CN104811146A - Anti-aberration frequency doubling interference locking amplification system based on reverse repeated m sequences - Google Patents

Abstract

The invention discloses a lock-in amplification system for anti-odd multiplier interference based on an inverse repeated m-sequence, which includes an inverse repeated m-sequence signal generation unit, a weak voltage signal to be measured, a chopper, a forward amplifier, a reverse amplifier, an analog switch, low-pass filter, DC amplifier and analog-to-digital converter; the inverse repetitive m sequence signal generation unit controls the workflow of the entire lock-in amplification system, collects input information, and uses codes to generate inverse repetitive m sequences; the chopper uses The weak voltage signal to be measured is modulated into an alternating signal; the forward amplifier, the inverting amplifier and the analog switch constitute a digital correlator; the low-pass filter is used as an integrator of the entire lock-in amplification system; the DC amplifier will The signal output by the low-pass filter is adjusted to be within the collection range of the analog-to-digital converter; the analog-to-digital converter is used to collect the output voltage of the DC amplifier. The lock-in amplification system can improve the signal-to-noise ratio and increase the anti-interference ability.

Description

A Lock-In Amplification System Against Odd-octave Interference Based on Inverse Repeated M-Sequence

technical field

The invention relates to the field of lock-in amplifiers, in particular to a lock-in amplifier system based on an anti-odd multiplier frequency interference based on an inverse repeated m-sequence.

Background technique

In engineering, when the detection range is small, the change is slow or the DC signal is usually selected, the lock-in amplification method is used for processing. That is, the weak and slow-changing signal is modulated into an AC signal, thereby avoiding the 1/f noise and the DC drift problem of the amplifier. Sometimes in order to simplify the design, a chopper is used to directly modulate the weak signal into a periodic square wave signal (the angular frequency is ω ₀ ), and then use the square wave signal of the same frequency and phase as a reference signal to drive the analog switch, and the positive half cycle is turned on Positive amplifier, negative half-period turn-on inverting amplifier (similar to rectifier circuit), then filter the pulsating signal output by PSD, filter out the high-frequency part, leaving only the low-frequency part related to the amplitude of the signal to be measured.

The lock-in amplifier can modulate a DC signal with a small amplitude or a slowly changing signal with a carrier (square wave or sine wave) with a fixed frequency and an average value of zero, and then use a reference signal (usually the same frequency as the modulation signal) after amplification. The same-phase square wave or sine wave) is processed through the analog circuit, and its output is only proportional to the signal to be measured. During the correlation processing, all noise signals that are not related to the modulation signal will be filtered out, which can greatly improve the Improve the signal-to-noise improvement ratio of the amplifier circuit. Because the reference signal and the modulation signal have the same frequency and phase, it is just like "locked", so it is called lock-in amplification, also called phase lock-in amplification.

The lock-in amplifier can be implemented with an analog multiplier, but it is troublesome to use an analog multiplier in an actual lock-in amplifier. Most of them use the switch modulation method for lock-in amplification. than) under the control of the square wave for polarity transformation. Existing lock-in amplifiers are very good at suppressing even-octave frequency interference, but their ability to suppress odd-octave frequency interference is limited, because the cumulative average value of odd-octave frequency signals in a square wave period is not 0, and cannot be completely canceled.

For example, suppose the modulating signal is r(t), and the weak signal to be detected is a (generally DC or approximately DC, treated as a constant), then the modulated signal is ar(t), and the noise signal is n(t ), then the input signal x(t)=ar(t)+n(t), the output result of the lock-in amplifier is the cross-correlation function of x(t) and r(t):

$\begin{array}{c}{R}_{\mathrm{xr}}=\underset{T\&Right\; Arrow;\∞}{\lim }\frac{1}{T}{\∫}_0^{T}x\left(t\right)r(t-\mathrm{\τ})\mathrm{dt}\\ =\underset{T\&Right\; Arrow;\∞}{\lim }\frac{1}{T}{\∫}_0^T[\mathrm{ar}\left(t\right)+\mathrm{no}\left(t\right)]r(t-\mathrm{\τ})\mathrm{dt}\\ =\underset{T\&Right\; Arrow;\∞}{\lim }\frac{1}{T}{\∫}_0^T\mathrm{ar}\left(t\right)r(t-\mathrm{\τ})\mathrm{dt}+\underset{T\&Right\; Arrow;\∞}{\lim }\frac{1}{T}\mathrm{no}\left(t\right)r(t-\mathrm{\τ})\mathrm{dt}\\ =a{R}_{\mathrm{rr}}+R_{\mathrm{rn}}\end{array}$

In the result, aR _rr is the result we require, which contains the component of the signal a to be tested, and R _rr is the autocorrelation function of the reference signal. As long as the phase difference between the reference signal and the modulation signal is fixed to zero, then R _rr is a Fixed value, so aR _rr only changes with the change of the signal a to be measured, and realizes the purpose of the lock-in amplifier.

