Overclocking to 128 MHz

When Justin used a 128 MHz clock for the ADC  I simply thought that it was not possible. I based this to the FX3014 specs that state the upper limit of GPIFII to 100MHz.  

He was right and this kind of overclocking is possible, maybe because we don't use the FX3 in an extended temperature range or maybe because the GIPIFII state machine we use is simple.

The transfer rate of the USB stream raises to 128 Msps with 16 bit samples i.e. 256 MBs. This requires a quite fast PC and it may raise some more problems in the transfer. 

Nevertheless it runs on my PC (I7-3770)  and offers another advantage other than wider bandwidth.

Howard Su noticed that the noise level when running in 128M mode decreases by some 3dB while the signals remain with the same amplitude. 

What's happening is that we have a 3dB decimation gain as explained in  https://www.analog.com/en/technical-articles/analog-tips-decimation-for-adcs.html :

" ...The SNR calculation of an ideal ADC must account for the processing gain of the filtered noise. Using a perfect digital filter, the processing gain due to the filtered noise will increase by 3 dB for every power-of-two reduction in bandwidth. "

This gain is related to the ratio between ADC bandwidth and demodulation bandwidth; it is not related to the sample rate IF bandwidth.

Bjarne Mjelde  reported the similar result during MW listening.

Please notice that the IF filter is wider in the 128M mode when the 2 MHz bandwidth is used.
The comparison is valid in the center of the picture from 300 kHz to 1700 kHz, outside of it there are fft/filtering artefacts.  Different filtering in the 2M band with 128M mode has to be tested.

Update 29/12/2020

On https://github.com/ik1xpv/ExtIO_sddc/releases/tag/v1.1.0 you can find the sources and the binary of the dll.
We tried to fix some of the bugs you reported of v.1.1 RC1. 
Thank you for the support and please continue to signal the bugs you find. We know they are there.
You can open an issue in https://github.com/ik1xpv/ExtIO_sddc/issues 
or post it in https://groups.io/g/NextGenSDRs or email to me ik1xpv[at]gmail.com,

Possibly we will have no new release coming before 2021 :-))
Best Season's Greetings ,


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