[HBR] HBR2K -- Chapter 14 -- Large Signal Performance, Part 4

[hbr@mailman.qth.net]

In a message dated 4/12/03 9:12:24 PM Eastern Daylight Time, 
waltah@earthlink.net writes:

> > I've thought of doing that sort of thing to match an xtal filter but being
>  > the lazy sort I've used LC matching instead. Some sets have used a
>  > slug-tuned coil and capacitive voltage divider arrangement (see ARRL
>  > handbook), others a tuned-toroid-with-link setup.
>  
>  One man's lazy is another's 'too hard for me.'   I tried that first, and 
>  tied myself in knots, trying to get a match.   With a cathode follower, 
>  everything can be calculated -- the source impedance is 
>  1/transconductance in parallel with the cathode resistor so even for a 
>  triode at a funny plate voltage you're in the ballpark to start.

Just for info, here's how I do it for high Z tube circuits matching to 
filters that want resistive terminations:

First, a suitable toroid is wound with suitable wire and tuned with a 
suitable trimmer so as to resonate at the IF. ("suitable" defined as "looks 
about right and could be obtained without too much trouble"). Number of turns 
on the toroid is noted. 

The trimmer is then removed from the circuit and the L and unloaded Q of the 
toroid measured at the IF. The equivalent R of the toroid is then calculated. 
The number of link turns is then determined from the "impedance varies as the 
turns ratio squared" formula. The link is then wound over the cold end of the 
tuned winding.

As a check, the Q of the main winding is measured with the link unterminated, 
then again with a nonreactive resistor connected across the link. The Q with 
the link terminated should be exactly half the Q with the link unterminated 
if the link turns are correct. 

Yes, there are some approximations in the process, but it has worked well for 
me.

Waddya think, sirs?

73 de Jim, N2EY