Sr830 dsp lock in amplifier manual

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Stanford Research Systems

· 1 mHz to 102.4 kHz frequency range

· >100 dB dynamic reserve

· 5 ppm/°C stability

· 0.01 degree phase resolution

· Time constants from 10 µs to 30 ks

(up to 24 dB/oct rolloff)

· Auto-gain, -phase, -reserve and -offset

· Synthesized reference source

· GPIB and RS-232 interfaces

· SR810 ... $3850

(U.S. list)

· SR830 ... $4750

(U.S. list)

The SR810 and SR830 DSP Lock-In Amplifiers provide high

performance at a reasonable cost. The SR830 simultaneously

displays the magnitude and phase of a signal, while the SR810

displays the magnitude only. Both instruments use digital

signal processing (DSP) to replace the demodulators, output

filters, and amplifiers found in conventional lock-ins. The

SR810 and SR830 provide uncompromised performance with

an operating range of 1 mHz to 102 kHz and 100 dB of drift-

free dynamic reserve.

Input Channel

The SR810 and SR830 have differential inputs with 6 nV/√Hz

input noise. The input impedance is 10 MΩ, and minimum

full-scale input voltage sensitivity is 2 nV. The inputs can

also be configured for current measurements with selectable

current gains of 10

6

and 10

8

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. 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 design of these lock-ins has such inherently

large dynamic reserve that no band pass filter is needed.

Extended 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

SR810 & SR830 DSP Lock-In Amplifiers

Digital Lock-In Amplifiers

SR810 and SR830 — DSP lock-in amplifiers

SR830 DSP Lock-In Amplifier

WHAT IS A LOCK-IN AMPLIFIER?

Lock-in amplifiers are used to detect and measure

very small AC signals - all the way down to a few

nanovolts! Accurate measurements may be made

even when the small signal is obscured by noise

sources many thousands of times larger.

Lock-in amplifiers use a technique known as

phase-sensitive detection to single out the compo-

nent of the signal at a specific reference frequency

AND phase. Noise signals at frequencies other

than the reference frequency are rejected and do

not affect the measurement.

Let's consider an example. Suppose the signal is a

10 nV sine wave at 10 kHz. Clearly some amplifi-

cation is required. A good low noise amplifier will

have about 5 nV/√Hz of input noise. If the amplifier

bandwidth is 100 kHz and the gain is 1000, then

we can expect our output to be 10 µV of signal

(10 nV x 1000) and 1.6 mV of broadband noise

(5 nV/√Hz x √100 kHz x 1000). We won't have

much luck measuring the output signal unless we

single out the frequency of interest.

If we follow the amplifier with a band pass filter

with a Q=100 (a VERY good filter) centered at

10 kHz, any signal in a 100 Hz bandwidth will be

detected (10 kHz/Q). The noise in the filter pass

band will be 50 µV (5 nV/√Hz x √100 Hz x 1000)

and the signal will still be 10 µV. The output noise

is much greater than the signal and an accurate

measurement can not be made. Further gain will

not help the signal to noise problem.

Now try following the amplifier with a phase-

sensitive detector (PSD). The PSD can detect the

signal at 10 kHz with a bandwidth as narrow as

0.01 Hz! In this case, the noise in the detection

bandwidth will be only 0.5 µV (5 nV/√Hz x √.01 Hz

x 1000) while the signal is still 10 µV. The signal to

noise ratio is now 20 and an accurate measure-

ment of the signal is possible.

What is phase-sensitive detection?

Lock-in measurements require a frequency refer-

ence. Typically an experiment is excited at a fixed

frequency (from an oscillator or function generator)

and the lock-in detects the response from the

SR830 BASICS

experiment at the reference frequency. In the dia-

gram below, the reference signal is a square wave

from a function generator. If the sine output from

the function generator is used to excite the experi-

ment, the response might be the signal waveform

shown below. The signal is V

The SR830 generates its own sine wave, shown

as the lock-in reference below. The lock-in refer-

The SR830 amplifies the signal and then multiplies

it by the lock-in reference using a phase-sensitive

detector or multiplier. The output of the PSD is

simply the product of two sine waves.

The PSD output is two AC signals, one at the dif-

If the PSD output is passed through a low pass

filter, the AC signals are removed. What will be

left? In the general case, nothing. However, if ω

, the difference frequency component

will be a DC signal. In this case, the filtered PSD

. This might be the sync output