And R _rn is the cross-correlation function between the noise signal and the reference signal. The physical meaning of cross-correlation is the similarity between the two signals. From the frequency domain, if the frequency components of the noise n(t) and r(t) overlap part, then this item is not zero, which interferes with the result. We have said that generally r(t) is a square wave with a duty cycle of 50%, and the frequency content of the square wave has odd harmonics such as ω ₀ , 3ω ₀ , 5ω ₀ ..., so if the noise n(t ) containing these odd harmonics will adversely affect the correlation results.

Therefore, in the existing lock-in amplifier, once the frequency of some interference signals is close to or equal to the modulating signal, it will have a bad influence on the results of the lock-in amplifier. When the frequency of the interfering signal is close to 3ω ₀ , 5ω ₀ , 7ω ₀ . However, once the interference signal frequency is very close to ω ₀ , the filter circuit is difficult to distinguish, and in many cases due to the coupling between devices, the interference signal is equal to ω ₀ , at this time the lock-in amplifier system can only distinguish the interference by phase signal and the signal to be tested.

Traditional lock-in amplifiers are all implemented with pure hardware electronic systems, which will bring a lot of inconvenience in the development, debugging and upgrading of equipment, and the reading and accuracy problems caused by traditional pointer dials should also be improved.

Therefore, it is necessary to design a brand-new lock-in amplification system, which must be able to resist odd frequency interference, high precision, convenient reading, and adjustable parameters.

Contents of the invention

Aiming at the deficiencies of the above-mentioned prior art, the present invention designs a lock-in amplification system based on an inverse repeated m-sequence to resist odd multiplier interference. The lock-in amplification system can greatly increase the anti-interference ability of the system and simplify the complexity of the system.

In order to solve the technical problems existing in the above-mentioned prior art, the concrete technical scheme that adopts is:

A lock-in amplification system based on reverse repetitive m-sequence anti-odd multiplier interference, including reverse repetitive m-sequence signal generation unit, weak voltage signal to be measured, chopper, forward amplifier, reverse amplifier, analog switch, low-pass Filters, DC amplifiers and analog-to-digital converters, where:

The inverse repetitive m-sequence signal generation unit is used to control the workflow of the entire lock-in amplification system, collect input information, use codes to generate an inverse repetitive m-sequence, control the analog-to-digital converter to collect the output voltage of each amplifier, and internally program a filtering algorithm To improve the signal-to-noise ratio of the entire lock-in amplifier system;

The chopper is used to modulate the weak voltage signal to be measured into an alternating signal, the modulated signal amplitude is the amplitude of the signal to be measured, and the waveform is an inverse repeating m sequence;

The forward amplifier, the inverting amplifier and the analog switch constitute a digital correlator;

The low-pass filter is used as an integrator of the entire lock-in amplification system;

The DC amplifier adjusts the signal output by the low-pass filter to the acquisition range of the analog-to-digital converter;

The analog-to-digital converter is used to collect the output voltage of the DC amplifier.

In a preferred solution, the generating unit of the inverse repetitive m-sequence signal includes a shift register and a feedback network, and the inverse repetitive m-sequence signal is generated by a shift register plus a feedback network.

In a further preferred technical solution, the inverse repeating m-sequence signal generating unit includes a single-chip microcomputer, a keyboard and an LCD display; the single-chip microcomputer is used to control the workflow of the entire lock-in amplification system, collect input information, and use codes to generate an inverse repeating m-sequence, Control the output voltage of the AD acquisition amplifier, and internally write a filtering algorithm to improve the signal-to-noise ratio of the entire lock-in amplification system. The reverse repetition m-sequence signal is generated by a single-chip microcomputer; the keyboard is used to input the length of the reverse repetition m-sequence that needs to be generated. and the sending frequency of the sequence; the LCD display screen is used for displaying a prompt interface during human-computer interaction, and for displaying a detection result during lock-in amplification detection.

By adopting the above-mentioned scheme, compared with the prior art, the lock-in amplification system based on the anti-odd multiplier interference of the reverse repeated m-sequence of the present invention has the following technical effects:

1. Change the reference signal of the lock-in amplifier from a square wave with a duty cycle of 50% to an inverse repetitive m-sequence with excellent autocorrelation, which greatly increases the anti-interference ability of the system.

