[BC] "Cusp Rotation"

[Thomas G. Osenkowsky]

The optimal sideband cusp rotation will be different
for every transmitter design. One must consider the
ATU phase shift, transmission line phase shift and
tx output network phase shift for a non-D system. 
For a DA system, we have the issue of pattern
bandwidth.

For the simple non-D case, usually a Tee network
is employed in the ATU. The rotation can be measured
at the ATU input terminals. The phase shift of the
transmission line (considering Velocity Factor) can be
calculated or the rotation measured at the tx antenna
terminal.

The transmitter output network is more complex than
a simple Tee network. It has the task of matching the
final amplifier(s) to the antenna and attenuating spurious
and harmonic energy. Most new transmitters combine
multiple PA stages to produce full power. The optimal
cusp rotation must appear at each PA device. The phase
shift between this point and the antenna terminal is the
unknown that is important. The manufacturer can provide
this data. Not all transmitters are the same! In the older
plate modulated rigs, it was common to use a -225 deg
network. This provided isolation between Plate Tuning
and Plate Loading. Some newer transmitters employ
90 degree, 135 degree or even 180 degree output
networks. Placing a symmetrical load to  the antenna
terminal can be the worst possible load to present to the
PA for a -225 degree network, but perfectlu acceptable
for a 180 degree network.

Tom Osenkowsky, CPBE


----- Original Message ----- 
From: "Kent Winrich" <kwinrich at gmail.com>
To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
Sent: Friday, March 16, 2007 11:48 AM
Subject: Re: [BC] "Cusp Rotation"


> You are expecting the manufacturers to provide this info??? 
> HAHAHAHA  They dont know half of what they put out.  Heck I couldnt 
> even get a simple thing like BTU output from Continental for their 
> 816HD (you know so I could spec out an HVAC system??) let alone a 
> bunch of measurement points on their own transmitter.
> 
> I like the idea of your Vector analyzer.  Sure would be easier than 
> going through the OIB and what have you that I did for my two HD 
> installs.  UGH.
> 
> Kent Winrich
> 
> 
> On 3/13/07, Phil Alexander 
> <<mailto:dynotherm at earthlink.net>dynotherm at earthlink.net> wrote:
> 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.
> 
> What is really needed is for manufacturers to include
> a dynamic measurement point(s) for the **actual**
> dynamic, complex impedance across the range of operating
> frequencies as seen at the **true** PA output before
> combining/matching to the Tx output spigot.
> 
> That is to say something approaching the idea of a
> **true** PA load sample that might be read by a RF
> vector analyzer.
> 
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