AC signal amplitude detection based on TPF111 chip

01Why measure the amplitude of AC signals?

  In the National College Student Intelligent Car Competition, there are groups that complete track tracking through electromagnetic navigation. This year (the 15th), a group that completes electromagnetic navigation through artificial intelligence has been added. The basis of these groups requires the ability to accurately measure the alternating magnetic field signal of the track detected by the I-shaped inductor. Although the magnetic field may be affected by various environmental magnetic field shielding, change, or cause magnetic field changes due to signal generators, the precise amplitude measurement of the detected AC magnetic field signal lays the foundation for subsequent applications.

  Previously, some methods for detecting the amplitude of 20kHz AC magnetic field signals were discussed in the blog post, including:

High frequency detection

  Compare the signal detection methods using diodes and single-supply op amps.

Precision software detection

  By sampling the AC signal and then calculating the amplitude of the signal;

Using AD8302 detection

  This is to measure the amplitude of the AC signal using Analog Devices' RF/IF amplitude and phase detection ICs;

Use a digital oscilloscope to measure the amplitude and phase of an AC signal

  A digital oscilloscope that can be connected to the Internet is used to collect high-frequency signals, and then the amplitude and phase of the AC signal are measured using software methods.

  Flexible and efficient electromagnetic signal detection methods can improve the accuracy of electromagnetic positioning in practical engineering applications.

02 Principle of amplitude measurement based on TPF111

  The TPF111 chip mentioned in this article was originally a low-cost video reconstruction filter designed specifically for consumer applications. It has the function of amplifying the input signal by 2 times (6dB). If coupled by capacitors, it can clamp the signal, that is, it can clamp the lowest value of the signal to about 200mV. Using this feature, the amplitude of the input AC signal can be measured.

  The figure below is a schematic diagram of the structure of the TPF111 internally clamping the video synchronization (horizontal synchronization, field synchronization) level (the lowest level of the signal) in the input signal. If the output signal is coupled to the input end through a capacitor, the internal level comparison and MOS tube discharge circuit will maintain the lowest level of the signal at a fixed potential.

▲ TPF111 sync tip clamp circuit

  If the input is a sinusoidal signal, and its lowest level is maintained at Vclamp, then the average value of the output voltage is equal to E+Vclamp. Therefore, by subtracting the fixed Vclamp from the DC component of the output signal, the amplitude E of the corresponding signal can be obtained. This is the basic principle of TPF111 to complete sinusoidal signal detection.

▲ The DC component corresponding to the signal after bottom clamping

  For basic functional experiments of TPF111, please refer to the blog post: Results of TPF111 Video Signal Amplifier Research.

0320kHz AC signal source with adjustable amplitude

  In order to verify the above-mentioned effect of TPF111 on the amplitude detection of sinusoidal signals, it is necessary to establish a 20kHz AC signal source whose amplitude can be controlled by a program. In this way, the relationship between the input and output of the TPF111 detection can be measured.

1. Basic Methods

  In the previous blog post, some basic methods for implementing AC signal sources were given, mainly including:

  (1) Use an AC signal source. In general, AC signal source devices have the function of controlling the output signal amplitude. For example, the DS345 signal source introduced in How to use a multimeter to measure random noise. However, this type of signal source lacks an external programming interface to change the output amplitude.

  (2) Use a mechanical rheostat. Of course, ordinary potentiometers can change the amplitude of the signal, but they also lack a programmable interface. The blog post on mechanical rheostats introduces the method of using a stepper motor to control a potentiometer. However, this method can only give a rough direction of the signal change and lacks a method for precise setting.

  (3) Use digital potentiometers. In the previous blog post X9C102, X9C103, X9C104 and AD5272 digital rheostat, two types of digital potentiometers and rheostats were introduced. However, due to the influence of parasitic capacitance, these devices have limitations on the frequency of the signal.

  (4) Use DAC to change the amplitude of the AC signal. The 20kHz AC signal source in this experiment borrows the method given in the blog post Can DAC8830 be used as a potentiometer?

▲ Polarity markings for various electrolytic capacitors

2. DAC8830 variable amplitude AC signal source

  Directly use DAC8830 to change the amplitude of AC signal. Due to the influence of DAC8830 output impedance, an external op amp is needed to improve the output load capacity of DAC8830. OPA4377 is used as DAC8830 output buffer, which can drive various detection loads of AC output signal.