2. After using the inverse repetitive m-sequence, the filter circuit originally designed to suppress odd-multiple frequency interference can be omitted, which simplifies the complexity of the system.

3. Use a single-chip microcomputer to generate an inverse repeating m-sequence, whose length and cycle are controlled by a program, flexible and changeable, and easy to implement.

4. The single-chip microcomputer performs digital filtering on the signal sampled by AD to further filter out high-frequency components and improve the signal-to-noise ratio improvement ratio.

5. LCD display is used to display system parameters and collection results, which is convenient and intuitive.

Description of drawings

Fig. 1 is the shift register plus feedback network that produces the reverse repetition m-sequence signal;

Fig. 2 is the system flow chart that the single-chip microcomputer produces the reverse repetition m-sequence signal;

Fig. 3 is the autocorrelation function schematic diagram of the square wave in the lock-in amplifier in the prior art;

Fig. 4 is a schematic diagram of the autocorrelation function of the reverse repeat m sequence in the lock-in amplification system of the present invention;

Figure 5 is a schematic diagram of the suppression of the second harmonic by a square wave;

Fig. 6 is the schematic diagram of the suppression of the 2nd harmonic by the reverse repeated m sequence;

Figure 7 is a schematic diagram of the suppression of the third harmonic by a square wave;

Fig. 8 is a schematic diagram of the suppression of the 3rd harmonic by the reverse repeated m-sequence;

FIG. 9 is a schematic structural diagram of a lock-in amplification system based on an inverse repeated m-sequence and resisting odd frequency interference in Embodiment 1. FIG.

detailed description

In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be further described in detail below in conjunction with specific examples. It should be understood that these descriptions are exemplary only, and are not intended to limit the scope of the present invention. Also, in the following description, descriptions of well-known structures and techniques are omitted to avoid unnecessarily obscuring the concept of the present invention.

As shown in Figure 1, the initial state of the linear feedback shift register is (a ₀ a ₁ …a _n-2 a _n-1 ), after a shift of linear feedback, the input state of the first stage at the left end of the shift register is

$a_{\mathrm{no}}=c_1{a}_{\mathrm{no}-1}+c_2{a}_{\mathrm{no}-2}+\·\·\&Center\; Dot;+c_{\mathrm{no}-1}{a}_1+c_{\mathrm{no}}{a}_0=\underset{i=1}{\overset{\mathrm{no}}{\mathrm{\Σ}}}{c}_i{a}_{\mathrm{no}-i}$

After k shifts, the input of the first stage is

$a_i=\underset{i=1}{\overset{\mathrm{no}}{\mathrm{\Σ}}}{a}_i{a}_{l-i}$

Wherein, l=n+k-1≥n, k=1, 2, 3, . . .

According to the recurrence relation It can be seen that the input of the first stage of the shift register is determined by the feedback logic and the original state of the shift register.

The reverse repeat m sequence can be formed by extending the obtained m sequence by 2 times and then inverting every other position. If the sequence is relatively long, multiple chips need to be combined with each other. The hardware generation of the inverse repeated m-sequence will not be described too much here.

When the reverse repeat m-sequence signal is generated by a single-chip computer program. Its generation principle is the same as the hardware shift register plus feedback network, except that the length N of the shift register, the initial state a _n , and the feedback network c _n are all controlled by the single-chip microcomputer program, and its flow chart is shown in Figure 2:

The initial value a _n of the register and the feedback network c _n are programmed in the ROM of the single-chip microcomputer in the form of a constant array. The user selects the appropriate feedback network through the keyboard and can simulate the shift register through the program to generate m-sequences of different lengths. The generated m-sequence has a length of N and is stored in RAM. Then it is expanded into a repeated m sequence with a length of 2N, and then reversed to generate an inverse repeated m sequence with a length of 2N.

The time-varying waveform of the reverse repeated m-sequence generated is r(t). r(t) is divided into two paths, one path is used to drive the chopper, and the weak signal a is modulated into an alternating signal ar(t) that changes according to the waveform of r(t), and ar(t) is simultaneously transmitted by the non-inverting amplifier and the inverting amplifier Amplify to generate two phase-symmetrical signals Aar(t) and -Aar(t) (A is the amplification factor of the amplifier). Another path r(t) drives the analog switch, so that the two symmetrical signals are turned on alternately and enter the low-pass filter. The process of alternate conduction realizes the multiplication of the signal Aar(t) and the reference signal r(t), the low-pass filter realizes the addition of the multiplication results, and the two processes jointly realize the correlation process (correlation is two sequences multiplied and then added).

