[hpsdr] Boat Anchors..PreAmps

[Phil Harman]

The ADC is a voltage driven device. It will overload (gracefully in the
case of Mercury) if the instantaneous vector sum of all the signals at its
input (i.e. after any filtering) exceeds a specified value.

Before starting the design of Mercury I was advised by a well know
mathematician that all the signals in the input bandwidth will add
together and overload the ADC. In which case don't bother trying to build
such a receiver.

I also asked Bill Sabin - he agreed with the analysis. But added that this
would happen once, for a second, in my life time.

The rest is history...

73 Phil...VK6APH







> ***** High Performance Software Defined Radio Discussion List *****
>
> If you look at OLD navy receivers you will often find two stages of RF
> amps before the mixer with a complex RF filter system.  More than weak
> signals would require in the bands then used or need for strong signal
> rejection.  Today's ships do have real problems with multiple signals
> though.  A lot of that stuff was to prevent LO leakage which might give
> the ship's position away.  The late W0CY here had several of those and
> they were magnificent mechanical devices with lots and lots of shielding
> and DC bypassing.
>
> In the case of our ADC's, a very valid purpose of the pre-amp is to
> protect the input from excess voltages.  I've blown two on a different RX
> using the same ADC (lightning once and a leaky TR relay the other).
>
> I'm pretty sure that our ADC is a voltage driven device.  Perhaps someone
> can explain whether we are looking at overload from the highest voltage in
> the band or total power across the band (input).  Or am I back to the dB
> power vs dB voltage bit?
>
> N0UU
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