CN100461629C - A digital phase-lock amplifier - Google Patents

Abstract

The related digital phase-locking amplifier for detecting and amplifying small signal comprises: a signal channel connecting the processor by and ADC, a reference channel, a signal processor, and a central controller, wherein the processor includes two-path multipliers, two-path integrators. a digital-controlled phase shifter to generate two-path outputs with one for multiplier and another first to a 90deg shifter then entering another multiplier. This invention takes the multiplication and integral calculation to the treated signal and reference signal.

Description

A kind of digital lock-in amplifier

Technical field

The present invention relates to small-signal and survey multiplying arrangement field, a kind of digital lock-in amplifier of saying so more specifically.

Technical background

In recent years, the Detection of Weak Signals technology is to utilize electronics, information theory and physical method, analyze principle and rule that noise produces, the characteristics and the correlation of research measured signal, an emerging technology-oriented discipline of the small-signal that detection is flooded by noise background, lock-in amplifier has been widely used in the various fields such as industrial production, scientific research, Medical Devices as the common instrument of Detection of Weak Signals.For example, molecule speed mass spectrometer, ESEM (SEM), soft X-ray excited electrical potential energy spectrometer (SXAPS), Auger (Auger) electrospectrograph etc. have all adopted lock-in amplifier in together.The most methods of simulation that adopt of existing lock-in amplifier realize.

Summary of the invention

The present invention has designed a kind of digital lock-in amplifier, and digital lock-in amplifier is the improvement of simulation lock-in amplifier.Use state-of-the-art Digital Signal Processing, the research signal is carried out analog/digital conversion, utilize characteristics such as digital signal good stability, antijamming capability is strong, the mathematical logic computing is convenient to carry out data processing then.

The present invention is achieved through the following technical solutions its goal of the invention.

The digital lock-in amplifier of the present invention's design, comprise signalling channel, reference channel, four parts of signal processor and master controller, signalling channel is connected with signal processor by A/D converter, described signal processor comprises the multiplier that two-way is connected with A/D converter, the two-way integrator that is connected with multiplier, digital phase shifter and 90-degree phase shifter, reference channel forms two-way output through digital phase shifter, one the tunnel directly as the input of multiplier, and another road multiplier is imported again through behind the 90-degree phase shifter in another road, and the two-way integrator is respectively through forming two-way output behind two D/A converters.Signal converts digital signal to by A/D converter after amplifying through signalling channel, multiplies each other and integral operation with the reference signal of reference channel in the input signal processor again.Be a digital operation process completely, this is the maximum characteristics of digital lock-in amplifier.Because general analog multiplier and other analogue devices be subjected to external disturbance easily, and have temperature problem such as waft, in high-speed computation, accuracy can reduce greatly.These shortcomings are non-existent fully in digital system.With respect to analog signal, the antijamming capability of digital signal is very strong, particularly for the signal that changes at a high speed.Master controller is used for the automatic control to each module, with the control end of signalling channel and signal processor be connected

Described signalling channel comprises the numerical control narrow band filter and the numerical control attenuator circuit of analog gain circuit, high Q value.Numerical control narrow band filter centre frequency is by the reference frequency of master controller control automatic tracking system, and the noise filtering beyond the reference frequency, the Q value reaches 1, has improved the input noise tolerance limit of system greatly, makes the instrument scope of application wider.

The frequency range of the reference signal of reference channel is 800hz～20khz, and this scope has been included general optical detection, and the band limits of Materials Measurement, satisfies the teaching of suitable scope and uses with research and development, measures and use.Disturb owing to avoided the low-frequency range of 50hz, 100hz, system configuration is simplified greatly, and therefore the performance of product improves greatly in measuring range, and also the various lock-in amplifiers than external are low for price simultaneously.

The square wave reference signal of the design's digital phase shifter and 90-degree phase shifter output is transformed into corresponding sinusoidal reference ripple by transducer and the cosine reference wave is imported multiplier again.The two-way multiplication is respectively signal and sinusoidal reference ripple, cosine reference wave multiply each other, and output two-way result.Because there are characteristics such as certain instability and phase delay in analog multiplier, therefore general lock-in amplifier all is to adopt the reference signal and the input signal of square wave to multiply each other.Yet such way can cause the multiple harmonic of reference frequency in the input signal is loaded in the multiplied result, causes multiplied result and actual signal intensity to have certain deviation, and accuracy reduces.The digital lock-in amplifier of the present invention's design is reduced to sine wave and cosine wave to reference signal according to the frequency and the phase place of reference square-wave signal, and then carries out multiplication with input signal, has so just solved the defective that harmonic wave loads.

