On Fri, 28 Feb 2014 13:10:34 +0000, gareth wrote:

"Stuart Longland VK4MSL" wrote in message
...
I'd like to reach up into the VHF; 2m if possible. Now I bought some
Si570 ICs the other day, and they're quite capable of reaching 160MHz.
So 144MHz is no problem. Taking the 3rd harmonic should get me to
70cm.

10.7 MHz is pretty standard for both*****, and there are helical cavity
filters for 145MHz,
as used in the "Simple Spectrum Analyser" design (G4PMK, Nov 1989
RadCom) Nor sure about 70cms, but there might be SAW filters, as used
for the 433.92 domestic products.

Yeah, not sure what the bandwidth is like on those. Here in Australia,
2m extends up to 148MHz, so I was looking to plonk a filter at 146MHz in-
circuit with a 2MHz bandwidth.

I found a calculator for helical resonators he http://coil32.narod.ru/
calc/helical_resonator-en.html

It looks simple enough (perhaps deceptively so).

Failing that. a cavity filter or interdigital filter, is a
possibility.The most professional design around is probably Scotty
Sprowls's spectrum analyser (http://scottyspectrumanalyzer.com) and that
uses a cavity filter for a 1.3GHz if,
with the second IF being 10.7 MHZ.

BUT BUT BUT as emphasized above, these are thoughts and not experiences!

***** I've a scrap main board from an FT780 which is a 70com rig but it
uses IFs of 56MHz, 10.7MHz and 455kHz

That would make sense then, I couldn't see how a 10.7MHz IF would work
for 70cm, but put a 30MHz IF in front of it, then no problem.