EMRFD Message Archive 12499
Message Date From Subject 12499 2016-03-12 04:35:48 Ken Chase RF chokes
I am working on a small signal amp. The design calls for a RF choke in the collector of a value of 120 uH at 9 MHZ. The amp is going to be for 7 MHZ, so do I need to scale this RF choke for 7 MHZ?
If so then 120 uH @ 9 MHZ on FT37-43 is 6786 ohms. Then for 7 MHZ this 6786 ohms on FT37-43 is 154 uH. Is my calculations correct?
12500 2016-03-12 09:45:21 Jim Strohm Re: RF chokes Jim N6OTQKen,Was the original amp designed with the aid of modern modeling tools, or just something that somebody threw together that worked? In other words ... do the transistor's input and output impedances support the 6.8Kohm choke value, or could it conceivably be something lower?I'm wondering whether maybe a 100 uH choke might have worked, but the designer only had a 120 uH in his junkbox. Hence the published value.If the choke really ought to be 6.8Kohm at 9 MHz, then your calculations look correct -- or close enough, anyway.
12501 2016-03-12 17:30:06 Ken Chase Re: RF chokes Hi Jim
The circuit is Wes' Hycas amp, so I assume it is correct. As it utrns out the reactnace between the two frequencies is not much, so I went in between turns wise.
73 Ken VA3ABN
12502 2016-03-13 10:01:23 Jim Strohm Re: RF chokes Jim N6OTQSounds good, Ken.As most of us should know by now, some component values aren't as critical as we make them out to be.