[hpsdr] HPSDR Preselectors and Front Ends

[Phil Harman]

Hi Peter,

Thanks for your post regarding a narrow band RF filter for Mercury. You are 
quite correct that  Graham, KE9H, and I did look at the Hilberling PT-8000 
input filter when we started the design for Alex.  I found that even with a 
very large core the filter would degrade the IP3 of Mercury. This seemed to 
be true of any design that used a resonant circuit for it's main 
selectivity.  I also found that measuring IP3 > +40dBm was very hard. In 
fact I had to build special test equipment to enable me to get meaningful 
readings, since even good quality commercial equipment was not giving me the 
required results.

In terms of the requirement for  narrower RF selectivity I guess we will 
need user feedback.  I live in a very quiet part of the world so don't have 
dozens of 59+ CW signals to contend with during a contest - in fact in this 
part of the sun spot cycle I have very few signals at all!

At the moment I'm not aware of a project to build a narrow RF filter so your 
design could be of interest to others. Perhaps we could support the 
selection of coil taps and capacitors from the Atlas bus (or Mercury) so a 
Basic Stamp  is not required.

If you let me have a copy of your notes I'll arrange for them to be placed 
on the Wiki.

Thanks again for telling us about your interesting project.

73's Phil...VK6APH


----- Original Message ----- 
From: "P M Gant" <ha5rxz at gmail.com>
To: <hpsdr at lists.hpsdr.org>
Sent: Sunday, March 01, 2009 8:15 PM
Subject: [hpsdr] HPSDR Preselectors and Front Ends


> ***** High Performance Software Defined Radio Discussion List *****
>
> Hello Everyone
>
> I joined the HPSDR group because my main area of ham radio research is
> high performance HF front ends. Here in Budapest the RF environment is
> very noisy so I have a very good test environment. Your
> preselector/bandpass filter module (Alexiares) is interesting but I
> wonder if more selectivity may be needed, especially when working CW?
> Looking at the whole system I imagine the following:
>
> Starting environment - DC to daylight
> 33 MHz Low Pass filter - DC to 33 MHz
> Filter through Alex module - 5 MHz width (approximate figure)
> Preselector? - 20 KHz width
> Mercury front end
>
> I spent yesterday reading all the posts on the Reflector concerning
> preselectors and I note that at one point you considered the design of
> the Hilberling PT-8000. Using this design and its large toroid I have
> been conducting experiments into preselector designs and I now have a
> working system. Taps on the coil are controlled by the same logic that
> drives the six Low Pass & High Pass filters but I only have three taps
> instead of five. Selection of the capacitors (3-512pf) is carried out
> by a BASIC Stamp which is driven by the main VCO / 1000 (45KHz to 75
> KHz) This frequency span gives me 30,000 1KHz slots, this is enough
> resolution. The biggest challenge was keeping the RF from the Stamp
> away from the receiver input signal but I have now solved this
> problem.
>
> Will there be a preselector for the HPSDR system in the future or will
> the group just use the Alexiares? You are welcome to a copy of my
> notes if you want them because I have no commercial interests, radio
> is just a hobby.
>
> Gant, Peter HA5RXZ
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