Lock-In Amplifiers DC-200 MHz

No. The range collectively spans 1 mHz to 200 MHz, but no single model covers that entire interval, and lock-in detection is based on an AC signal referenced to a known modulation frequency. The SR865A reaches 4 MHz from 1 mHz, the SR860 reaches 500 kHz, and the SR844 covers the RF band from 25 kHz to 200 MHz; the remaining models occupy lower and mid-frequency ranges. The demodulated result may be a DC output, but that does not make the input a true DC measurement. Use the actual reference-frequency limits in the variant comparison when selecting an instrument.

The primary selection difference is the upper frequency limit: 4 MHz for the SR865A and 500 kHz for the SR860. Both models start at 1 mHz, provide 1 nV to 1 V voltage output scales, include current-input scales down to 1 fA and specify 2.5 nV/√Hz voltage noise at 1 kHz. They also share 120 dB typical dynamic reserve, 1 µs to 30 ks time constants, FFT displays, 1 Mpoint capture and data streaming at rates up to 1.25 MHz. Choose the SR865A when the reference, detected harmonic or anticipated future work may exceed 500 kHz. For measurements remaining below that limit, the SR860 provides a closely aligned measurement, analysis and automation workflow.

Choose the SR844 when the signal or reference frequency exceeds the 4 MHz ceiling of the SR865A, or when a 50 Ω RF signal path is required. Its 25 kHz to 200 MHz band can be used with either a 50 Ω input or a 1 MΩ + 30 pF input, allowing selection between RF matching and higher input impedance. Minimum full-scale sensitivity varies with frequency, from 100 nV below 1 MHz to 10 µV near the 200 MHz limit. Dynamic reserve is available up to 80 dB, while filter bypass provides X and Y update rates of 10 to 20 µs. The SR844 has no built-in current input, so current-output detectors may require a suitable external transimpedance or RF preamplifier.

The SR2124 is intended for measurements in which continuous digital clock activity could couple interference into a sensitive experiment. Its clock-stopping architecture disables digital clock operation once a setting change is complete, and a front-panel lockout can prevent accidental reactivation during a measurement. The instrument covers 0.2 Hz to 200 kHz, provides dual-phase detection, 2.5 nV/√Hz voltage noise and selectable 10⁶ or 10⁸ V/A current-input gain. Its tuneable band-pass filter can raise the maximum achievable dynamic reserve to 100 dB, while F, 2F and 3F detection supports harmonic experiments. Remote commands temporarily reactivate the controller, so their timing should be considered in particularly noise-sensitive installations.

The input should be matched to whether the detector behaves primarily as a voltage source, current source or impedance-matched RF source. High-impedance voltage sources can use the 10 MΩ inputs of the SR865A, SR860, SR2124 and SR850, or the 100 MΩ AC-coupled input of the SR510; however, cable and source capacitance may then restrict bandwidth. Current-output detectors can connect to the built-in current inputs on all models except the SR844, with the available gain, input range and noise depending on the selected instrument. The SR844 provides a 50 Ω input for matched RF systems and a 1 MΩ input for higher-impedance sources. Selection should therefore consider source impedance, capacitance, required bandwidth and input noise rather than sensitivity alone.

Dynamic reserve determines how much larger an interfering signal can be than the selected full-scale signal before measurement accuracy is affected, while the output filter controls noise bandwidth and response time. The SR865A and SR860 specify 120 dB typical reserve, the SR850 exceeds 100 dB, and the SR2124 can reach 100 dB when its band-pass filter is engaged. Longer time constants reduce equivalent noise bandwidth but slow the response to signal changes, frequency sweeps or feedback-loop corrections. The SR865A and SR860 provide time constants from 1 µs to 30 ks with RC, Gaussian and phase-linear filter choices, while the SR844 offers a filter-bypass mode for faster updates. In practice, use the smallest input range that avoids overload, then choose the time constant and slope around the required settling time and signal bandwidth.

The reference system must match the experiment’s modulation arrangement as well as its frequency. The SR865A and SR860 support internal, external, dual-frequency and chopper modes, together with harmonic detection where the multiplier N is less than 99 and the detected frequency remains within the instrument limit. The SR2124 supports internal, external and rear-panel VCO operation with F, 2F and 3F detection, while the SR850 supports higher-order harmonic detection up to its 102.4 kHz ceiling. For automation, the SR865A and SR860 provide GPIB, RS-232, USB and Ethernet, whereas the other models use combinations of GPIB, RS-232, Centronics or fibre-optic control. Choose the detection mode and interface together, particularly where data streaming, legacy software or digital-noise isolation is important.

A detector cable should be discharged and connected to the amplifier before the detector or its high-voltage supply is switched on. An unterminated photomultiplier cable can accumulate a charge large enough to damage a sensitive front-end amplifier, while the SR865A and SR860 specify maximum inputs of 1 V peak or 1 µA peak. Select a voltage or current input according to the detector model, and use a suitable preamplifier when additional isolation, gain or bandwidth control is required. A 50 Ω termination can support high-frequency transmission but may add more current noise than a high-impedance or transimpedance arrangement. Detector bias isolation, cable capacitance, grounding and the amplifier damage threshold should therefore be reviewed before installation.