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Boosting Maximum Frequency on the WGM-201

Posted by Peter Hiscocks on 2/10/2015 to Products
The WGM-201 hardware is actually capable of much higher output frequencies than the defaults programmed into the software. Here is how to increase the maximum frequency.

Run the program normally. Set the waveform type to 'sine' and look at the maximum output frequency, shown in the box at the top of the frequency slider.  If it's 3000000 (3MHz) then you can modify the code to go to 10000000 (10MHz) or even higher. The DDS (direct digital synthesis) generator in the WGM-201 is clocked at 138.208MHz, so there will still be nearly 14 samples per cycle of the output waveform at output frequency 10MHz.

The source code was installed, along with the executable, when you installed the software.  To modify the code, first locate the directory that contains the source code for the WGM-201.

Then, using your favourite editor, open the file 'functionGen.tcl'.  Locate the line 'set maxFrequency 3000000'. I like to change things by commenting out the existing line and putting in a new line. That makes it easier to back out if the changes don't work. To do that, put a hashmark # at the beginning of that line, turning it into a comment. Then add the new line, so you now have this:

# set maxFrequency 3000000
set maxFrequency 10000000

Save that source code. Restart the program. You should now see that the maximum frequency shown in the box at the top of the frequency slider is 10MHz.  We tested operation at 10MHz and found the output dropped by about 0.5db between 1 and 10MHz.  You can push it to even higher frequencies if you can tolerate some further decrease in amplitude.

What about more complex wave shapes? Waveforms such as square and triangle contain multiple harmonics of the fundamental and these harmonics must be reproduced correctly to generate the correct waveform shape.  For example, a square wave includes the fundamental at frequency f, the third harmonic at 3f, the fifth harmonic at 5f and so on.  Operating at a fundamental frequency of 3MHz works fine for those waveforms.  However, at 10MHz some of the higher order harmonics are attenuated and the waveform is then distorted.  As well, because the frequency is generated by DDS (direct digital synthesis), the square wave has visible jitter at the zero crossings.  With this software modification, the maximum frequency is primarily limited by the operator requirement for waveform fidelity.

We'll eventually modify the software so that 10MHz is the maximum frequency for sine waves and 3MHz for other waveforms. In the meantime, if you need the higher frequency output, you can modify the software as I describe.

Can this same trick be applied to the CGR-101? Yes, you can extend the frequency, but not quite as far.  The DDS clock frequency in the CGR-101 is 100MHz, a bit lower than the WGM-201.  (Watch here for announcement of the new CGR-201, which has a much higher DDS clock frequency.)

In the source code directory for the CGR-101, find and modify the file 'waveform.tcl' Change two lines:

set maxFrequencyLimit 3000000 => set maxFrequencyLimit 10000000
set maxFrequency 3000000 => set maxFrequency 10000000

The output frequency is now extended. It drops off by about 3db at 6MHz.  The maximum useable frequency will depend on the operator requirement for waveform amplitude.

Can the Network Analyser maximum frequency be extended as well? Yes, it's a matter of changing some lines in the file 'netalyser.tcl', and the VNA now operates to a maximum of about 6MHz.  The change in amplitude is largely irrelevant because the network analyser calculates the *ratio* of input to output signal.  

The changes to the software are a little too extensive to list here.  Contact us if you'd like the modified file.
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