Assuming that the noise signal is n(t), then the input signal x(t)=ar(t)+n(t), the output of the lock-in amplifier is the cross-correlation function of x(t) and r(t):

$\begin{array}{c}{R}_{\mathrm{xr}}=\underset{T\&Right\; Arrow;\∞}{\lim }\frac{1}{T}{\∫}_0^{T}x\left(t\right)r(t-\mathrm{\τ})\mathrm{dt}\end{array}$

$\begin{array}{c}=\underset{T\&Right\; Arrow;\∞}{\lim }\frac{1}{T}{\∫}_0^T[\mathrm{ar}\left(t\right)+\mathrm{no}\left(t\right)]r(t-\mathrm{\τ})\mathrm{dt}\\ =\underset{T\&Right\; Arrow;\∞}{\lim }\frac{1}{T}{\∫}_0^T\mathrm{ar}\left(t\right)r(t-\mathrm{\τ})\mathrm{dt}+\underset{T\&Right\; Arrow;\∞}{\lim }\frac{1}{T}\mathrm{no}\left(t\right)r(t-\mathrm{\τ})\mathrm{dt}\\ =a{R}_{\mathrm{rr}}+R_{\mathrm{rn}}\end{array}$

In the result, aR _rr is the result we require, which contains the component of the signal a to be tested, and R _rr is the autocorrelation function of the reference signal. As long as the phase difference between the reference signal and the modulation signal is fixed to zero, then R _rr is a Fixed value, so aR _rr only changes with the change of the signal a to be measured, and realizes the purpose of the lock-in amplifier. Because the inverse repeating m-sequence r(t) is a pseudo-random signal without the _ω0 corner frequency like a square wave signal, the odd harmonic problem of a conventional lock-in amplifier is impossible. At the same time, comparing the autocorrelation function of the reverse repeated m-sequence and the square wave, it can be found that the reverse repeated m-sequence has excellent autocorrelation, which makes the chance of mutual influence of multiple devices working at the same time becomes very small.

In addition to easily generating the required sequence, the single-chip microcomputer can also be matched with analog-to-digital converters of different precision to meet different measurement requirements. The single-chip microcomputer converts the analog signal output by the lock-in amplifier into a digital signal through AD and stores it in RAM for digital filtering processing to further improve the anti-interference ability. At the same time, the obtained results are displayed on the LCD screen, which is clear and intuitive, and is convenient for the operator to read and record.

As shown in Figure 3, the autocorrelation function of the square wave and the autocorrelation function of the inverse repeated m sequence in Figure 4, when the interference signal is the same as the reference signal, the cross-correlation term R _rn between the noise and the reference signal mentioned earlier becomes the autocorrelation of the reference signal Correlation function, since the signal to be measured and the reference signal are synchronous (generated by the same source), from the autocorrelation function diagram, whether it is a square wave or an inverse repetitive m sequence, they can reach the maximum value, and give full play to the lock-in amplifier. effect. Generally, the phase difference between the noise and the reference signal is random. In order to reduce the influence of the noise on the lock-in amplifier, the autocorrelation function of the reference signal must have the following characteristics: the amplitude is very high in some very narrow areas, and the lock-in amplifier can be fully utilized. The effect of the amplifier is zero in other areas, so that the external noise has a greater probability of falling on this part, reducing the impact on the output result. Figure 4 just satisfies this characteristic. In a period, only a very small number of interference signals can fall in the region with a large amplitude, and most of them fall in the region with an amplitude close to zero. However, Figure 3 is obviously worse, only a small part of the area can be close to zero, and most of the areas have a relatively large range.

The four pictures shown in Figures 5 to 8 show the suppression capabilities of even frequency multiplier interference and odd frequency multiplier interference in the square wave and reverse repeated m sequences, respectively.

For the convenience of explanation, the reverse repeated m-sequence with a length of 14 is selected, and for comparison, the square wave takes 7 cycles.

As shown in Figure 5, the suppression of the second harmonic by the square wave, and Figure 6 shows the suppression of the second harmonic by the reverse repetition of the m-sequence. It can be seen that after the product is accumulated for a long time, the interference generated by the harmonic is just offset. , the average value is zero, and both modes can be suppressed well.

As shown in Figure 7, the square wave suppresses the third harmonic. After the product, there are 28 positive half waves and only 14 negative half waves. After the cancellation, there are still 14 positive half waves. The average value is not is zero, which interferes with the output result.

