On Fri, 04 Mar 2005 01:13:39 GMT, "Thomas Magma"
wrote:

Wow this is a long thread. Don't really know where I should put my two bits
in, but here it goes.

I have designed several RF PA sections in the past. 500MHz at about 50W.
Pretty easy stuff if you have the right tools and know how to use them. The
tools I like using for matching the power output FET is two triple stub
tuners. One on the input of the FET and one on the output. So it
goes...pre-amp (50 ohm output) - stub tuner - FET - stub tuner - 50 ohm
dummy pad - spectrum analyzer. Then just tune the stubs for the performance
you desire, these include: efficiency (thermal issues), harmonic content,
spurious emissions, load VSWR considerations, cold start, ect. Then remove
the FET and look into the triple stub tuners with the network analyzer.
Model and duplicate the network out of discrete components that can handle
the voltage/power, send the design off to the enviro test lab, and head home
early for the day.

Cheers,
Thomas

Hi Thomas,

Thanx, your two bits were worth more than the academic plug nickel.
This is something that our original poster should hearken to as his
needs were obviously production oriented. Bench experience will trump
cut-and-paste theory in a heart-beat.

However, triple stub is pretty aggressive. How long did it take you
to flatten response?

73's
Richard Clark, KB7QHC

I read in sci.electronics.design that Richard Clark
wrote (in ) about '1/4 vs
1/2 wavelength antenna', on Thu, 3 Mar 2005:
On Thu, 3 Mar 2005 20:53:48 +0000, John Woodgate
wrote:

Doesn't everyone know that an audio amplifier that id designed to feed
an 8 ohm load MUST have an output source impedance of 0.0000001 ohms or
less.

Hi John,

I hope that was a joke.

If you read the whole paragraph, you will see.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I read in sci.electronics.design that Tom Ring
wrote (in ) about '1/4 vs
1/2 wavelength antenna', on Thu, 3 Mar 2005:

He forgot to mention that for that output impedance to be relevant, you
need superconducting wire to the speakers as well as superconducting
voice coils.

See the last sentence, about the effect of an **8 ohm** source impedance
on damping.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

"Richard Harrison" wrote:
Radio transmitters don`t produce significant harmonics. It`s the law.
They are linear power sources. We can and do tune them for all the power
they will produce under their particular operating conditions of drive
and d-c power supply. They operate at more than 50% efficiency which
means that they don`t take power 100% of the time, but are switched-off
during part of the r-f cycle. Output impedance is thus an average over
the entire cycle. It`s OK. We have no harmonics. Gaps are filled by the
tank circuit and other filters.
_______________

Note that without adjustment, modern, solid-state FM broadcast transmitters
can (and do) provide 80% or better PA efficiency into a 50 ohm load across
20% bandwidth, with no tank circuit or other in-band filter(s).

If this is done in a commercial service, certainly it could be done in
amateur radio devices. Physics is not application-selective.

Posters of various forms of "Absolute Truths" to the contrary might well do
a bit more research.

RF

Response is flattened through gain controlling the pre-amp from a look-up
table held in the micro's EEPROM. The alignment procedure is automated using
the HB-IP bus from the spectrum analyzer and a computer. The
computer/analyzer also looks for harmonic content and spurious emissions
during this procedure. Think it takes about ten seconds to do this.

"Richard Clark" wrote in message
...
On Fri, 04 Mar 2005 01:13:39 GMT, "Thomas Magma"
wrote:

Wow this is a long thread. Don't really know where I should put my two
bits
in, but here it goes.

I have designed several RF PA sections in the past. 500MHz at about 50W.
Pretty easy stuff if you have the right tools and know how to use them.
The
tools I like using for matching the power output FET is two triple stub
tuners. One on the input of the FET and one on the output. So it
goes...pre-amp (50 ohm output) - stub tuner - FET - stub tuner - 50
ohm
dummy pad - spectrum analyzer. Then just tune the stubs for the
performance
you desire, these include: efficiency (thermal issues), harmonic content,
spurious emissions, load VSWR considerations, cold start, ect. Then
remove
the FET and look into the triple stub tuners with the network analyzer.
Model and duplicate the network out of discrete components that can
handle
the voltage/power, send the design off to the enviro test lab, and head
home
early for the day.

Cheers,
Thomas

Hi Thomas,

Thanx, your two bits were worth more than the academic plug nickel.
This is something that our original poster should hearken to as his
needs were obviously production oriented. Bench experience will trump
cut-and-paste theory in a heart-beat.

However, triple stub is pretty aggressive. How long did it take you
to flatten response?

