Model 420 lock-in amplifier

Dual phase analogue lock-in amplifier

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Scitec Instruments Model 420 is a dual phase analogue lock-in amplifier. It is suitable for making amplitude and phase measurements.


  • Dual phase instrument
  • Differential or single-ended input
  • Gain settings from 3 µV to 1 V
  • 10 Hz to 100 kHz
  • High performance wide bandwidth input gain stage
  • Analogue meter for display of output signals
  • Output offset controls
  • Output time constants from 100 µs to 30 s
  • 1F and 2F reference signal operation
  • 90° step and fine phase control

Scitec Instruments lock-in amplifiers are available as both single phase and dual phase instruments. Single phase lock-in amplifiers have a single demodulator and can be used to make both amplitude and phase measurements. However a manual set-up procedure is required both initially and following any phase change of the output signal. Dual phase lock-in amplifiers have two demodulators which operate with a 90° phase separation. In-phase and out-of-phase components are measured simultaneously which simplifies the process of making amplitude and phase measurements. In addition the Model 420 has circuitry for calculating the vector magnitude from the output of the two demodulators. The Model 420 instruments can be used to make measurements of the input signal at both the first and second harmonic of the reference signal.

A key figure of merit used for lock-in amplifiers is dynamic reserve. The dynamic reserve of a lock-in amplifier is defined as the ratio of the noise to signal that is allowed before saturation occurs. The dynamic reserve for the Model 420 is 60dB, allowing an input signal buried in noise 1000 times larger to be recovered.



Input Signal Channel

The input signal channel amplifies the input signal to a level suitable for the demodulator. High performance, low-noise, broad-band amplifiers are used throughout. The input circuitry can accept a differential or single-ended input via the front panel signal input BNC.

  • Input: High or low impedance differential or single ended via front panel BNC
  • Sensitivity: 3 µV to 1 V (for 1 V output) switched in 1, 3, 10 steps
  • Input Impedance: 10¹²W//1 nF, dc coupled
  • Frequency: 10 Hz to 100 kHz
  • Maximum Inputs: ±16 V before input protection circuitry comes into operation. The input BNC has been tested for electrostatic discharge damage
  • Input Noise Voltage: Other lock-in amplifier manufacturers give input noise values but without providing any definition of this specification. Indeed there is no meaningful definition of this specification in relation to lock-in amplifiers. As such, Scitec Instruments does not provide noise values and other manufacturers data sheets are considered misleading. Further details about input noise are available on request
  • Gain Accuracy: 1%
  • Gain Stability: 200 ppm/°C
  • Dynamic Reserve: 60 dB limited by a maximum signal input noise voltage of 10 V


The output of the signal input stage is processed using two very high bandwidth demodulators each operating 90° apart from each other to produce the X and Y signals.

Low Pass Filter

The X and Y outputs from the demodulators are passed through two first order low pass filters and then amplified. The X and Y signals are combined to produce R, the modulus signal, where R=(X2+Y2)1/2 before output via a front panel BNC.

  • Time Constant: 100 µs to 30 s in 1, 3, 10 steps
  • All Outputs: +/-1 V output corresponds to full scale input. Short circuit protection included.
  • Front Panel Output: X, Y or R switchable output
  • Rear Panel Outputs: X, Y and R separate outputs
  • Offset: Up to 1x full scale for both X and Y channels, switchable on or off

Reference Channel

  • Frequency: 10 Hz to 100 kHz
  • Input Impedance: 5.6 M ac coupled
  • Trigger: Sine – 100 mV rms min (15 V max.) Pulse – 5 V, mark to space ratio in the range 20:1 to 1:20.
  • Acquisition Time: 10 s max.
  • Phase Control: 90° steps + fine shift in range 0° – 100°
  • Phase Drift: 0.1°/°C

The datasheet for the Model 410 and Model 420 lock-in amplifiers is available in pdf format:

Model 410 and Model 420 datasheet