Standard Waveforms - Square & Pulse - Duty Cycle & Pulse Width
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On this post I'm trying to illustrate the performance of the standard square & pulse signals throughout the range of duty cycle and pulse width settings offered by the instrument. The following AG1012F settings are the same for all the tests:
Channel = CH1
Output Load = 50 ohms
Amplitude = 1Vpp
Offset = 0mV
The AWG is connected to my SDS7102 DSO via a 20" RG58/U BNC cable terminated at the DSO end with a 50 ohm feedthrough terminator. If you try these tests on your unit, keep in mind that using longer cables or some RG58 A/U cables (stranded center conductor) will probably cause lower amplitude readings due to cable losses, specially at the higher frequencies.
The SDS7102 settings can be read from the images, they are located just below the scope's graticule.
For all the tests Ch1 is set to 200mV/Div and a 0.00 divisions offset. The timebase setting is below the Y axis and is labeled (M:). The number on the bottom right of the graticule is the trigger frequency which in this case should be the same as the signal frequency. There are various signal measures on the bottom left of the screen.
As implemented in the AG1012F, there are many similarities between the standard square & pulse signals. The main difference is that for pulse the duty cycle or pulse width is adjustable throughout the whole 5MHz range. In contrast, the square signal only offers duty cycle adjustments. In addition, for frequencies of 1MHz and above the square signal duty cycle is fixed at 50%. However, when the frequency is below 1MHz, the instrument's behavior seems to be identical for both square and pulse signals when the duty cycle setting is changed.
The behavior of the square signal at some particular settings was explored on the previous post. On this post I'd like to concentrate on the behavior of the pulse signal throughout its whole range. However, keep in mind that this behavior equally applies to square waves at frequencies below 1MHz.
The following gives the specifics for each test:
Test #1, Image of the AG1012F set to Standard square, Frequency = 5MHz, Duty Cycle fixed at 50%.
Test #2, Image of the SDS7102 while viewing the output of the AG1012F during Test #1.
These tests show that in this case the image displayed by the AG1012F accurately describes its output waveform including the Duty Cycle. This is typical of all square wave settings from 1MHz to 5MHz. In this range the Duty Cycle can not be changed and is fixed at 50%.
Test #3, Image of the AG1012F set to Standard pulse, Frequency = 5MHz, Duty Cycle set to 50%.
Test #4, Image of the AG1012F set to Standard pulse, Frequency = 5MHz, Pulse Width set to 100 ns.
Test #5, Image of the SDS7102 while viewing the output of the AG1012F during Tests #3 & #4.
These tests show that in this case the image displayed by the AG1012F does not accurately describe its output waveform's Duty Cycle or Pulse Width. In fact, although at this frequency the AG1012F can be set to duty cycles ranging from 20% to 80% in 0.1% increments, or a Pulse Width ranging from 40 ns to 80 ns in 1 ns increments, there are only four possible distinct Duty Cycles or Pulse Widths outputs available: 20% (40 ns), 40% (80 ns), 60% (120 ns) & 80% (160 ns).
The following tests illustrate what happens when you set the Duty Cycle anywhere between 20% and 39.9% or the Pulse Width anywhere between 40 ns and 79 ns. As shown by Test #10, the AG1012F output stays at 20% (40 ns) throughout this range.
Test #6, Image of the AG1012F set to Standard pulse, Frequency = 5MHz, Duty Cycle set to 20%.
Test #7, Image of the AG1012F set to Standard pulse, Frequency = 5MHz, Pulse Width set to 40 ns.
Test #8, Image of the AG1012F set to Standard pulse, Frequency = 5MHz, Duty Cycle set to 39.9%.
Test #9, Image of the AG1012F set to Standard pulse, Frequency = 5MHz, Pulse Width set to 79 ns.
Test #10, Image of the SDS7102 while viewing the output of the AG1012F during Tests #6 through #9.
When the Duty Cycle is increased to 40% or the Pulse Width is increased to 80 ns, as illustrated by Test #13, the AG1012F output finally changes to the next possible Duty Cycle/Pulse Width available.
Test #11, Image of the AG1012F set to Standard pulse, Frequency = 5MHz, Duty Cycle set to 40%.
Test #12, Image of the AG1012F set to Standard pulse, Frequency = 5MHz, Pulse Width set to 80 ns.
Test #13, Image of the SDS7102 while viewing the output of the AG1012F during Tests #11 & #12.
The above discrepancies between the AG1012F screen display and the output waveform are more evident at the higher frequencies. For example, if you lower the frequency to 25 kHz, Which has a 40000 ns period, there is a change in the pulse width for each 0.1% Duty Cycle increment. So as far as the Duty Cycle is concerned, the AG1012F screen display matches the output waveform in this case.
However, the Pulse Width at the output of the AG1012F never changes in 1 ns increments, so the screen display only matches the output waveform periodically as far as the Pulse Width is concerned. Owon has set this interval or segment to a minimum of 40 ns, the segment length can reach up to about 48 ns depending on the waveform's period. So you can expect the screen display to actually match the output waveform about every forty to forty eight 1 ns increments.
There is one other pitfall in this area that can cause trouble for a user that relies on the screen display as an indication of the output waveform. At some frequencies the lowest Duty Cycle or Pulse Width setting does not produce an output waveform at all. The following tests illustrate what happens at one of these frequencies.
Test #14, Image of the AG1012F set to Standard pulse, Frequency = 4.9MHz, Duty Cycle set to 19.6%.
Test #15, Image of the AG1012F set to Standard pulse, Frequency = 4.9MHz, Pulse Width set to 40 ns.
Test #16, Image of the SDS7102 while viewing the output of the AG1012F during Tests #14 & #15.
Test #17, Image of the AG1012F set to Standard pulse, Frequency = 4.9MHz, Duty Cycle set to 20%.
Test #18, Image of the AG1012F set to Standard pulse, Frequency = 4.9MHz, Pulse Width set to 41 ns.
Test #19, Image of the SDS7102 while viewing the output of the AG1012F during Tests #17 & #18.
The obvious conclusion is that the AG1012F screen display is not a dependable indication of the output waveform as far as the Duty Cycle and Pulse Width are concerned. One workaround is for the user to monitor the AWG with a scope as during these tests. I'm also working on an spreadsheet that simulates the AG1012F behavior and predicts within 0.1% of the Duty Cycle or within 3 ns of the Pulse Width the segment boundaries for a particular frequency. The last attachment shows the spreadsheet results when you enter a frequency of 4.9MHz. If there is any interest I will make the spreadsheet available in one of my next posts.