▲ Variable amplitude signal source experimental circuit board based on DAC8830

stm32cmd(‘set 7fff’)

  The following is a 20kHz signal source with adjustable output amplitude obtained using DAC8830. Its output is buffered by OPA4377 to improve the load capacity.

▲ Use DAC8830 to output AC signals of different amplitudes

#!/usr/local/bin/python

# -*- coding: gbk -*-

#============================================================

# TEST1.PY                     -- by Dr. ZhuoQing 2020-06-20

#

# Note:

#============================================================

from headm import *

from tsmodule.tsvisa        import *

from tsmodule.tsstm32       import *

#------------------------------------------------------------

gifid = 5

#------------------------------------------------------------

printf(meterval())

tspgiffirst(gifid)

step = 100

for i in range(step):

    setnum = int(0xffff * i / step)

    stm32cmd('set %x'%setnum)

    time.sleep(.2)

    tspgifappend(gifid)

printf('a')

#------------------------------------------------------------

#        END OF FILE : TEST1.PY

#============================================================

  The following three figures show the relationship between the DAC8830 set value and the measured amplitude of the output AC signal. It can be seen that there is a linear relationship between the set value and the output voltage amplitude in a large range and a small range. It is only when the value is relatively small that the output AC signal amplitude will have certain fluctuations and nonlinearity.

▲ Setting value and output AC signal voltage

▲ Setting value and output AC signal voltage

▲ Setting value and output AC signal voltage

04 Experimental plan

  The amplifier circuit with capacitor coupling is designed using TPF111U, as shown in the figure below. It can be seen that the TPF111 package is very small, which reduces the size of the circuit board occupied by the detection solution.

▲ Experimental TPF111U schematic diagram and experimental circuit board

  Note: The package of TPF111U is SC70

  The figure below shows the relationship between the input signal and the TPF111 output signal. It can be seen that the output signal is twice (6dB) amplified by the input signal. And the lowest value of the output signal remains basically unchanged during this process, indicating that the TPF111 has a clamping function for the lowest point.

  When the amplitude of the input signal exceeds half of the TPF111 operating voltage, the output signal will be saturated and distorted.

▲ The relationship between TPF111U output and input signals

05 Experimental Results

  When the input AC signal is 0:V, the bias voltage output by TPF111 is: 0.437V. This is Vclamp.

  Next, a 20kHz alternating signal with variable signal amplitude is added through DAC8830. The output DC component of TPF111 is measured through the DC range of the universal voltage. The following figure shows the relationship between the effective value of the input AC signal and the DC component of the output signal.

  When the effective value of the input signal is 0.85V, the output and input are basically linear. When the input signal exceeds 0.85V, the output voltage slows down. This is because the peak value of the input signal is too large, causing TPF111 to saturate.

▲ Relationship between input AC signal and TPF111U output DC signal

  When the input signal is less than 0.5V, the relationship between input and output is plotted below. It can be seen that the output voltage is basically linear with the input, eliminating the dead zone effect when using a diode for detection.

▲ Relationship between input AC signal and TPF111U output DC signal

  When the signal is less than 0.05V, the signal output shows a relatively slow trend. The following is an amplified curve of the signal when it is 0.01V. It can be seen that although this part of the signal shows more nonlinearity, there is still no obvious dead zone. This shows that the use of TPF111 detection has a stronger sensitivity.

▲ Relationship between input AC signal and TPF111U output DC signal

■ Conclusion

  TPF111 was originally used for video amplification. This article discusses the use of its low-level clamping function to detect the input sinusoidal AC signal. Through the measurement of actual signals, it is shown that the use of the DC component of the TPF111 output to detect the amplitude of the input sine wave has basically no dead zone influence, and the detection sensitivity is very high. In the high-frequency detection blog, the single-power supply op amp LMV321 is introduced for half-wave amplification for detection. In comparison, the TPF111 detection has a larger linear range.

  Since the output of TPF111 has a fixed DC component, it is necessary to collect and save the DC voltage value in advance. This component is subtracted from the final measurement result to obtain the amplitude information that is proportional to the input AC signal amplitude.