|[AMRadio] Stagger Tuning IF transformers with scope|
garyschafer at comcast.net
Tue Feb 1 23:28:02 EST 2005
To do it with the scope you need a sweep generator synced with the scope
horizontal trace. Then hook the scope vertical to the output of the IF
stage. It is difficult to do without some sort of marker generator for
the two frequencies you want for the bandwidth. You could use one signal
generator and move it to each band edge and put a mark on the scope screen.
Another way to stagger tune the IF is with just a signal generator and
the S meter on the receiver. Assuming the receiver has good dial
calibration of 1khz or better. Actually you don't even need a signal
generator. You can use the crystal calibrator for the signal source.
Tune in the calibrator signal and find the center of the band pass by
maximum signal strength. Note that frequency on the dial. Now tune to
one side and see what frequency where the S meter drops about 1 S unit.
Do the same in the other direction. Add the two differences together and
that is how wide the If is now.
To widen it, tune to where you want the new band edge on one side. Now
tune one of the IF cans to get a peak at that frequency. Move the dial
to the other side of the signal by the same amount frequency wise and
tune a different IF can for a peak there. Note that the total signal
will be down some from when you started and the S meter will not read as
high as originally.
Repeat these same steps with other IF cans. Then recheck where the 1 S
unit down points are on both sides of the signal. Also see that the
difference comes out to the frequency where the middle should be.
Sometimes it is easier to stagger the two coils in each can. Tune one
high and one low.
Do this a little at a time and recheck your 1 s unit down points often.
That will give you an idea of the progress.
When done tune across the signal and watch the S meter for "ripple". You
will see it go up and down an S unit or so when tuning from one side to
the other. You don't want to get too much ripple in there or it will
cause distortion. You can play with the tuning a little to smooth out
Once you get the feel for how much things change you can do a pretty
good job usually. Sometimes you run low on gain. Some receivers have
such sharp IF cans that you can not get rid of the ripple when
staggering them very far without loosing a lot of gain.
Ka9p at aol.com wrote:
> It's embarrassing to ask, but I want to stagger tune the IF transformers in a
> 51J for a little broader AM response with my scope, and realized I don't know
> exactly what to do - I assume you use a sweep generator or FM sig wider than
> the bandpass, but don't understand how to use the scope to look at the
> response (and to think I got my license when you had to know something) - if there's
> a reference on line somewhere or someone can give me a hint I'd really
> appreciate it. Thanks, Scott
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