[hpsdr] ADC Overload (again)

Graham / KE9H KE9H at austin.rr.com
Sun Aug 2 08:52:01 PDT 2009


Both the preamp and the attenuator are rugged, and will definitely not 
be harmed
by levels that just make the "Overload" light come on.  The primary 
protection is
the the clamp diode at the input connector, which very hard clamps at 5 
peak to peak at the input.

I have a single empirical data point, from testing of PennyWhistle, and 
the coax cables between the RFPA and Alex, where I accidentally keyed 16 
into the front end of the Mercury, for some 30+ seconds.

The Mercury data converter and attenuator were not hurt, but the input clamp
diode did get hot enough to smoke and smell.  Testing showed that Mercury
worked normally, but that one of the four diodes inside the clamp had 
causing the clamp to now clamp at 2 volts in one direction and 4 in the 
so I did replace the clamp diode as a result. (The overload light comes 
on around
1.8 Volts peak to peak, so, it was still clamping above the overload 
point, and
not degrading receiver performance.)

So, my personal conclusion is that signals that cause the "Overload" 
light to
pulse on and off are no where near levels that will harm anything, and that
in the event of severe overload, the clamp diode works, and will sacrifice
itself before the input on Mercury is harmed.

The overload light just means that the wideband input to the radio is
high enough that the data converter is starting to go non-linear.

No data in the event of a direct lightening strike. You will be on your
own if that happens.

If you know that it is 540 kHz to 1600 kHz broadcast band overload
that is causing the problem, then a simple 1.7 MHz High Pass Filter
will fix things.  If shortwave broadcast is causing the problem, then
you will need to block those.  I have seen my overload light blink on
nearby lightening strikes, when Alex was not in the line, so with an
unfiltered wideband input to the Mercury. 

Having Alex in the line supplies both the 1.7 MHz high pass and
band limits the input on all ham bands, as well as provides two
additional voltage clamp diodes in line with the input
to the receiver.

I am personally impressed with how rugged, just the basic Mercury
front end is.

--- Graham / KE9H


Richard Ames wrote:
> ***** High Performance Software Defined Radio Discussion List *****
> Phil -
> In this message:
> http://lists.openhpsdr.org/htdig.cgi/hpsdr-openhpsdr.org/2009-January/007932.html
> You said "you may see smoke from the preamp" when the ADC Overload is
> flashing.  Here in Sydney I occasionally see it flashing when the
> attenuator is out of the circuit ('Preamp on'). I suspect this is due to
> generally high noise level + broadcast transmitters about 10kms away.
> This afternoon with 80m having S9 noise - it was on all the time.
> How concerned should I be???  I often need the 'preamp on' in the
> amateur bands.....
> Regards, Richard.

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