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On Fri, 04 Mar 2005 01:13:39 GMT, "Thomas Magma"
wrote:

Wow this is a long thread. Don't really know where I should put my two bits
in, but here it goes.

I have designed several RF PA sections in the past. 500MHz at about 50W.
Pretty easy stuff if you have the right tools and know how to use them. The
tools I like using for matching the power output FET is two triple stub
tuners. One on the input of the FET and one on the output. So it
goes...pre-amp (50 ohm output) - stub tuner - FET - stub tuner - 50 ohm
dummy pad - spectrum analyzer. Then just tune the stubs for the performance
you desire, these include: efficiency (thermal issues), harmonic content,
spurious emissions, load VSWR considerations, cold start, ect. Then remove
the FET and look into the triple stub tuners with the network analyzer.
Model and duplicate the network out of discrete components that can handle
the voltage/power, send the design off to the enviro test lab, and head home
early for the day.

Cheers,
Thomas

Hi Thomas,

Thanx, your two bits were worth more than the academic plug nickel.
This is something that our original poster should hearken to as his
needs were obviously production oriented.

I doubt you understand what he wrote. I can't fathom why you would be concerned
with the OP when your own difficulties are so acute.

Bench experience will trump
cut-and-paste theory in a heart-beat.

How would you know?

However, triple stub is pretty aggressive. How long did it take you
to flatten response?

How long will it take you to figure out that he wrote not a wisp of a word on
what the "output-Z" of the amplifier is? He did write that he determines how
the amp was loaded to acheive power, something I've been saying is the prime
concern.

On Fri, 04 Mar 2005 16:47:05 GMT, "Thomas Magma"
wrote:

Response is flattened through gain controlling the pre-amp from a look-up
table held in the micro's EEPROM. The alignment procedure is automated using
the HB-IP bus from the spectrum analyzer and a computer. The
computer/analyzer also looks for harmonic content and spurious emissions
during this procedure. Think it takes about ten seconds to do this.


Hi Thomas,

10 seconds to adjust all 6 stubs?

73's
Richard Clark, KB7QHC

Richard Fry wrote:
"Physics is not application-selsctive."

True. The laws of physics are inviolable.

The FM amplifier does not need linearity. Amplitude distortion is
irrevelant. Severe clipping to remove amplitude variations is common
practice. Phase/frequency shift is the modulation of interest.

Clipping generates harmonics and FCC rules limit harmonic transmission
in all services. Any manufacturer wants to require the fewest user
adjustments. I`m not surprised that tuned frequency selective circuits
are minimized.

I would be surprised if some final filter were not used to guarantee
compliance with the rules.

Best regards, Richard Harrison, KB5WZI

No the triple stub tuners are only for development. Production boards have
discrete components to form the match network. Power levelling or
"flattening the response" is computer adjusting the output power to
compensate for the reactive components to ensure a constant output power
over the entire band of the radio. We also put in a small temperature
compensation coefficient into the EEPROM because the PA tends to put out
more power when it is cold.

"Richard Clark" wrote in message
...
On Fri, 04 Mar 2005 16:47:05 GMT, "Thomas Magma"
wrote:

Response is flattened through gain controlling the pre-amp from a look-up
table held in the micro's EEPROM. The alignment procedure is automated
using
the HB-IP bus from the spectrum analyzer and a computer. The
computer/analyzer also looks for harmonic content and spurious emissions
during this procedure. Think it takes about ten seconds to do this.


Hi Thomas,

10 seconds to adjust all 6 stubs?

73's
Richard Clark, KB7QHC

On Fri, 4 Mar 2005 11:11:32 -0600, (Richard
Harrison) wrote:
"Physics is not application-selsctive."
True. The laws of physics are inviolable.
....
I would be surprised if some final filter were not used to guarantee
compliance with the rules.

Hi Richard,

Yes, that would be the technical marvel of the ages, but just like our
rigs, even the biggest FM transmitters bend to the necessity for
output filtering:
http://www.broadcast.harris.com/prod...%20Bro%2DB.pdf

There is an amusing claim, however, for their power module(s)
"Each module is conservatively rated to produce
850W of power into a system VSWR of 1.5:11."

Not a very good copy editing job is my guess.

Looking at the "efficiency" side of the equation is simple here too:
Power Consumption (nominal)
• Z2CD: 4.0kW at 2.2kW output power
55%
• Z3.5CD: 6.1kW at 3.75kW output power
61%
• Z5CD: 7.9kW at 5kW output power
63%
• Z7.5CD:11.7kW at 7.5kW output power
64%
• Z10CD: 15.3kW at 10kW output power
65%
• ZD20CD:31kW at 20kW output power
65%

73's
Richard Clark, KB7QHC