Lock-in Amplifier Module(锁相放大器模块)

LIM100.DOC

Lock-in Amplifier Module LIM-100

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Wide frequency range: 100 Hz to >1 MHzWide Gain Range: 200 to 200 000Low noise: 4.5 nV/√HzVoltage Range: ± 5 V to ± 12 V

Small Size: Module for Front End Applications

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Functional Description

The LIM-100 is a complete single phase lock-in-amplifier module for the frequency rangefrom 100 Hz to 1 MHz. It contains a low noise preamplifier with an adjustable low pass filter,a TTL controlled phase sensitive rectifier, an adjustable output filter and an output amplifierwith adjustable gain. A block diagram of the LIM-100 is shown in the figure below.

activehigh/lowReferenceInputReferenceChannelReference IndicatorLEDReferenceOutputSignalInputProg. GainPreamplifierLow PassFilterPhasesensitiveRectifierOutputFilterOutputAmplifierSignalOutputGain1Gain2RangeCutoffFrequencyPreamplifierOutputFilterTime ConstantOutputOffsetOutputGainThe preamplifier has a selectable gain of 100, 1000 or 10 000. A subsequent low pass filter isprovided to suppress noise components above the interesting frequency range. Controlled bythe reference signal, the phase sensitive rectifier periodically inverts the amplified inputsignal. For input signals components having the same frequency as the reference signal, theresult is a DC signal which depends on the input signal amplitude and the phase differencebetween the input signal and the reference signal. Signals with other frequencies areconverted into AC signals, which are suppressed in the subsequent output filter. Thus thesystem works as a filter / rectifier combination with a centre frequency determined by thereference signal and a bandwidth set by the output filter.

The overall gain of the LIM-100 is the product of preamplifier gain and output gain and canbe as high as 200 000. To avoid overload by wideband noise, the low pass filter can be setfrom 1.5 kHz to 150 kHz and from 150 kHz to 15 MHz by a switch and a potentiometer. Theamplified and filtered signal is available at a monitor output connector.

The reference channel accepts TTL or CMOS levels. A LED is provided to indicate whether areference signal is present. The reference signal can be inverted by the 'active low/high'switch. This switch can be used to reverse the polarity of the output signal. The internalreference signal is available at the 'Reference Output' connector.

The output filter time constant is adjusted by a potentiometer from 30 ms to 1 s (other valuesavailable on request).

The output amplifier provides an additional signal gain from 2 to 20 (adjustable by apotentiometer). An offset adjustment is provided to compensate offsets due to amplifieroffset and switching transients at high reference frequencies.

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Specification

Signal Channel

Input Impedance

Preamplifier Gain (Switch Selectable) Preamplifier Bandwidth

1MΩ / 30 pF*

100 / 1000 / 10000*Gain = 100: 5 MHzGain = 1000: 1.5 MHzGain = 10000: 1.1 MHz50 Hz*

100 Hz: 9 nV / √Hz1kHz: 6 nV / √Hz10kHz: 4,5 nV / √Hz2 fA / √Hz

1.5 kHz to 150 kHz and

150 kHz to 5 MHz (continuouslyadjustable)10 ms to 1 s*

2 to 20 (continuously adjustable)Vee + 2V to Vcc -2V, from 1 kOhmVee + 2V to Vcc - 2V, max. 20 mA± 2 V

100 kΩ, pull up or pull down*H or L activeTTL / CMOS< 2nsLEDCMOS+5 V to +12 V-5 V to - 12 VSMA

111 x 60 x 31 mm

Low Cut-off Frequency

Input Voltage Noise (Source Impedance 50Ω)

Input Current Noise (100 Hz) Low Pass Filter

(2 Ranges Switch Selectable)Output Filter Time ConstantOutput Amplifier Gain

Preamplifier-Out Monitor SignalOutput Signal (1 kΩ Load)

Output Signal (50 Ω Load, Vcc = -Vee = 12V)Reference ChannelInput Impedance

Polarity (Switch Selectable)Threshold Phase StabilityReference Indicator

Reference Monitor OutputPower SupplyVccVeeMechanical DataConnectorsDimensions

* Other Values on Request - please specify

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Operating Instructions

Controls

The potentiometers for the low pass filter of the preamplifier, the output amplifier gain andoffset and the output filter are accessible through holes in the case. To set the switches for thepreamplifier gain, the low pass filter range and the reference polarity the case of the LIM-100must be opened. The location of the switches and potentiometers on the LIM-100 board isshown in the figure below.

