1. FIToolbox - PWM transmit
29/10/2016
FIToolbox - PWM transmit
FIToolbox - PWM transmit
Diagnostic Sonar Ltd.
Dave Lines

2. FIToolbox - PWM transmit
29/10/2016
FIToolbox - PWM transmit
Background
Requirement is to demonstrate the ability of the FIToolbox to produced shaped excitation on each transmit channel by means of Pulse Width Modulation (PWM)
A utility has been implemented that converts a desired waveform into a 3-level PWM signal
It produces a graph showing the filtered PWM output and the filtered version of the desired waveform, allowing comparison
The filter is Band Pass and attempts to mimic the combined effect of the transducer response and any system filtering
The utility can export the PWM output as a Pulse definition file (.pul) for import into the FIToolbox applications

3. Experimental verification
29/10/2016
FIToolbox - PWM transmit
Experimental verification
A wideband 5MHz 0.5inch single element unfocused transducer was used
The echo signal from the air interface at the end of an acoustic delay line was recorded for different PWM excitations
The band pass filter (BPF) in the PWM utility was set from 3MHz to 7MHz to try to match the transducer response
Minimum pulse width was set to 4 samples = 20ns and this will limit the rendition of low amplitude parts of the signal at 5MHz
A single cycle at 5MHz was used as a reference to see how well the utility output matched the measured echo

4. PWM – 5MHz 1 cycle – No Window
29/10/2016
FIToolbox - PWM transmit
PWM – 5MHz 1 cycle – No Window

5. Tx/Rx: 1 cycle 5MHz – No Window
29/10/2016
FIToolbox - PWM transmit
Tx/Rx: 1 cycle 5MHz – No Window

6. FIToolbox - PWM transmit
29/10/2016
FIToolbox - PWM transmit
Shaped waveforms
The following waveforms were generated in the utility and the matching pulse-echo responses are shown:
5MHz 10 cycles without amplitude shading
5MHz 1000 samples at 5ns with Hanning envelope shading
5MHz 1600 samples at 5ns with amplitude shading as two concatenated Hanning windows

7. PWM – 5MHz 10 cycle – No Window
29/10/2016
FIToolbox - PWM transmit
PWM – 5MHz 10 cycle – No Window

8. Tx/Rx: 10 cycle 5MHz – No Window
29/10/2016
FIToolbox - PWM transmit
Tx/Rx: 10 cycle 5MHz – No Window

9. PWM – 5MHz – Hanning Window
29/10/2016
FIToolbox - PWM transmit
PWM – 5MHz – Hanning Window

10. Tx/Rx: 5MHz – Hanning Window
29/10/2016
FIToolbox - PWM transmit
Tx/Rx: 5MHz – Hanning Window

11. PWM – 5MHz – Twin Hanning Window
29/10/2016
FIToolbox - PWM transmit
PWM – 5MHz – Twin Hanning Window

12. Tx/Rx: 5MHz – Twin Hanning Window
29/10/2016
FIToolbox - PWM transmit
Tx/Rx: 5MHz – Twin Hanning Window

13. FIToolbox - PWM transmit
29/10/2016
FIToolbox - PWM transmit
Conclusions
A utility has been created for converting a desired excitation signal into a 3 level pulse file that can then be loaded into the FIToolbox applications
A range of excitation amplitude shapes have been generated and the utility screenshots show:
Pulse shape generated
Expected result after filter matching transducer & system
A 5MHz transducer was used to produce matching pulse echo responses for each excitation and verifies the approach
Control of low amplitude parts of the envelope is constrained by the minimum pulse width (20ns) but this limitation is reduced at lower frequencies
The FIToolbox supports different excitation waveforms for each transmit channel and each transmission and so can be used in place of multiple arbitrary waveform generators