[linrad] Re: Delta44

[<leif@xxxxxxxxxx>]

> My limited understanding is that the mods to the card only lower the
> noise floor across the BW.
No. The most important thing is that the design error - or I should
rather call it a mishap in production - makes the Delta44 very
sensitive to currents on the computer chassis. Such currents
are normal and the original design of the Delta44 has separated the
analog ground properly from the outer screen (the metal of the d-sub
in contact with the computer chassis). That is quite clear from
the PCB layout.

The error is that somewhere along the line a d-sub that is soldered
to the ground was chosen rather than an isolated one for which
the PCB was made.

> As a beginner, I will mod my Delta-44 later, after I am fully familiar
> with Linrad and hardware.
This is a good decision. First get familiar to it all:-)

Then, in case you see a difference when the cable is connected
as compared to when you run the Delta44 with its 15-pin d-sub
open you could start to worry about modifications.

The other mods remove some spurs an will improve the dynamic
range a bit by lowering the noise floor but that is only a few dB.
The ground loop error can cause very strong false signals if
you do not have a very carefully designed star ground for everything
and even with careful grounding you can easily get interference
30 dB above the noise floor on the Linrad screen.

> A ground loop is possible at DC or 50/60 Hz. Start off collocating the
> equipment
> and ensuring a common ground and power sources.
> Ground loop problems would probably show up at DC (0 Hz) and not at both
> the ends of the BW
> Or spectra at 50/60 Hz and harmonics. Or even wideband noise.
Yes, but you may pick up signals around 40 kHz from a computer screen
or a switch power supply.

>
> When starting any experimental system, I make all connections when
> powered OFF.
> Then I apply power to all, then start up the application.
>
> Do other spectrum analyzer applications show the same problem ?
>
> I suspect a setting inside the Linrad application software.
That is not possible (as far as I know)

The key information is that the false signal disappears when
the Delta44 input is left open.


73

Leif / SM5BSZ
>


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