[hpsdr] Dither and Random should be enabled for best performance of Mercury

[Phil Harman]

> ***** High Performance Software Defined Radio Discussion List *****
>
> All,
> Phil Harman reminded me about Dither and Random last week, but I must
> confess I forgot about it this week until I started getting IMD
> Dynamic Range numbers that were about half of what I expected. As a
> result, I have added this to the section on Receiver Performance, IMD
> Dynamic Range on the Wiki:
>
> Important note: To get good performance with respect to the IMD
> Dynamic Range tests, be sure to check "Dither Enabled" and "Random
> Enabled" in the PowerSDR >Setup > HPSDR > Mercury Options panel. If
> this is not done your IMD Dynamic Range will be reduced by about 50%.
>
> Ron, W9KFB
> Indianapolis

Hi Ron,

Thanks for your work on measuring the performance of Mercury. Ron is quite
correct that if you measure the IMD performance in the lab without Dither
and Random enabled then you get lower figures than expected.

However, the performance when connected to an antenna and in the presence
of in-band and out-of-band signals also need to be considered. Unlike
conventional analogue receivers, large signals (within certain limits)
actually improve the IMD performance of DDC based receivers.

We are about to publish some ground breaking work by Walter, HB9JG, on the
effects of dither, random and band signals on the IMD performance of
Mercury.

This is quite possibly the first time such and analysis has been
undertaken for an Amateur DDC receiver and makes interesting reading.
Walter is presently formatting his document for inclusion on the Wiki and
will announce when it is available.

73's Phil...VK6APH


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