in direct digital synthesis (DDS) source generates a very low distortion (-80 dBc) reference signal. Single frequency sinewaves can be generated from 1 mHz to 102 kHz with 4 1/2 digits of frequency resolution. Both frequency and amplitude can be set from the front panel or from a computer. When using an external reference, the synthesized source is phase locked to the reference signal.
Unlike some lock-in amplifiers, the SR810 and SR830 are simple to use. All instrument functions are set from the front panel keypad, and a spin knob is provided to quickly adjust parameters. The SR830 has two data displays which can be quickly configured to show X, Y, R, Θ, Xnoise, Ynoise or either of the two aux inputs. The SR810 has a single data display. Both the SR810 and SR830 have an additional LED display which can be set to display the reference frequency, phase, or amplitude.
Up to nine different instrument configurations can be stored in non-volatile RAM for fast and easy instrument setup. Standard RS-232 and GPIB interfaces provide communication with computers. All functions can be controlled and read through the interfaces.
Auto Gain, Phase and Offset
Auto-functions allow parameters needing frequent adjustment to automatically be set by the instrument. Gain, phase, offset and dynamic reserve are each quickly optimized with a single key press. The offset and expand features are useful when examining small fluctuations in a measurement. The input signal is quickly nulled with the auto-offset function, and resolution is increased by expanding around the relative value by up to 100 times. Harmonic detection is no longer limited to the 2F component. Digital design allows any harmonic (2F, 3F, ... nF) up to 102 kHz to be measured without changing the reference frequency.
Analog Inputs and Outputs
Both instruments have a user-defined output for measuring X, R, X-noise, Aux1, Aux 2 or the ratio of the input signal to an external voltage. The SR830 has a second user-defined output that measures Y, Θ, Y-noise, Aux 3, Aux 4 or ratio. The SR810 and SR830 both have X and Y analog outputs (rear panel) that are updated at 256 kHz. Four auxiliary 16-bit inputs are provided for general purpose use and can be read from the front panel or the computer interface. Four programmable 16-bit outputs provide voltages from -10.5 V to +10.5 V and can be set via the front panel or computer interfaces.
The SR810 has an 8,000 point memory buffer for recording the time history of a measurement at rates up to 512 samples/sec. The SR830 has two 16,000 point buffers to simultaneously record two measurements, like R and Θ. Data is transferred from the buffers using the computer interfaces. A trigger input is also provided to externally synchronize data recording.
The SR810 and SR830 DSP Lock-In Amplifiers from Stanford Research Systems Systems offer outstanding performance, features, and value. Specification by specification, feature by feature, no other lock-in can compare.
|Voltage inputs||Single-ended or differential|
|Sensitivity||2 nV to 1 V|
|Current input||106; or 108; V/A|
|Voltage input||10 MΩ + 25 pF, AC or DC coupled|
|Current input||1 kΩ to virtual ground|
|Gain accuracy||±1 % (±0.2 % typ.)|
|Noise||6 nV/√Hz at 1 kHz|
|0.13 pA/√Hz at 1 kHz (106] V/A)|
|0.013 pA/√Hz at 100 Hz (108] V/A)|
|Line filters||50/60 Hz and 100/120 Hz (Q=4)|
|CMRR||100 dB at 10 kHz, decreasing by 6 dB/oct above 10 kHz|
|Dynamic reserve||>100 dB (without prefilters)|
|Frequency range||0.001 Hz to 102.4 kHz|
|Reference input||TTL or sine (400 mVpp min.)|
|Input impedance||1 MΩ, 25 pF|
|Phase resolution||0.01° front panel,0.008° through computer interfaces|
|Absolute phase error||<1°|
|Relative phase error||<0.001°|
|Orthogonality||90° ± 0.001°|
|Int. reference||Synthesized, <0.0001° rms at 1 kHz|
|Ext. reference||0.005° rms at 1 kHz, 100 ms, 12 dB/oct|
|Phase drift||<0.01°/°C below 10 kHz,|
|<0.1°/°C, 10 kHz to 100 kHz|
|Harmonic detection||2F, 3F, ... nF to 102 kHz (n<19,999)|
|Acquisition time||(2 cycles + 5 ms) or 40 ms, whichever is greater|
|Digital outputs and display||no drift|
|Analog outputs||<5 ppm/°C for all dynamic reserve settings|
|Harmonic rejection||-90 dB|
|Time constants||10 µs to 30 ks (6, 12, 18, 24 dB/oct rolloff). Synchronous filtering available below 200 Hz.|
|Range||1 mHz to 102 kHz|
|Accuracy||25 ppm + 30 µHz|
|Frequency resolution||4½ or 0.1 mHz whichever is greater|
|Distortion||-80 dBc (f < 10 kHz)|
|-70 dBc (f > 10 kHz) at 1 Vrms amplitude|
|Amplitude||0.004 to 5 Vrms into 10 kΩ (2 mV resolution), 50 Ω output impedance, 50mA maximum current into 50 Ω|
|Amplitude stability||50 ppm/°C|
|Outputs||Sine and TTL (when using an external reference, both outputs are phase locked to the external reference)|
|Sweeps||Linear and log|
|Channel||4½-digit LED display with 40-segment LED bar graph. X, R, X-noise, Aux 1 or Aux 2. The display can also be any of these quantities divided by Aux 1 or Aux 2.|
|Offset||X, Y, R can be offset up to ±105 % of full scale.|
|Expand||X, Y, R can be expanded by 10× or 100×.|
|Reference||64½-digit LED display|
|Inputs and Outputs|
|CH1 output||±10 V output of X, R, X-noise, Aux 1 or Aux 2. Updated at 512 Hz.|
|X, Y outputs||In-phase and quadrature components|
|Rear panel||±10 V, updated at 256 kHz|
|Aux. A/D inputs||4 BNC inputs, 1 mV resolution, ±10 V, sampled at 512 Hz|
|Aux. D/A outputs||4 BNC outputs, 1 mV resolution, ±10 V|
|Sine out||Internal oscillator analog output|
|TTL out||Internal oscillator TTL output|
|Data buffer||8k point buffer. Data is recorded at rates to 512 Hz and read through the computer interfaces|
|Trigger In (TTL))||Trigger synchronizes data recording|
|Remote pre-amp||Provides power to the optional SR550, SR552 and SR554 preamps|
|Interfaces||IEEE-488.2 and RS-232 interfaces standard. All instrument functions can be controlled and read through IEEE-488.2 or RS-232 interfaces.|
|Power||40 W, 100/120/220/240 VAC, 50/60 Hz|
|Dimensions||17" × 5.25" × 19.5" (WHL)|
|Warranty||One year parts and labor on defects in materials and workmanship|