[hpsdr] HPSDR Preselectors and Front Ends

[P M Gant]

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