• Low-noise, all analog design • No digital interference • 0.2 Hz to 200 kHz measurement range • Low-noise current and voltage inputs • Harmonic detection (f, 2f, or 3f) • Selectable input filtering SR124 Analog Lock-In Amplifier- For over a half century, the lock-in amplifier has been the instrument of choice for measuring small AC signals in the presence of noise. Early instruments were designed with analog electronics, multi-gang mechanical switches, needle indicators, etc., and measurements were often monitored with chart recorders. Microprocessor based designs emerged in the 1980s, and by the early 1990s even the lock-in’s analog demodulators were replaced by high-resolution ADCs and digital signal processors (DSP). Remote computer control, digital readouts and user-friendly front panels all resulted. The capabilities of the modern DSP lock-in amplifier in stability, dynamic reserve, and flexibility were revolutionary, making it a mainstay for researchers and engineers across multiple fields. But in moving forward, something was left behind. For a core group of users, including low-temperature researchers in particular, the new instruments became a potential source of high-frequency interference. This is best reflected in the persistence of one instrument — the PAR124A — still actively used by many researchers decades after discontinuation by its long-gone manufacturer. Back to Analog Recognizing that one size shouldn’t have to fit all, SRS is proud to introduce the SR124 Analog Lock-In Amplifier. Inspired by the best of an earlier generation’s lock-ins, but availing itself of today’s low-noise analog components and design methodologies, the SR124 is an uncompromising tour de force in low-noise, high-performance analog instrumentation. SRS Stanford Research Systems phone: (408)744-9040 www.thinkSRS.com

Architecture The SR124 design follows two basic themes. First, the signal path is entirely built from low-noise analog electronics: the best JFETs, transistors, op-amps, and discrete components. Second, configuration control is managed by a microcontroller whose system clock only oscillates during the brief moments needed to change gains or filter settings. This “clock-stopping” architecture, first introduced by SRS in the SR560 Voltage Preamplifier, eliminates the inconvenience and reliability issues associated with mechanical panel controls, and makes full remote operation of the SR124 possible....

first chosen in the reference section of the front panel — either manually or by the auto Ref Range feature. In external reference mode, the SR124 will lock to external signals with at least 100 mVpp amplitude. The reference oscillator will phase-lock to the positive zero-crossings of the external reference input. Locking at f, 2f (second harmonic), or 3f (3rd harmonic) is supported. Operated in internal mode, the reference frequency is set from the front panel with ±0.1 % accuracy. In addition to internal and external reference modes, the SR124 also has a built-in voltage controlled oscillator...

Signal Channel Voltage inputs Sensitivity Current input Input impedance Voltage Current Gain accuracy Gain stability Noise (rms) Signal filters Flat Band pass High pass Low pass Notch CMRR Dynamic reserve Low noise Normal High reserve Reference Channel Frequency range Reference input Input impedance Phase resolution Phase accuracy Harmonic detection Demodulator Output stability Low noise Normal High reserve Time constants Single-ended or differential 100 nV to 500 mV 106 or 108 V/A 10 MD + 35 pF, AC or DC coupled 1 kD to virtual ground ±1 % 100 ppm/°C (flat mode, normal reserve) 2.5 nV/VHz at...