[AMRadio] Stagger Tuning IF transformers with scope

[Edward B Richards]

Hi Guys;

You can broaden out sharp IF transformers by adding a resistor across one
or both windings, lowering the Q. Experiment with different values of
resistance to see the effect. As the Q goes down, so will the gain. Good
luck.

73, Ed Richards K6UUZ


On Tue, 01 Feb 2005 23:28:02 -0500 Gary Schafer <garyschafer at comcast.net>
writes:
> Hi Scott,
> 
> 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 
> the ripple.
> 
> 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.
> 
> 73
> Gary  K4FMX
> 
> 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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