The present invention is described multiplier, integrator, and digital phase shifter, 90-degree phase shifter and transducer adopt programmable logic device to be packaged into a slice chip.Main arithmetic element is made into integrated chip, has very high antijamming capability.

Master controller is connected with the output that a LCDs is used for showing continuously two D/A converters.Adopt Trendline to show interior operating state of system's a period of time intuitively, for production surveying works such as infrared plated film, ac magnetic susceptibility have brought very big convenience; Simultaneously also be the information of a kind of hommization of bringing to operating personnel.

Analog gain circuit is provided with gain results output, and the numerical control narrow band filter is provided with automatic tracking filter output, and multiplier is provided with the output of phase sensitivity multiplication, and integrator is provided with integral result output.Handled easily personnel control and detection system operating state.

Analog gain circuit, numerical control narrow band filter and A/D converter are provided with the signal that is connected with master controller and overflow the test side, guarantee that signal is undistorted.

Description of drawings

Fig. 1 is a module diagram of the present invention;

Fig. 2 is the detailed system structure chart of Fig. 1;

Fig. 3 is a lock-in amplifier performance parameter graph of a relation;

Fig. 4 is the influence figure of narrow band filter to overload level.

Embodiment

The present invention is described further below in conjunction with accompanying drawing.

Digital lock-in amplifier as shown in Figure 1, comprise signalling channel 1, reference channel 2, signal processor 3 and 8 four major parts of master controller, the detection signal of simulation is from signalling channel 1 input, after signalling channel 1 amplification, convert digital signal to by A/D converter 4, input signal processor 3.Reference signal is amplified, after the shaping, filtering, is formed square wave input input signal processor 3 through reference channel 2.By signal processor 3 detection signal and reference signal are carried out forming two-way output after the computing, export the simulation output that two D/A converters 5 and 5 ' are transformed into X passage and Y passage, show continuously through master controller 8 and by LCDs 6.Master controller 8 is used for the automation control of system, is connected with the control end of signalling channel 1 with signal processor 3.

The particular hardware structure chart of digital lock-in amplifier of the present invention as shown in Figure 2.Signalling channel 1 comprises the numerical control narrow band filter 12 and the numerical control attenuator circuit 13 of analog gain circuit 11, high Q value.The major function of signalling channel 1 is that the detection signal of importing is carried out analog gain, because the amplitude of the detection signal of input is all smaller usually, generally at the mv or the uv order of magnitude, and signal all is imbedded in the various noises usually, therefore analog gain partly need be accomplished: the noise that internal system produces is low, and (frequency signal in addition of 800hz～10khz) filters the measuring frequency scope as far as possible.After the signal input, system can carry out high multiple gain (the native system maximum gain is 180dB), and the linearity that requires to gain is than higher.System has from the motion tracking narrow band filter, and promptly the reference frequency of numerical control narrow band filter 12 centre frequency automatic tracking systems is filtered external noise, has improved the input noise tolerance limit of system greatly, makes the instrument scope of application wider.Full scale sensitivity FS of the present invention is 10nv, and the system dynamics deposit is 53dB.

The problem of selecting for use about the Q value of numerical control narrow band filter 12.Lock-in amplifier is that a kind of high efficiency weak signal is extracted instrument, in fact is exactly the instrument of resisting various interference and noise.The ability of therefore resisting noise is the key parameter of lock-in amplifier.This parameter can be described with dynamic deposit.

The decibel value of the ratio of the incoming level FS when the overload level OVL that dynamic deposit is defined as lock-in amplifier exports with full scale, promptly

Dynamically deposit=201g (OVL/FS) (dB)

Concrete dynamic deposit, the relation between OVL, the FS are as shown in Figure 3.As shown in the figure, owing to contain noise in the input signal, and the power of noise is bigger than the power of actual signal usually.Can be learnt that by Fig. 3 dynamically deposit is big more, the ability that system suppresses noise is just strong more.In fact, because lock-in amplifier selects to suppress noise by frequency plot, therefore dynamic deposit is with frequency change.Among Fig. 4, left figure does not add numerical control narrow band filter 12, and signal is easy to just overflow.Right figure has added numerical control narrow band filter 12, and on the frequency range of certain width, OVL has had certain lifting.And the Q value of numerical control narrow band filter 12 is high more, and the scope that OVL promotes is big more, and middle depression is narrow more, shows that dynamic range has increased.Yet, because there is certain unsteadiness in common numerical control narrow band filter 12, cause the drift of centre frequency, the output signal instability, so the Q value of narrow band filter can not be too high.The digital lock-in amplifier of the present invention's design has added numerical control narrow band filter 12 systems.This system is undertaken digital control by single-chip microcomputer, guaranteed the stable of centre frequency fr, and filter has than higher Q value simultaneously, has reached the unification of system with stability.

