High Frequency Lock-In Amplifier
SR844 200 MHz, dual phase RF lock-in amplifier
SR844 200 MHz Lock-In Amplifier
The SR844 is the widest bandwidth lock-in amplifier · 25 kHz to 200 MHz frequency range
available. Unlike simple down-converters, no additionalinstruments are required. And unlike analog lock-ins, no · 80 dB dynamic reserve
manual frequency range switching is necessary.
· Time constants from 100 µs to 30 ks
The SR844 provides uncompromised performance with a (6, 12, 18 or 24 dB/oct rolloff)
frequency range of 25 kHz to 200 MHz and up to 80 dB ofdrift-free dynamic reserve. It includes the many features, ease · "No Time Constant" mode
of operation and programmability that you've come to expectfrom SRS DSP lock-in amplifiers.
(10 µs to 20 µs update rate)
Digital Technology
· Auto gain, phase, reserve and offset
The SR844 uses the same advanced DSP technology found in · Two 16-bit DACs and ADCs
the SR850, SR830 and SR810 lock-in amplifiers. DSP offersmany advantages over analog instrumentshigh dynamic · Internal or external reference
reserve, low zero-drift, accurate RF phase shifts andorthogonality, and digital output filtering.
· GPIB and RS-232 interfaces
Signal Input
The SR844 has both 50 Ω and 1 MΩ inputs. The 1 MΩ inputis used with high source impedances at low frequencies, orwith a standard 10× scope probe. The 50 Ω input provides thebest RF signal matching. Up to 60 dB of RF attenuation or20 dB of RF gain can be selected in 20 dB increments. Full-scale sensitivities range from 1 Vrms (+13 dBm) to 100 nVrms · SR844 . $7950 (U.S. list)
(−127 dBm). Gain allocation can be optimized to provide upto 80 dB of dynamic reserve.
Stanford Research Systems
SR844 200 MHz Lock-In Amplifier
Ratio mode is used to normalize the signal to an externallyapplied analog voltage. It is useful to eliminate the effect of The SR844 offers both external and internal reference operation. In both cases, the entire 25 kHz to 200 MHzfrequency range is covered without any manual range Transfer function measurements can be easily made from the selection. The external reference input has an auto-threshold front panel by a programmable scan of up to 11 frequencies.
feature which locks to sine, square or pulsed signals. The Setups and offsets are recalled at each frequency in the scan.
internal reference is digitally synthesized and is adjustablewith 3-digit frequency resolution.
Analog Inputs and Outputs
Harmonic detection of the 2F component is available for both The two displays each have a user-defined output for internal and external reference modes.
measuring X, Y, R, R(dBm), θ, and X-noise or Y-noise. Twouser-programmable DACs provide −10.5 V to +10.5 V outputs A reference output (1.0 Vpp square wave into 50 Ω), which is with 1 mV resolution. These outputs may be set from the front phase synchronous with the lock-in reference, is available in panel or via the computer interfaces.
In addition, there are two general-purpose analog inputs.
Output Filters
These are 16-bit ADCs which can be displayed on the frontpanel, read over the interface or used to ratio the input signal.
Time constants from 100 µs to 30 ks can be selected with achoice of 6, 12, 18 or 24 dB/oct rolloff. For high bandwidth, Internal Memory
real-time outputs, the filtering can be by-passed entirely. Inthis "No Filter" mode, the effective time constant is about The SR844 has two 16,000 point memory buffers for 30 µs with the analog outputs updating every 10 to 20 µs.
recording (rates to 512 samples/s) the time history of eachdisplayed measurement. Data may be transferred from the Ease of Operation
buffers using either interface. A trigger input is also providedto synchronize data recording with external events.
The SR844 is easy to use. All instrument functions are setfrom the front-panel keypad, and the knob is used to quicklyadjust parameters. Up to nine different instrumentconfigurations 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 dataacquisition systems.
Useful Features
Ordering Information
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 The offset and expand features are useful for evaluating smallfluctuations in your signal. The input is nulled with the auto-offset function and output expand increases the resolution byup to 100×.
Stanford Research Systems
SR844 Specifications
Signal Channel
−9.5 dBc @ 3 × ref, −14 dBc @ 5 × ref, Displays
ing and computation of R. The ratio input is normalized to 1 V and has a Reference Channel
Channel 1 and Channel 2 Outputs
Acquisition time <10 s (auto-ranging, any frequency) 25 kHz to 200 MHz square wave1.0 Vpp nominal into 50 Ω Auxiliary Inputs and Outputs
Rear panel (TTL) 25 kHz to 1.5 MHz, 0 to +5 V Demodulator
IEEE-488.2 and RS-232 interfaces are standard. Digital displays have no zero-drift.
drift for all dyn. reserve settings.
Stanford Research Systems
SR844 200 MHz Lock-In Amplifier
About RF Lock-In Amplifiers
The SR844 RF lock-in amplifier utilizes a combination of low-pass filtered (with 6, 12, 18 or 24 dB/oct filter slopes) analog and digital techniques to obtain maximum allowing the original signal amplitude and phase to be performance over a wide frequency range. Since it is not feasible to use pure digital techniques at the SR844'smaximum 200 MHz operating frequency, analog down- Note that the SR844 uses a square wave mixer, not a sine conversion is used to transform the signal frequency to a range wave mixer like other SRS lock-in amplifiers. This is because precision sine generation is impractical with currenttechnology over the SR844's operating range. The effect of A block diagram of the SR844 lock-in amplifier is shown using a square wave mixer is that the lock-in will respond not below. The RF input signal passes through adjustable RF only at the reference frequency, but at all the Fourier attenuators and gain stages depending on the selected input components of the square wave reference. Since a square sensitivity. The signal is then mixed with two reference wave consists of odd harmonics with amplitudes 1/3, 1/5, 1/7, signals which differ in phase by 90°.
etc., the SR844 will respond at odd multiples of the referencefrequency as well as at the reference frequency as itself. This The reference signals are generated from either the external usually does not present a problem as long as it is understood.
reference input or the built-in frequency synthesizer, using aphase-lock loop (PLL) circuit. If the reference frequencywere exactly at the signal frequency, the output of the mixerswould be at DC. Since it is difficult to build drift and offsetfree amplifier chains, the SR844 chops the reference signals ata chopping frequency (fc) which is chosen to be fast relativeto the fastest time constants, yet slow relative to the inputsignal frequency. The IF amplifier and filter chain can now beAC coupled, eliminating DC offset and drift problems.
Once the in-phase and quadrature IF signals have beenamplified and low-pass filtered, the signals are digitized bytwo precision 16-bit analog-to-digital converters. The digitalIF signals are ratioed (if ratio mode is selected) and digitally SR844 block diagram
Stanford Research Systems


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