Low PassFilterReferenceH/LOutputGainOutputOutputOffsetFilterSignalInputGND+VccGND-Vee10010MHzGain1FilterkHz100Gain210PreamplifierReferenceOutputInputReferenceOutputSignalOutputFigure 1: LIM ControlsInputs

To operate the LIM-100, the input signal is connected to the 'Signal Input' and the referencesignal to the 'Reference Input'. The internal connection of the signal input and the referenceinput is shown in the figure below.

+5VCi30pFC110 nR3470D1..D8R1100k(standard)+5VD1R3470HCT14R11.05 MR220 MSignal InputR2100k(alternate)D2Reference InputFig. 2: Input Configuration- 4 -

The signal input is shown in the left part of figure 2. C1 prevents DC signals from theamplifier input. The input impedance of the circuit is 1 MΩ parallel with 30 pF. Whenconnecting the LIM-100 to high impedance signal sources, the high input resistance inconjunction with the input capacitance Ci (and the cable capacitance) forms a low pass filter.To avoid high frequency cut-off in these cases, an additional termination resistor isrecommended. The input capacitance can, however, be utilized to reduce the crest factor ofextremely fast signals (e.g. from photomultipliers).

The reference input is shown on the right of figure 2. The input accepts TTL or CMOS levels.In the standard configuration, it has a 100 kΩ pull up resistor (R1). To derive a referencesignal from mechanical chopper devices, a simple fork or reflex optocoupler can be used withits output connected to the reference input. An alternate configuration uses R2 instead of R1thus pulling down the unconnected input to the 'Low' state.

To avoid damage due to overvoltage, both inputs are protected by safety diodes. The diodeswithstand a current of 1 A for 1 us. Due to the 470 Ω series resistors the inputs withstandtransients up to several 100 V.Outputs

To check gain and bandwidth of the preamplifier or the phase relationship between input andreference signal, a preamplifier and a reference output are provided. Furthermore, thereference output can be used to cascade several LIM modules by connecting one referenceoutput to the next reference input. The reference input to output delay is approximately 15 ns.The phase shift associated with this delay is negligible in most cases.

The signal output of the LIM-100 has 50 Ω output impedance and can deliver up to 40 mAoutput current. If the module output is connected to a 50 Ω load, the maximum output voltagetherefore is 2 V and the gain is reduced by a factor of 2.Operating Recommendations

As long as the noise in the input signal is comparable to the signal amplitude, all you have todo is to set the desired gain and output time constant.

However, to get maximum performance from the LIM-100 with signals lost in noise orsignals with a high crest factor, the following recommendations should be observed.

The worst case is a weak signal lost in a noise spectrum which is similar to the spectrum ofthe signal. In this case the preamplifier gain should be set to a value which just avoidssaturation by the highest noise peaks. To provide maximum dynamic range, the LIM-100should be operated with ± 12 V supplies. The preamplifier output signal should be checked byan oscilloscope and adjusted to approximately ± 5 V. To achieve the desired output voltage,the output gain has to be increased or only a fraction of the maximum output level used. Withthe maximum output gain of 20 and an output level of ± 5 V the 'dynamic reserve' is 20. If anoutput voltage of ±1 V is used the dynamic reserve is increased to 100.

Fortunately, the noise spectrum usually is much wider than the signal spectrum. In this casethe noise above the signal (reference) frequency can be cut off by the low pass filter of thepreamplifier. This yields an additional gain in the dynamic reserve equal to the square root ofthe ratio of the noise bandwidth to the signal frequency. A special case of this kind aresignals from photomultipliers that consist of short, random pulses due to the detection ofsingle photons. The spectrum of the PMT signal is constant up to 100 MHz or more, whereasthe signal frequency can be in the kHz range. The processing of such signals does not causeany problems if proper filtering is used.

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Technical Support

We are pleased to support you in all problems concerning the measurement of fast electricalor optical signals. This includes the installation of the LIM-100 module, its application toyour measurement problem, the technical environment and physical problems related to shorttime measurement techniques. Simply call us.Becker & Hickl GmbH

Intelligent Measurement and Control SystemsNahmitzer Damm 3012277 Berlin

Tel. +49 / 30 / 787 56 32FAX +49 / 30 / 787 57 34email: info@becker-hickl.dehttp://www.becker-hickl.de

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