The frequency range of the reference signal of input reference channel 1 is 800hz～20khz, and this scope has been included general optical detection, and the band limits of Materials Measurement, satisfies the teaching of suitable scope and uses with research and development, measures and use.Disturb owing to avoided the low-frequency range of 50hz, system configuration is simplified greatly, and therefore the performance of product improves greatly in measuring range, and also the various lock-in amplifiers than external are low for price simultaneously.Reference signal is through forming square-wave input signal processor 3 after signalling channel 1 amplification, shaping and the filtering.

Signal processor 3 comprises the multiplier 31 that two-way is connected with A/D converter 4,31 ', the two-way integrator 32 that is connected with multiplier 31,32 ', digital phase shifter 33 and 90-degree phase shifter 34, reference channel 2 forms two-way output through digital phase shifter 33, one tunnel input as multiplier 31, another road multiplier 31 ' is imported again through behind the 90-degree phase shifter 34 in another road, the square wave reference signal of digital phase shifter 33 and 90-degree phase shifter 34 outputs is transformed into corresponding sinusoidal reference ripple by transducer 35 and the cosine reference wave is imported multiplier 31 again, 31 ', two-way integrator 32,32 ' respectively through two D/A converters 5,5 ' back forms two-way output.Described multiplier 31,31 ', integrator 32,32 ', digital phase shifter 33,90-degree phase shifter 34 and transducer 35 adopt programmable logic device to be packaged into a slice chip.

Signal processor 3 is cores of lock-in amplifier.Phase Lock Technique is exactly that signal and reference signal after the gain are carried out multiplying, carries out low-pass filtering, i.e. integral operation then.

The waveform equation of supposing input is:

S _i(t)＝A _isin(ωt+φ)+B _i(t) (1)

Wherein ω is the reference frequency of system, A _iSin (ω t+ φ) is a signal section, B _i(t) be noise section

Reference signal is:

S _r(t)＝A _rsin(ωt) (3)

Input signal and reference signal multiply each other in multiplier and are obtained by the integrator integration:

$S_0=\underset{n\→\∞}{\lim }\frac{1}{\mathrm{nT}}\underset{0}{\overset{\mathrm{nT}}{\∫}}{S}_i\left(t\right)S_r\left(t\right)\mathrm{dt}$

$=\underset{n\→\∞}{\lim }\left\{\frac{1}{\mathrm{nT}}[\underset{0}{\overset{\mathrm{nT}}{\∫}}{A}_i{A}_r\sin (\mathrm{\ωt}+\mathrm{\φ})\sin \left(\mathrm{\ωt}\right)\mathrm{dt}+\underset{0}{\overset{\mathrm{nT}}{\∫}}{A}_r{B}_i\left(t\right)\sin \left(\mathrm{\ωt}\right)\mathrm{dt}]\right\}$

$=\frac{1}{2}{A}_i{A}_r\cos \mathrm{\φ}---\left(4\right)$

By result's equation as can be seen, result and time t are irrelevant, and with noise B _i(t) irrelevant.Just amplitude and the relative phase difference with signal has relation.

What use in the native system is binary channels phase sensitivity related operation, and the concrete operation result is as follows.

Reference signal becomes in the dual channel system

S _r(t)＝A _rsin(ωt) (5)

S′ _r(t)＝A _rcos(ωt) (6)

Measured signal and reference signal multiply each other in multiplier and are obtained by the integrator integration

$S_0=\underset{n\→\∞}{\lim }\frac{1}{\mathrm{nT}}\underset{0}{\overset{\mathrm{nT}}{\∫}}{S}_i\left(t\right)S_r\left(t\right)\mathrm{dt}$

$=\underset{n\→\∞}{\lim }\left\{\frac{1}{\mathrm{nT}}[\underset{0}{\overset{\mathrm{nT}}{\∫}}{A}_i{A}_r\sin (\mathrm{\ωt}+\mathrm{\φ})\sin \left(\mathrm{\ωt}\right)\mathrm{dt}+\underset{0}{\overset{\mathrm{nT}}{\∫}}{A}_r{B}_i\left(t\right)\sin \left(\mathrm{\ωt}\right)\mathrm{dt}]\right\}$

