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146MHz Digital Pre Distortion Tests

I'm a bit late in adding this (Oct 2016) but thought it was a useful bit of information regarding spectral cleanliness on this part of 2m!

Charles added an option in the DATV-Express FPGA some time back (Jan 2015) to allow for amplitude correction offsets to be programmed in, fixed Digital Pre Distortion (AM Only DPD).
We had no way of making this happen in real tome as the Express does not have a receiver, but manual adjustment of the correction offsets allows us to compensate for some of the Amplitude (AM) non linearity in the amplifiers we are using.
I think this is in the current version of the Linux firmware/server code but not windows.

I played around for some time with the 'ramp' mode in the Express where is does a linear ramp in amplitude repeatedly and then measured this curve at the output of the amplifier using an rtlsdr, the dynamic range was not great but it gave some useful information.
You can easily see that the amplifiers are not very linear.
Trying to calculate the required offsets to apply is quite a complex task (for me and i failed miserably trying!) so please ignore the lookup 'LUT' curve in the following image!

Amplifier before Pre-Distotion

The DVB-S signal without Pre Distortion:
Before DPD

So instead i just generated a bit of python code to make an adjustable correction factor curve that i could load into the Express and 'tune' to match my amplifier..

DPD table creation

It did not take too long to make a noticable improvement in the IMD products of the amplifier.

Below shows after applying the correction curve shown above.
After DPD

The next plot is of the amplifier when the DPD table was active, you can see that it is only slightly more linear compared with the plot above but the DVB spectrum is improved quite a lot.

After DPD power plot

In practice it was possible to achieve about 6-8dB of IMD improvement most of the time, this was an exceptional result!
Any changes of the amplifier gain/tuning reduces the effect but it was always an improvement when enabled.
Using this particular correction table for different amplifiers also has a positive effect so they must have had a similar curve.

The next step with this is for Charles to do some more hard work (again) with the LimeSDR and create some real time Digital Pre-Distortion :-)

Last page added:16/08/17 15:45