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7124 Precision DSP Lock-in Amplifier

Model 7124 – OverviewSignal Recovery 7124-315
The 7124 is the only DSP lock-in with a fiber optically separated analog front end giving you the best of both technologies.  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 More recently the analog phase sensitive detectors forming the heart of the instrument have been replaced by DSP (digital signal processing) designs, further improving performance.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 the next generation amplifier from the widely popular PARC-124. 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.

 

Signal Channel – Remote Connection Unit (7124RCU)
Voltage Input
ModesA only, -B only or Differential (A-B)
Frequency Response0.5 Hz < F < 150 kHz (–3dB)
Full-scale Sensitivity2 nV to 1 V in a 1-2-5 sequence (e.g. 2 nV, 5 nV, 10 nV, 20 nV, etc.)
Impedance – FET input10 MΩ // 25 pF, AC or DC coupled
Indepance – Bipolar input10 kΩ // 25 pF, input must be DC coupled
Max safe input±12.0 V
Voltage Noise5 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
ModeLow Noise (10E8 V/A) or Wide Bandwidth (10E6 V/A)
Full-scale Sensitivity
Low Noise2 fA to 10 nA in a 1-2-5 sequence
Wide Bandwidth2 fA to 1 µA in a 1-2-5 sequence
Frequency Response (-3 dB):
Low Noise0.5 Hz < F < 500 Hz minimum
Wide Bandwidth0.5 Hz < F < 50 kHz minimum
Impedance
Low Noise< 2.5 kΩ @ 100 Hz
Wide Bandwidth< 250 Ω @ 1 kHz
Noise
Low Noise13 fA/√Hz @ 500 Hz
Wide Bandwidth130 fA/√Hz @ 1 kHz
Gain Accuracy±2.0% typ, midband
Either Input Mode
Line FilterFilter can be set to attenuate 50/60 Hz, 100/120 Hz, or both frequency bands
Max. Dynamic Reserve> 100 dB
GroundingBNC 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 Impedance1 kΩ
Signal Channel – Main Console
Auxiliary Input
ModeSingle-ended voltage mode input
Impedance10 MΩ // 25 pF
Maximum Safe Input±12.0 V
Full-scale1 V
Sensitivity
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 Impedance1 kΩ
Reference Channel
Reference Input – Remote Connection Unit or Main Console
TTL Input (rear panel)
Frequency Range0.5 Hz to 150 kHz
Analog Input (front panel)
Impedance1 MΩ // 30 pF
Sinusoidal Input Level1.0 V rms*
Frequency Range0.5 Hz to 150 kHz
Squarewave Input Level250 mV rms
Frequency Range2 Hz to 150 kHz
Reference Channel
Phase Set Resolution0.001° increments
Phase Noise at 100 ms TC, 12 dB/octave slope:
Internal Reference< 0.0001° rms
External Reference< 0.01° rms @ 1 kHz
Orthogonality90° ± 0.0001°
Acquisition Time
Internal Referenceinstantaneous acquisition
External Reference2 cycles + 1 s
Reference Frequency Meter Resolution4 ppm or 1 mHz, whichever is the greater
Demodulator and Output Processing
Output Zero Stability
Digital OutputsNo zero drift on all settings
DisplaysNo zero drift on all settings
DAC Analog Outputs< 100 ppm/°C
Harmonic Rejection-90 dB
Output Filters
Time Constant10 µs to 100 ks in a 1-2-5 sequence
Slope (roll-off):
TC < 5 ms6 or 12 dB/octave
TC > 5 ms6, 12, 18 or 24 dB/octave
Synchronous FilterAvailable for F < 20 Hz
OffsetAuto and Manual on X and/or Y: ±300% full-scale
Phase Measurement Resolution< 0.01º
Reference MonitorTTL signal at current reference frequency, internal or external
Oscillator – General
Frequency
Range0.5 Hz to 150 kHz
Setting Resolution1 mHz
Absolute Accuracy± 50 ppm
Amplitude
Range1 mV to 5 V
Setting Resolution1 mV
Output Impedance50 Ω
Sweep
Frequency Sweep
Output Range0.5 Hz to 150 kHz
LawLinear or Logarithmic
Step Rate1 kHz maximum (1 ms/step)
Amplitude Sweep
Output Range0.000 to 1.000 V rms
LawLinear
Step Rate
Main Console20 Hz maximum (50 ms/step)
RCU1 Hz maximum (1 s/step)
Oscillator Output – Remote Connection Unit
Amplitude
Accuracy±1.0% typ
Stability100 ppm/ºC
Distortion (THD)-80 dB @ 1 kHz and 100 mV rms
Oscillator Output – Main Console
Amplitude
Accuracy±0.2% typ
Stability50 ppm/ºC
Distortion (THD)-80 dB @ 1 kHz and 100 mV rms
Data Storage Buffer
Size100,000 data points
Max. Storage Rate
Fast Modeup to 1 MHz (X1, Y1, X2, Y2, ADC1, Demod I/P 1, Demod I/P 2)
GeneralMain Console7124 RCU
Power Requirements
Voltage110/120/220/240 VAC110/120/220/240 VAC
Frequency50/60 Hz50/60 Hz
Power40 VA max15 VA max
RCU with option 7124/99
Voltage±24.0 V DC
Current+300 mA / –170 mA
Dimensions
Width15½” (390 mm)15½” (390 mm)
Depth7¼” (185 mm)7¼” (185 mm)
Height
With feet7¼” (185 mm)3″ (75 mm)
Without feet6½” (170 mm)2½” (64 mm)
Weight12.8 lb (5.8 kg)7.9 lb (3.6 kg)
Preliminary specifications subject to change without notice

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