[hpsdr] ALEX - Input protection

[Henry Vredegoor]

Hello Graham, All,

Thank you for your reply, I see how you are going to implement this on ALEX.

Just for the sake of understanding and learning:

I was somewhat aware of the energy distribution in frequency of static noise
caused by lightning etc., but in my previous post I was referring to the
direct DC path to ground, the same idea of the solution as pointed to by
Ray, as posted by Achim DH2VA, not the transformer effect of filtering
itself.
But interesting to read and learn anyway!!

If I understand you correctly, you are using 2 X 10 Kohm resistors for this
(I am surprised that these can be so high in value!), but wouldn't such a
direct DC path to ground work better? (the lower the impedance to ground for
DC / "LF" static, the better?)

It was a (too-) quickly typed posting I guess, because I meant of course the
"ground" or "return" OF THE ANTENNA SYSTEM being connected to the "ground"
connection of the input winding of the broadband transformer.
In an ideal situation this antenna system "ground" or "return" could be
"isolated" from the shack/receiver/safety ground; the output of the
broadband transformer could then be connected to the shack/receiver/safety
ground on one side so that unbalanced filters could still be used?

Also would a galvanic isolation be useful wrt. avoiding ground loops in a
HPSDR setup or is this a non-issue in Mercury/Penelope setups and only of
importance if ALEX would be used as a BPF in a Softrock/Janus/Ozy like
setup?

Another thing I am wondering about is if it would be beneficial if a step-up
of impedance on the output of such a transformer (say 600 Ohms or so) would
give better results for the passive filters?
I played around somewhat with filter simulation software and if I remember
(and simulated!) correctly, it seemed to be of influence. Or am I wrong?



73's,

Henry.




> -----Original Message-----
> From: Graham Haddock [mailto:KE9H at austin.rr.com] 
> Sent: maandag 6 augustus 2007 19:43
> To: Henry Vredegoor
> Cc: hpsdr at hpsdr.org
> Subject: Re: [hpsdr] ALEX - Input protection
> 
> 
> Henry:
> Comments embedded below.
> 
> Henry Vredegoor wrote:
> > Would a (broadband) input transformer help in this respect?
> >   
> It depends.  Most of the energy generated by a lightning 
> strike is between
> 5 kHz and 300 kHz.  Only a very small percentage is above 1 
> MHz.  So, if the
> bandwidth of the "broadband" transformer had a low frequency cut-off
> of around 160 meters, and the transformer provided a very low
> impedance to ground below 500 kHz, then it could help a lot.  
> If it was
> truly broadband, passing energy down to the tens of kHz, then 
> it would not
> help much at all, other than to provide a DC path to ground.
> 
> In most ham radios, it is the narrow per-band bandpass filters that
> limits the energy getting to the front end of the radio, not 
> really the
> voltage limiter at the input.
> 
> The goal of MERCURY seems to be to provide a very broadband receiver,
> enabling all kinds of experiments, test equipment operation and
> multi-channel and multi-band receive, which is somewhat at odds
> with the transient protection.
> > A permanent connection to ground of one side of the 50 Ohm 
> input winding
> > would discharge the antenna system permanently.
> >   
> ALEX will have some 10K resistors to ground to bleed off any DC static
> build up.
> > Also as ground loop prevention?
> > Could be part of some filter in ALEX ? 
> >   
> All of the filters are unbalanced, with one-side hard grounded.
> > IP3 of the device could be a problem.
> >   
> Surprisingly, the IP3 performance of the ferrite input/output
> transformers that Phil and I looked at for some alternative
> ALEX designs had extremely good IP3 performance,
> that is, above +50 dBm.  This surprised me in light of the
> way that the powdered iron toroid inductors do limit
> IP3.
> 
> 
> So, I will have a bidirectional voltage clamp on each ALEX receiver
> input that will start clamping around 3 volts, and still clamp
> below 20 volts, even in the presence of tens of amperes of
> transient.  This is not lightning protection, you still need to
> shunt the lightning energy to ground before it ever gets to
> your equipment.  This is transient protection to limit the
> input voltage to sensitive semiconductor switches, attenuator,
> and amplifiers, as best we can.
> 
> --- Graham / KE9H
> 
> ==
> 
> 
> 


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