[R-390] IF Deck Problems Revisited

[2002tii]

Roger wrote:

> The point that comes to mine is that an inch of wire and a by pass cap
> gets resonant at 455 (OK so 410 in your deck).

It would have to be an awfully large bypass capacitor. Small wire has a
high frequency inductance of about 20 nH per inch, so with typical
bypass values of .1 uF to .01 uF the resonant frequency will be 3-10
MHz. To get it down to 455 kHz, you would need about 8 uF. (This is the
very reason that you don't see values much above .1 uF used as bypass
caps at HF, and even smaller at VHF. Along with very short leads.
A 50 pF cap standing up on 1/2" leads resonates at around 150 MHz.)

See <http://www.ee.scu.edu/eefac/healy/indwire.html> for formulae to
calculate the inductance of a straight wire.

> So a poor connection on a ground point leaves a cap and its wire
> hanging like an antenna to some point. Then to really mess with us it
> behaves like a resonant circuit and injects the most unwanted signal
> back into the Deck some where.

A series LC is a low-impedance, high-current circuit element. If the
nether end is disconnected, it can't draw any current, so it isn't an
efficient radiator.

Now, a bad ground could cause the capacitor to quit bypassing, which
could easily cause oscillation. But in this case, it was probably the
unshielded IFs.

Best regards,

Don