[BC] "Cusp Rotation"

[Jeff Welton]

Phil;

I have to respectfully disagree on one point you raise below and 
offer an adversarial perspective on another.  Other points have been 
duly taken under advisement.... Once the thread dies, I'll condense 
it and pass it over to Engineering, so they can use the input from the field.

You suggest phase shifting to put the PA load so that it appears 
"horns up" (oriented around a 12:00 axis).  This is counter to what I 
have been taught and seen where the horns should be oriented to the 
left or right (3:00 or 9:00 axis... what Gray calls drunken smiley 
faces).  Either 3:00 or 9:00 rotation will work for IBOC, although a 
3:00 orientation provides a bit of pre-emphasis that can be 
beneficial to the analog.  Incidentally, among other literary 
endeavours that are about to be, or have already been, published, 
Bobby Cox at Kintronic Labs wrote a very detailed and well researched 
article about this topic, available on their website:
http://www.kintronic.com/site/techpapers/TP-AM_Case_Studies.pdf

I don't disagree with the concept of having a measuring point for the 
PA outputs... the trick is to make it physically close enough to the 
actual PA output point(s) so that it doesn't induce any phase delay 
of its own, or to provide a correction chart.  In addition, due to 
safety concerns, it would need to be either interlocked or shielded 
to prevent direct human contact with the hot conductor.  All of this 
is going to affect equipment cost.

Our position historically has been that since the antenna system 
needs to be optimized for HD Radio anyway, the cost to the station is 
less if we provide the phase delay information publicly so the 
consultant can factor that into the optimization process, as opposed 
to adding cost to the transmitter.  Correct me if I'm wrong here, but 
if you have a Smith chart plot with specific starting, ending and 
center points, how is it harder to set them up with a network 
analyzer so that they are oriented around, for example 11:00 as 
opposed to 3:00?  Obviously antenna work is not my strong suit, so 
this is a chance for me to learn something from the perspective of 
those who are doing it regularly.

Regarding the anto phase correction and/or manual rotators in the 
transmitter network, there are various arguments on both sides of the 
fence.  One argument against would be the need for measuring a 
complex impedance at carrier and at both sidebands, then either 
displaying it, or using it to drive the auto circuit.  However.... 
what happens if (oh my beating heart) the load is near perfect and 
the levels are too close to be measured?  What happens if the load is 
really bad and the correction circuit goes off the rails trying to 
fix something that won't pass HD, no matter how much 
tweaking?  Designing for all these possibilities again impacts cost.

The problem as I see it - and this is probably a topic for a 
different thread - is that we have to design the units to keep the 
guys who have to install them and work on them happy... but we also 
have to keep the price low enough to make the guys who pay for them 
happy as well.  Unfortunately, in the majority of cases, those guys 
are not the same person and frequently they don't share the same 
objectives.  That's probably one of the biggest challenges any 
manufacturer has to face.  There's no question that we could design a 
box that would perform virtually any task any engineer could ever ask 
for - but if it's twice as much as another box that makes the same 
output power, regardless of the added features, it's not going to 
sell well enough to keep it in the stable.

Enough bandwidth, I'm drifting off the topic and I need to get to 
work; the primary purpose of my response was to get your thoughts on 
the orientation of the cusp and to seek input on the design concepts.

Best,

Jeff Welton
Technical Sales Representative
Nautel
+1.902.823.3900 x 127
www.nautel.com


-----Original Message-----
From: Phil Alexander

This whole "phase rotation" / "cusp rotation" business
drives me slightly nuts, because what we are really
attempting (as best we can) is indirectly "de-rotating"
or moving the **actual** PA load at the true output of
the PA to R = x j0 at the operating frequency with the
sideband frequencies "horns up." Where "x" is the true
output resistance of the power amplifier(s). IOW it is
almost like what we did by tuning and loading a tube
box "back in the day" by operating slightly off
resonance for best load to yield best modulation b/w,
except today we must approximate it and set it in the
blind.