[BC] ATU broadband tuning

[Alan Alsobrook]

Dave Dunsmoor wrote:
> How do you guys who know this stuff well, adjust an ATU for not only low
> reflected power at carrier, but also at +/- some amount of khz to effect a
> clean match for good sideband response? I'm not asking for "secrets" if you
> have specifics that you like to keep in your toolbox, but in general what is
> the "art beyond the science" of broadbanding an ATU?

Most likely the reason no one answered is that there is no one answer. 
if you have a narrow banded ATU then you first should determine why it 
narrow banded. If it's the load that is narrow then you might could add 
  a lumped impedance to somewhat counteract it. (this is often a -x in 
front of the T network).  If it's not the load then often time it is 
because somewhere along the way the Q of the matching network was made 
too high.  As an example if you need a .002uF cap in the shunt leg but 
you only have a .00051, we all know that by using a bit more coil that 
.00051 can me made to look like a .002 on carrier and give you the 50j0 
you wanted to see. Yet as you shift off frequency because the higher Q 
of that branch of the network the Z shifts much more rapidly.

So the answer is multi fold.  We spend about half a day on this in the 
AM class. (Hopefully we will be able to have another one when things 
improve). No one answer works in all cases.

Also about rotation, what you are looking is the transmitter has a 
matching network somewhat similar to an ATU. That network will have an 
impedance shift across the frequencies as well. What you are looking to 
do is to cause the shift in the ATU to match that of the output network 
of the transmitter, and then it works much better across the band. Many 
transmitters have different shifts thus you need to consult with the 
manufacture to determine the optimal shift for each transmitter. While 
this has always been true it didn't become a concern until IBOC arrived 
on the scene. For the most part if the shift isn't the same no one will 
ever know on analog transmissions.

  --
Alan Alsobrook CSRE AMD CBNT
St. Augustine Fl. 32086 904-829-8885
aalso at Bellsouth.net