[BC] "Cusp Rotation" (long winded, sorry)

[Jeff Welton]

Phil;

>Yes, I'm familiar with the article although I haven't 
>read it for a couple of years. As I recall, that article 
>was based on a Nautel wasn't it?

It was, on our ND1, 1kW unit - the predecessor to the J1000.

>However, that is oriented to the Tx output, thus not 
>relevant to what I was trying to say.

Well, that's the part where I was originally (possibly <g>) disagreeing...

Phase delay through the ND1 final filter from PA output to TX output connector, when the filter is properly tuned, is 180 degrees.  Thus the Smith chart is going to look exactly the same at both points.  Granted, that is an exception (for us anyway), only our 1kW transmitters exhibit this characteristic.  My point was that a horns up orientation at the PA output is not the ideal.  Horns left is the best, horns right a close second.  At the transmitter output, it could be just about anything - on this point we agree; if the manufacturers don't provide the phase delay data, your job becomes much more difficult without a field measurement of phase delay.

> No, a sample is what I said. That shouldn't be much hotter
>than the BNC for driving a mod. mon.

Fair enough.  It would need to be divided down through a high impedance purely resistive network, but I understand better now.  FWIW, on our XL and XR series transmitters, it's a simple procedure to get a scope probe directly on the PA outputs - tech support would be happy to walk you through it (or I could - I haven't forgotten that much yet!).  On the ND series it's a touch more complicated, but since we can easily get to the module output and since the components between the PA output and the module output are fixed devices, we can get within a degree of offering a correction curve - as that model is out of Production, it would have to be done by request, but it's not a difficult thing to do on our end - we have the modules in stock at all times.  Alternately, a wire soldered to the PA output and passed through the front of the module would give a sample point, but it's definitely not ideal (or even desirable with the unit running at full power).

>Yes, BUT, the actual phase delay and the design phase
>delay depend on what happens in tuning during test at
>the factory, and they may not be quite the same. As I
>understand it, the only way Nautel can provide the exact
>rotation is by customer request for factory measurement
>during final test. (And I understand you do that upon 
>request, so I do appreciate that point.)

Again, thanks for the clarification.  On this point we agree.  Incidentally, it's not done on request - every AM transmitter shipped from Nautel in the past 2 years or so (I don't remember the exact date) has the factory measured output network phase shift included with the final test results and configuration sheets that are shipped with the box (usually in a Ziplok bag taped inside the transmitter).  Granted we haven't done a fantastic job of telling you it's there, but it sure should be for all current production AM transmitters at or below the 50kW level.

>Second, consider that shutting down a array for measurement
>is not a trivial event because it means taking the station
>off the air and getting someone with the right tools to do
>the measurement. Once upon a time this was the case with
>common point impedance measurements, but that brought 
>about the common point version of the OIB and eliminated
>the problem.

Valid point... BUT... how are you going to measure the sideband impedances with the common point OIB if you can't sweep the frequency + and - 15kHz?  I know I can change the transmitter frequency by that much, since all current production models are driven by a DSP based synthesizer, frequency selectable in 100 Hz steps, but wouldn't the FCC frown on that? (sorry, that was a touch of sarcasm - I'm still working on that <g>)  My point is that, during optimization for IBOC, at some point the system is going to have to go down regardless, both for measuring and verification of tuning.  I think the point we are debating is whether it's more expedient to just leave it down while we tune it up, or if it's better to take it down, sweep it, bring it back online and adjust it hot, then take it down and sweep it again.  Once more, this is an area where I'm the first to admit I'm lacking in knowledge, so any clarifications are welcomed.

>Third, consider that transmitter frequencies are changed 
>in the field, and the best way of assuring the best match
>to the PA(s) would be measurement at their output.

This is a very valid point and one that I admit I hadn't considered during this debate.  A frequency change, depending on component tolerances will definitely change the phase delay to some extent through all of our output networks except the 1kW units, which are designed to be exactly 180 degrees when tuned.  While phase delay between PA output and filter output can be easily measured in the field, this does point to a useful application of a PA output sample.  That's the kind of input I'm seeking - thank you.

>Understood and agreed. However, and this is the point to consider, as
>matching to the actual PA(s) go(es) so goes the bit error rate of
>digital transmission. In the digital world, as Bobby Cox's paper shows,
>audience is essentially an inverse function of bit error rate.

On this point we agree wholeheartedly.  BER is very directly linked to ideal cusp rotation and can make the difference between a 50 mile range on one 5kW DA and not being able to receive the station with the truck parked 5 feet from the base of the high power tower on another 5kW DA.  This is not an exaggeration, I really have seen both cases, firsthand.

>the first Tx manufacturer who does it economically would gain what
>the guys who pay for the iron call a "market advantage."

Without letting any cats out of any bags, I think it is safe to say that we at Nautel, and probably every other manufacturer involved in producing equipment to handle all these ones and zeroes is aware of this.  As you've said before, it may not be the best system in the world, but it's what we've been given and it's up to us to make it work.  

We've got more tools to bring life to more ideas these days than at any point in history and it's a wildly exciting time to be along for the ride.  Some of the ideas are practical, some are not - sorting the wheat from the chaff is almost as much work as the actual implementation.  

>Thanks for the reply Jeff, I always enjoy your insightful viewpoint.
>BTW congrats on the new job. It is a great move on Nautel's part
>to put knowledgeable people in the field when others are making all
>the wrong moves.

Thanks for the compliment, I always enjoy a debate on any topic with someone who has thought out their stance and has the history and data to defend it.  I'm not sure about the "knowledgeable" part though, the more I learn, the more I realize I don't know nearly as much as I used to think I did!

Best Regards,

Jeff Welton
Technical Sales Representative
Nautel 
+1.902.823.3900 x 127
www.nautel.com
?
Making Digital Radio Work. 

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