Signal Recovery- Dual Phase Lock-in Amplifier (Model 7124)

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Model 7124 – Overview

Since their invention back in the 1960's, lock-in amplifiers have been used whenever the need arises to measure the amplitude and/or phase of a signal of known frequency in the presence of noise. Unlike other AC measuring instruments they have the ability to give accurate results even when the noise is much larger than the signal – in favorable conditions even up to a million times larger.

Early instruments used analog technology, with manual controls and switches, and with output readings being taken from large panel meters. Later, microprocessors were added to give more user-friendly operation, digital output displays, and to support computer control. More recently the analog phase sensitive detectors forming the heart of the instrument have been replaced by DSP (digital signal processing) designs, further improving performance.

But the addition of this digital technology has had one unfortunate side effect, which is that the instrument itself can act as a source of digital clock and switching noise, which is typically coupled back into the experiment via the signal or internal oscillator connectors. This noise is of course rejected by the lock-in and generally does not impair its performance, but the power it dissipates in the sample or device under test can cause serious problems. This is particularly the case in low temperature physics experiments.

SIGNAL RECOVERY is therefore proud to introduce the model 7124 precision lock-in amplifier, which is particularly suited to such work. It uses a unique analog fiber optic link to interconnect a remote connection unit (RCU), to which the experiment is connected, and a main instrument console. In normal operation there are no digital clock signals within the RCU, and so it can emit no switching noise.

This architecture gives an instrument with all the advantages of the latest DSP technology for signal detection, and a powerful processor for easy user operation, as well as the low noise performance that until now has only be available in instruments of all-analog design.

Signal and Reference Connections
In normal use the 7124's signal and reference connections are made at the RCU. The signal input can be switched to operate in single ended or differential voltage mode, or in current mode with a choice of two transimpedance settings. It can also be used to switch between two single-ended voltage signals, for simple sequential measurement, under computer control, of two inputs.  In cases where further preamplification is needed then one of the SIGNAL RECOVERY remote preamplifiers can be used, with its output connected to the 7124's single ended voltage input. This flexible choice of input modes allows the best possible connection to be made to the experiment. The RCU also has both general purpose analog and TTL logic reference inputs, as well as the output for a precision DDS oscillator that generates a sinewave signal of adjustable frequency and amplitude. 

The RCU is connected to the main instrument console via a 16ft (5 m) fiber cable bundle that carries the applied signal and oscillator outputs in analog form, and control signals to the RCU and the reference signal from it, in digital format. However, the RCU contains no digital clocks or oscillators that are running continuously, and so unlike all other lock-in amplifiers that use any form of logic control, there is no digital switch noise present at the signal connectors.

  • No digital clock or switching noise present at the RCU connectors
  • 0.5 Hz to 150 kHz operating frequency range
  • Voltage and current mode inputs
  • 1.0 MHz main ADC sampling rate
  • 10 µs to 100 ks output filter time constants
  • Dual Reference, Dual Harmonic and Virtual Reference operating modes
  • USB, RS232, and Ethernet computer interfaces

Since their invention back n the 1960's, lock-in amplifiers have been used whenever the need arises to measure the amplitude and/or phase of a signal of known frequency in the presence of noise.

