[Hammarlund] Re: Sweep Method for Alignment - ringing.

[Kenneth G. Gordon]

On 9 Apr 2006 at 19:32, Julian Bunn wrote:

> By the bandwidth, do you mean the width of the IF tuning peak?

Roughly, yes. Usually, the IF bandwidth is measured at the 3db down 
points. I.e., 3 db down on either side of the exact center peak.

> If so
> then this width is roughly 1/5 of my sweep frequency range i.e. about
> 20kHz. It varies (narrows) as the IF tuning is done.

That 20 Khz is at the "60 db down" point, or greater...or at least it 
BETTER be, I.e., at the bottom of the trace where it is widest. That 
point should not be used to adjust to. What Andy and I are talking about 
is the much more narrow point only a short distance down from the top.

That 20 Khz point is WAAAAAY "down" and it should take an extremely 
strong signal outside the IF passband, i.e. outside your curve, to "sneak 
under" that point. I.e., to be heard in the receiver output.

> 
> (My goal when tuning the IF is to maximise the height of the peak,
> which is achieved when the peak is at its narrowest, and so, I
> suppose, when its bandwidth is at a minimum?)

Well, yes, partly. However, you must also adjust for maximum symetry 
of the IF passband shape.

> I have a feeling I may be missing understanding something important
> here ... if the width of the peak isn't the bandwidth, then what is
> it, and how do I change it to observe the ringing effect you and Ken
> are talking about?

What you are missing is a clear understanding of what is the exact 
definition of "IF bandwidth". IF bandwidth is always measured at a 
particular point on the IF curve, usually the "3 db down", or -3db,  
points. The absolute peak of the IF bandpass curve is the 0 db point. 
Any point lower down on the curve, where the IF response is less 
sensitive, is "down", or negative. Use of the word "down" actually 
comes from the curve itself, since those points are "down" on the sides 
of the curve.

You can calculate the -3db points from the voltage information on your 
scope screen. Again, take the absolute peak of the IF curve as the 0 db 
point, and calculate "down" from there.

Now, at narrow IF bandwidths, when ringing occurs, one of the effects 
is that the IF bandwidth trace will appear on the screen to be much 
wider than it actually is. Another effect is that the trace can be an odd, 
non-symetrical, shape. A third can be that it can be difficult to effect the 
shape of the IF passband curve in any repeatable manner.

The IF passband MUST be symetrical, unless you want it specifically to 
be non-symetrical for some particular purpose, i.e. to reject a much 
stronger signal on one side or the other of the IF passband curve.

Ringing can be caused not only by too fast a sweep, but also by a 
signal with too great an amplitude within the IF passband. "Ringing" is 
simply another way of saying "shock excitation" to an "almost 
oscillating" condition. In the same way that a bell rings when struck, a 
narrow selectivity IF strip, orany filter, can "ring" if "struck" by any of 
several events occuring in its passband. The same thing holds true for 
any high-selectivity component, i.e. a single crystal, or crystal or 
mechanical filter, or an active filter. We can visualize an IF strip as a 
filter against anything outside its passband.

Therefore, we must not only keep the sweep frequency as slow as 
possible, but also the amplitude as low as possible. This is one of the 
reasons why even using the simple output meter method, we are 
admonished to keep the signal generator output as low as we need to 
just get a good reading.

Ken Gordon W7EKB