Re: Receiver front-end filter #rcvr


Glen Leinweber
 

Ton,
Yes, Hans would be less-aghast at tiny hyper-abrupt varicaps than that monster
dual-gang variable capacitor - as Cristi points out, its size compares with the
whole receiver module!
The physical scale of Hans designs amazes me - his board layout is as crowded
as a rush-hour bus on this receiver module.
In general, electronic design trends toward microscopic size where robots
out-perform shaky human hands. Hans obviously uses CAD to design his boards,
using component placement tools effectively. My industrial PCB layout days were
with tape & vellum on a drafting table, when half-watt resistors were standard, and
generally lay flat: double-sided boards with plate-thru holes were exotic.

I'd want to do some careful IP3 measurements when substituting varicaps for that
very linear mechanical variable capacitor. If any large unwanted signals sneak in,
they'd cause spurious mayhem. The Tayloe mixer in these radios is well-renowned
for its spurious rejection. Why taint it with a potentially weak pre-filter?

Be aware that this tune-able filter does make compromises that the QRP-Labs
fixed-tuned band filter modules avoid. As stated, bandwidth at the high-frequency
end of the tuning range is very wide, with inferior stop band attenuation.

Join QRPLabs@groups.io to automatically receive all group messages.