Model 7124 Specifications

Signal Channel – Remote Connection Unit (7124RCU)
Voltage Input
Modes A only, -B only or Differential (A-B)
Frequency Response 0.5 Hz < F < 150 kHz (–3dB)
Full-scale Sensitivity 2 nV to 1 V in a 1-2-5 sequence (e.g. 2 nV, 5 nV, 10 nV, 20 nV, etc.)
Impedance – FET input 10 MΩ // 25 pF, AC or DC coupled
Indepance – Bipolar input 10 kΩ // 25 pF, input must be DC coupled
Max safe input ±12.0 V
Voltage Noise 5 nV/√Hz @ 1 kHz
C.M.R.R. > 100 dB @ 1 kHz degrading by no more than 6 dB/octave with increasing frequency
Gain Accuracy ±1.0% typ, ±2.0% max
Distortion -90 dB THD (60 dB AC gain, 1 kHz)
Current Input
Mode Low Noise (10E8 V/A) or Wide Bandwidth (10E6 V/A)
Full-scale Sensitivity
   Low Noise 2 fA to 10 nA in a 1-2-5 sequence
   Wide Bandwidth 2 fA to 1 µA in a 1-2-5 sequence
Frequency Response (-3 dB):
   Low Noise 0.5 Hz < F < 500 Hz minimum
   Wide Bandwidth 0.5 Hz < F < 50 kHz minimum
   Low Noise < 2.5 kΩ @ 100 Hz
   Wide Bandwidth < 250 Ω @ 1 kHz
   Low Noise 13 fA/√Hz @ 500 Hz
   Wide Bandwidth 130 fA/√Hz @ 1 kHz
Gain Accuracy ±2.0% typ, midband
Either Input Mode
Line Filter Filter can be set to attenuate 50/60 Hz, 100/120 Hz, or both frequency bands
Max. Dynamic Reserve > 100 dB
Grounding BNC shield can be grounded or floated via 1 kΩ to ground
Signal Monitor Amplitude ±1 V FS. This is the signal after preamplification and filtering, but before transmission over the optical link
Output Impedance 1 kΩ
Signal Channel – Main Console
   Auxiliary Input
     Mode Single-ended voltage mode input
     Impedance 10 MΩ // 25 pF
     Maximum Safe Input ±12.0 V
     Full-scale 1 V
   Signal Monitor Amplitude ±1 V FS. This is the signal received from the Remote Connection Unit immediately prior to conversion by the main ADC
   Output Impedance 1 kΩ
Reference Channel
Reference Input – Remote Connection Unit or Main Console
TTL Input (rear panel)
Frequency Range 0.5 Hz to 150 kHz
Analog Input (front panel)
   Impedance 1 MΩ // 30 pF
   Sinusoidal Input Level 1.0 V rms*
   Frequency Range 0.5 Hz to 150 kHz
   Squarewave Input Level 250 mV rms
Frequency Range 2 Hz to 150 kHz
Reference Channel
Phase Set Resolution 0.001° increments
Phase Noise at 100 ms TC, 12 dB/octave slope:
   Internal Reference < 0.0001° rms
   External Reference < 0.01° rms @ 1 kHz
Orthogonality 90° ± 0.0001°
Acquisition Time
   Internal Reference instantaneous acquisition
   External Reference 2 cycles + 1 s
Reference Frequency Meter Resolution 4 ppm or 1 mHz, whichever is the greater
Demodulator and Output Processing
Output Zero Stability
   Digital Outputs No zero drift on all settings
   Displays No zero drift on all settings
   DAC Analog Outputs < 100 ppm/°C
Harmonic Rejection -90 dB
Output Filters
   Time Constant 10 µs to 100 ks in a 1-2-5 sequence
   Slope (roll-off):
     TC < 5 ms 6 or 12 dB/octave
     TC > 5 ms 6, 12, 18 or 24 dB/octave
Synchronous Filter Available for F < 20 Hz
Offset Auto and Manual on X and/or Y: ±300% full-scale
Phase Measurement Resolution < 0.01º
Reference Monitor TTL signal at current reference frequency, internal or external
Oscillator – General
   Range 0.5 Hz to 150 kHz
   Setting Resolution 1 mHz
   Absolute Accuracy ± 50 ppm
   Range 1 mV to 5 V
   Setting Resolution 1 mV
   Output Impedance 50 Ω
   Frequency Sweep
     Output Range 0.5 Hz to 150 kHz
     Law Linear or Logarithmic
   Step Rate 1 kHz maximum (1 ms/step)
   Amplitude Sweep
     Output Range 0.000 to 1.000 V rms
     Law Linear
   Step Rate
   Main Console  20 Hz maximum (50 ms/step)
RCU 1 Hz maximum (1 s/step)
Oscillator Output – Remote Connection Unit
Accuracy ±1.0% typ
Stability 100 ppm/ºC
Distortion (THD) -80 dB @ 1 kHz and 100 mV rms
Oscillator Output – Main Console
   Accuracy ±0.2% typ
   Stability 50 ppm/ºC
   Distortion (THD) -80 dB @ 1 kHz and 100 mV rms
Data Storage Buffer
Size 100,000 data points
Max. Storage Rate
   Fast Mode up to 1 MHz (X1, Y1, X2, Y2, ADC1, Demod I/P 1, Demod I/P 2)
Main Console 7124 RCU
Power Requirements
Voltage 110/120/220/240 VAC 110/120/220/240 VAC
Frequency 50/60 Hz 50/60 Hz
Power 40 VA max

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