$=\frac{1}{2}{A}_i{A}_r\cos \mathrm{\φ}---\left(7\right)$

${\mathrm{<figure-callout\; id="1"\; label="S"\; filenames="C200710027065E00121.png,C200710027065E00131.png"\; state="\{\{state\}\}">S</figure-callout>}}_1=\underset{n\&RightArrow;\&infin;}{\lim }\frac{1}{\mathrm{nT}}\underset{0}{\overset{\mathrm{nT}}{\&Integral;}}{S}_i\left(t\right){S\&prime;}_r\left(t\right)\mathrm{dt}$

$=\underset{n\&RightArrow;\&infin;}{\lim }\left\{\frac{1}{\mathrm{nT}}[\underset{0}{\overset{\mathrm{nT}}{\&Integral;}}{A}_i{A}_r\sin (\mathrm{\&omega;t}+\mathrm{\&phi;})\sin \left(\mathrm{\&omega;t}\right)\mathrm{dt}+\underset{0}{\overset{\mathrm{nT}}{\&Integral;}}{A}_r{B}_i\left(t\right)\cos \left(\mathrm{\&omega;t}\right)\mathrm{dt}]\right\}$

$=\frac{1}{2}{A}_i{A}_r\sin \mathrm{\&phi;}---\left(8\right)$

Therefore, when two passages are carried out computing according to following algebraic expression, just can obtain and the irrelevant result in phase angle.

$\sqrt{{\mathrm{<figure-callout\; id="1"\; label="S"\; filenames="C200710027065E00121.png,C200710027065E00131.png"\; state="\{\{state\}\}">S</figure-callout>}}_1^2+S_0^2}$

$={\left(\frac{1}{2}{A}_i{A}_r\cos \mathrm{\&phi;}\right)}^2+{\left(\frac{1}{2}{A}_i{A}_r\sin \mathrm{\&phi;}\right)}^2---\left(9\right)$

$=\frac{1}{2}{A}_i{A}_r$

Be divided by by (7) and (8) formula, can obtain the relative phase difference of input signal and reference signal.Can obtain the amplitude of input signal by (9) formula.

In digital lock-in amplifier of the present invention, all above computings are all finished in signal processor 3 and master controller 8 as can be seen, are digital operation processes completely, and this is the maximum characteristics of digital lock-in amplifier.Because general analog multiplier and other analogue devices be subjected to external disturbance easily, and have temperature problem such as waft, in high-speed computation, accuracy can reduce greatly.These shortcomings are non-existent fully in digital system.With respect to analog signal, the antijamming capability of digital signal is very strong, particularly for the signal that changes at a high speed.

The core of digital processing has used high performance digital signal processor as master controller 8, and programmable logic device is as signal processor 3, and digital signal channel sample speed is 1mhz, and data length is 12BIT.Adopt digital operation process completely, and overall process is digital control.Such data processing method has guaranteed that there are good stability and measurement precision in system, and these characteristics are very important in the small-signal fields of measurement.

This digital phase sensitivity correlator system mainly is made up of A/D conversion, programmable logic device, three parts of D/A conversion.Wherein A/D converter 4 and two D/A converters 5,5 ' conversions all are 12 the high accuracy conversion chips that adopts new shape, and the system data sample rate is 1MHz.At the conversion portion of former and later two numerals with analogue data, all carried out sampling filter with high-operational amplifier, at utmost reduce the phenomenon of sampling distortion.

The programmable logic device chip is the core of whole phase sensitivity correlator.FPGA inside, the function of mainly finishing has:

According to the control of master controller 8, the square wave by 33 pairs of inputs of digital phase shifter carries out phase shift, and phase shifting accuracy reaches 0.5 degree.Resolve through the later reference square wave of phase shift, produce corresponding sinusoidal reference ripple and cosine reference wave by transducer 35.Control multiplier 31,31 ' carries out the 12bit multiplying that two-way speed is 1MHz.Two-way multiplier 31,31 ' multiplies each other signal and sinusoidal reference ripple, cosine reference wave respectively, and output two-way result.Because there are characteristics such as certain instability and phase delay in analog multiplier, therefore general lock-in amplifier all is to adopt the reference signal and the input signal of square wave to multiply each other.Yet such way can cause the multiple harmonic of reference frequency in the input signal is loaded in the multiplied result, causes multiplied result and actual signal intensity to have certain deviation, and accuracy reduces.The design's digital lock-in amplifier is reduced to sine wave and cosine wave to reference signal according to the frequency and the phase place of reference square-wave signal, and then carries out multiplication with input signal, has so just solved the defective that harmonic wave loads.

