Quantitative Analysis of the FP-PWM-520 Pulse-Width Modulated Output Channels Signal

Publish Date: Sep 06, 2006 | 7 Ratings | 3.29 out of 5 |  PDF

Overview

The FP-PWM-520 is a FieldPoint pulse-width modulated (PWM) output module with eight PWM sourcing output channels. These channels can be configured using a period (one millisecond period resolution) and a 0 to 100% duty-cycle (12-bit resolution) setting to determine when to switch to the ON or OFF state.

The purpose of this document is to present the results of a quantitative analysis performed on the PWM output signal for the following parameters:
- Noise observed on the output signal
- Analysis of the pulse pattern and determination of the best set of pulse parameters that describes the pulse for various periods and duty cycles.

Table of Contents

  1. 1) Setup of Testing Configuration
  2. 2) Results and Conclusions
  3. 2.1) Waveform Samples
  4. 2.2) Noise Analysis
  5. 2.3) Pulse-Width Modulated Signal Analysis

1. 1) Setup of Testing Configuration

The Hardware used in this investigation consisted of:
- FP-2000 Network Interface Module used as an Ethernet interface
- FP-PWM-520
- Analog Devices 955 Power Supply 5VDC/1A
- Data Acquisition Board NI PCI-6070E, 12-bit, 1.25 MS/s, 16 Analog Input Multifunction DAQ
- Connector Block CB-50LP

The PWM channel 0 output signal was measured with the analog input channel 0 of the DAQ board, using lead wires from the FP-PWM-520 to the CB-50LP placed at close distance (approximately six inches). The hardware was set up in a low-noise environment and no additional signal conditioning was used.

The LabVIEW 6.1 software was used to both control the PWM output and perform the Data Acquisition and analysis of the signal.

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2. 2) Results and Conclusions


The data collected during the testing is presented in this paragraph. Following is a graphical presentation of waveform samples obtained, noise analysis of the signal and finally a presentation of the pulse pattern analysis results

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3. 2.1) Waveform Samples


Figure 1 shows the waveform graphs obtained for the PWM outputs of different duty-cycle rates and periods






Figure 1. Waveform graphs obtained for a) Duty Cycle 10%, Period 800 ms, b) Duty Cycle 50%, Period 1200 ms and c) Duty Cycle 90%, Period 1600 ms PWM outputs.

The figure shows that the waveform shapes match the different duty-cycle rates and periods set as input parameters for the module. A more extensive analysis will be presented in paragraph 2.3, presenting the pulse parameters for various periods and duty-cycles. The figure also indicates a small presence of noise at the top and bottom of the pulse width modulated signal. The objective of the next paragraph is to present a more detailed analysis of the noise observed on the signal.

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4. 2.2) Noise Analysis


Resolution of the data acquisition card is calculating by dividing the voltage output range by the maximum number of divisions of the 12-bit card, i.e. 212 = 4096 divisions. With the range set up to be 10V, the resolution of the measurement is R = . Figure 2.2.1 illustrates that the maximum amplitude of noise observed at the top (5V) is 2 x R, while this amplitude is only of R for the bottom (0V) of the pulse-width modulated output.



Figure 2. Noise Amplitude observed respectively at the top and bottom of the pulse-width modulated output

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5. 2.3) Pulse-Width Modulated Signal Analysis

The Analysis of the pulse pattern and determination of the best set of pulse parameters was obtained using the Pulse Parameters.VI (not in Base Package) from the LabVIEW Signal conditioning Palette on a train of pulses of indicated duty cycle and Period of at least 7 pulses.
When the signal contains a train of pulses, the Pulse Parameters VI uses the train to determine overshoot, top, amplitude, base,and undershoot but uses only the first pulse in the train to establish slew rate, risetime, falltime and duration. The following table summarizes the result obtained for various periods and duty cycles.

Table 1.



- risetime is the time required to rise from 10% amplitude to 90% amplitude on the rising edge of the pulse.
- falltime is the time required to fall from 90% amplitude to 10% amplitude on the falling edge of the pulse.
- duration is the difference between the falling edge time and the rising edge time at which 50% amplitude occurs.
- top is the line that best represents the values when the pulse is active, high, or on. For a negative-going pulse, top is less than base and results in a negative amplitude. For a positive-going pulse, top is larger than base and results in a positive amplitude.
- base is the line that best represents the values when the pulse is inactive, low, or off and is the level closest to zero. For a negative-going pulse, base is larger than top and results in a negative - amplitude. For a positive-going pulse, base is less than top and results in a positive amplitude.
- amplitude is the difference between top and base.
- undershoot is the difference between the baseline and the minimum value in the pulse.
- overshoot is the difference between the maximum value in the pulse and the topline.
- slew rate is the ratio between (90% amplitude - 10% amplitude) and the risetime.

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