As shown in Figure 8, it is the suppression of the 3rd harmonic by the reverse repetition m sequence. After the product, there are 22 positive half waves and 20 negative half waves. After the cancellation, only 2 positive half waves are left, such as the square wave The 14 in the state is reduced by a factor of 7. This is only the inhibitory ability when the sequence length is 14. If the length of the reverse repeat m sequence is increased, the inhibitory ability can be greatly improved.

Embodiment 1: as shown in Figure 9: a kind of lock-in amplification system based on the anti-odd multiplier interference of inverse repeated m sequence, comprises MCU2 (single-chip microcomputer, or be called micro control unit), keyboard 1, LCD display screen 3, to-be-tested Weak voltage signal 4, chopper 5, forward amplifier 6, reverse amplifier 7, analog switch 8, low-pass filter 9, DC amplifier 10 and analog-to-digital converter 11, wherein:

The MCU2 is used to control the workflow of the entire lock-in amplifier system, collect input information, use codes to generate reverse repeat m sequences, control AD11 (analog-to-digital converters) to collect the output voltages of each amplifier, and internally write a filtering algorithm to improve the entire lock-in. Signal-to-noise ratio of the amplification system;

The keyboard 1 is used to input the length of the inverse repeat m sequence that needs to be generated, and the sending frequency of the sequence;

The LCD display screen 3 is used for displaying a prompt interface during human-computer interaction, and for displaying detection results during lock-in amplification detection;

The chopper 5 is used to modulate the weak voltage signal to be measured into an alternating signal, the modulated signal amplitude is the amplitude of the signal to be measured, and the waveform is an inverse repeat m sequence;

The forward amplifier 6, the inverting amplifier 7 and the analog switch 8 constitute a digital correlator;

The low-pass filter 9 is used as an integrator of the entire lock-in amplification system;

The DC amplifier 10 adjusts the signal output by the low-pass filter 9 into the acquisition range of the analog-to-digital converter 11;

The analog-to-digital converter 11 is used to collect the output voltage of the DC amplifier 10 . the

It should be understood that the above specific embodiments of the present invention are only used to illustrate or explain the principle of the present invention, and not to limit the present invention. Therefore, any modification, equivalent replacement, improvement, etc. made without departing from the spirit and scope of the present invention shall be included within the protection scope of the present invention. Furthermore, the appended claims of the present invention are intended to cover all changes and modifications that come within the scope and boundaries of the appended claims, or equivalents of such scope and boundaries.

Claims (3)

1. A lock-in amplification system based on the anti-odd multiplier interference of the reverse repeating m-sequence, is characterized in that: it comprises the reverse repeating m-sequence signal generating unit, weak voltage signal to be measured, chopper, forward amplifier, reverse Amplifiers, analog switches, low-pass filters, DC amplifiers, and analog-to-digital converters, where: The inverse repetitive m-sequence signal generation unit is used to control the workflow of the entire lock-in amplification system, collect input information, use codes to generate an inverse repetitive m-sequence, control the analog-to-digital converter to collect the output voltage of each amplifier, and internally program a filtering algorithm To improve the signal-to-noise ratio of the entire lock-in amplifier system; The chopper is used to modulate the weak voltage signal to be measured into an alternating signal, the modulated signal amplitude is the amplitude of the signal to be measured, and the waveform is an inverse repeating m sequence; The forward amplifier, the inverting amplifier and the analog switch constitute a digital correlator; The low-pass filter is used as an integrator of the entire lock-in amplification system; The DC amplifier adjusts the signal output by the low-pass filter to the acquisition range of the analog-to-digital converter; The analog-to-digital converter is used to collect the output voltage of the DC amplifier. 2. A kind of lock-in amplification system based on the anti-odd multiplier interference of the reverse repetition m sequence according to claim 1, it is characterized in that: the reverse repetition m sequence signal generating unit comprises a shift register and a feedback network, the The reverse repeating m-sequence signal is generated by a shift register plus a feedback network. 3. a kind of lock-in amplifying system based on the anti-odd multiplier interference of reverse repeating m sequence according to claim 1, is characterized in that: described reverse repeating m sequence signal generation unit comprises single-chip microcomputer, keyboard and LCD display; The single-chip microcomputer is used to control the workflow of the entire lock-in amplifier system, collect input information, use the code to generate reverse repeating m-sequence, control the output voltage of the AD acquisition amplifier, and internally write a filtering algorithm to improve the signal-to-noise ratio of the entire lock-in amplifier system. The inverse repeat m sequence signal is generated by a single-chip microcomputer; the keyboard is used to input the length of the inverse repeat m sequence that needs to be generated, and the sending frequency of the sequence; the LCD display screen is used for man-machine interaction to display a prompt interface for locking and zooming in The detection result is displayed during detection.