Cooperate master controller 8, programmable logic device is travel(l)ing phase automatically, make the phase place of reference signal identical with phase of input signals, guarantee that the output amplitude of x passage in the output signal is exactly the integral multiple of input signal, the output amplitude of y passage levels off to 0 simultaneously.The integrator 32,32 ' of digital phase-sensitive correlator also is to be integrated in the programmable logic device chip internal, and digital integration can accurately be controlled the time of integration, has broken the error problem time of integration of traditional analog integrator.

Owing to adopt D/A chip at a high speed, operating rate is identical with the speed of A/D conversion.Therefore, system can also realize the form output of the real-time multiplication result of two-way multiplier with analog signal.Master controller 8 is connected with a LCDs 6 and is used for showing continuously two D/A converters 5,5 ' output.Can judge the reliability that obtains final integral result so that the staff checks the effect of lock-in amplifier multiplication like this; Be convenient to understanding, the study of college student simultaneously for the lock-in amplifier operation principle.Native system has adopted 320 * 240 LCDs, 6 output results, and LCDs 6 output contents also have maximum characteristic except general various parameter values: the Trendline that shows x passage and the final integration of y passage.This Trendline has shown the operating state in system's a period of time intuitively, for production surveying works such as infrared plated film, ac magnetic susceptibility have brought very big convenience; Simultaneously also be the information of a kind of hommization of bringing to operating personnel.

This digital lock-in amplifier all has the signal overflow mechanism in the whole process of signalling channel, the assurance signal is undistorted, as is provided with amplifying signal and overflows test side 81, narrow band filter signal and overflow test side 82 and overflow test side 83 with the A/D switching signal and be connected with master controller 8.

Simultaneously, there are a plurality of simulation output points in system, is provided with gain results output 71 as analog gain circuit 11, and numerical control narrow band filter 12 is provided with automatic tracking filter output 72, multiplier 31,31 ' is provided with phase sensitivity multiplication output 73,73 ', and integrator 32,32 ' is provided with integral result output 74,74 '.These output points all are very helpful for staff's detection system operating state and college experiment teaching.

Master controller 8 is provided with also that external interface 62 is connected with computer and the incoming end 61 of control panel.Made things convenient for operator monitoring and system has been provided with.

Claims (8)

1. digital lock-in amplifier, comprise signalling channel (1), reference channel (2), (8) four parts of signal processor (3) and master controller, it is characterized in that signalling channel (1) is connected with signal processor (3) by A/D converter (4), described signal processor (3) comprises multiplier (31), (31 ') that two-way is connected with A/D converter (4), the two-way integrator (32), (32 ') that are connected with multiplier (31), (31 '), digital phase shifter (33) and 90-degree phase shifter (34); Reference channel (2) forms two-way output through digital phase shifter (33), one the tunnel directly as the input of multiplier (31), import another road multiplier (31 ') again behind another road process 90-degree phase shifter (34), two-way integrator (32), (32 ') form two-way output through two D/A converters (5), (5 ') back respectively, and the control end of signalling channel (1) and signal processor (3) is connected with master controller (8).

2. digital lock-in amplifier according to claim 1 is characterized in that described signalling channel (1) comprises analog gain circuit (11), numerical control narrow band filter (12) and numerical control attenuator circuit (13).

3. digital lock-in amplifier according to claim 2 is characterized in that the frequency range of the reference signal of described reference channel (2) is 800hz～20khz.

4. digital lock-in amplifier according to claim 3 is characterized in that the square wave reference signal of digital phase shifter (33) and 90-degree phase shifter (34) output is transformed into corresponding sinusoidal reference ripple by transducer (35) and the cosine reference wave is imported multiplier (31), (31 ') again.

5. digital lock-in amplifier according to claim 4, it is characterized in that described multiplier (31), (31 '), integrator (32), (32 '), digital phase shifter (33), 90-degree phase shifter (34) and transducer (35) adopt a slice programmable logic device to realize.

6. according to claim 1 or 2 or 3 or 4 or 5 described digital lock-in amplifiers, it is characterized in that master controller (8) is connected with the output that a LCDs (6) is used for showing continuously two D/A converters (5), (5 ').

7. according to claim 2 or 3 or 4 or 5 described digital lock-in amplifiers, it is characterized in that analog gain circuit (11) is provided with gain results output (71), numerical control narrow band filter (12) is provided with automatic tracking filter output (72), multiplier (31). (31 ') is provided with phase sensitivity multiplication output (73), (73 '), and integrator (32), (32 ') are provided with integral result output (74), (74 ').

8. according to claim 2 or 3 or 4 or 5 described digital lock-in amplifiers, it is characterized in that analog gain circuit (11), numerical control narrow band filter (12) and A/D converter (4) are provided with the signal that is connected with master controller (8) and overflow the test side.

Images