Thomas Magma wrote:
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 was asking how long it took you to tune the triple stub filters
during devolpment.

I am curious about the exact nature of the impedance transmformation
these devices provided.

jk

"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, 04 Mar 2005 17:39:04 GMT, "Thomas Magma"
wrote:

No the triple stub tuners are only for development.

Hi Thomas,

I thought 10 seconds was awful quick. How long would it take to
flatten the response when manually adjusting the triple stub tuners?

What merit did you find with triple that could not be found with
double stub tuners?

73's
Richard Clark, KB7QHC

"Richard Harrison" wrote
The FM amplifier does not need linearity. Amplitude distortion is
irrevelant. Severe clipping to remove amplitude variations
is common practice.

Not so. You confuse receivers with transmitters. Limiting is supplied by
the IF strips of FM receivers to reduce/remove AM components on the incoming
wave, but FM broadcast transmitters are operated well below any
limiting/clipping level, and that is probably true of ham FM txs also.

Synchronous and asynchcronous AM are low in broadcast FM tx RF stages (the
FCC spec is -50dBc), but not because the FM amplifiers are "clipping."
Broadcast FM txs easily can be adjusted over an output power range of ~25%
to 105% or more simply by adjusting drive to the PA (keeping constant PA
volts). This technique often is used for output power control/VSWR
foldback, actually.

Clipping generates harmonics and FCC rules limit harmonic
transmission in all services. .. I would be surprised if some
final filter were not used to guarantee compliance with the rules.

You are confused again. I wrote that no "tank circuit or in-band filter(s)"
were necessary to achieve the high efficiency I described. Your post I was
responding to states that a "tank circuit and other filters" are necessary
for high efficiency -- that is not true.

Harmonics are present at the PA output of an FM transmitter, but "clipping"
is not the process whereby they are generated, as I state above. They are
reduced to legal values using a lowpass/harmonic filter. The FCC
attenuation spec for harmonics and spurs more than 600kHz from Fc is 80dB
below the unmodulated carrier.

The lowpass/harmonic filter does not improve efficiency--it has a small
amount of insertion loss in the FM band.

RF

Hi gwhite,

I would have to agree with you on most everything you have said through this
thread. I once saw my boss (with his "PHD") try to model and match a power
amp based on the small signal parameters off the datasheet. He insisted that
the stated input and output impedances were characteristic parasitics of
that amp and wouldn't change between a small or large signal. It was kind of
pathetic to watch him struggle for over a month on the matching network, and
I think he had resorted to guessing in the end.

I've often questioned why manufactures put small signal parameters on their
datasheets? Makes no sense to me. Even if they do publish some large signal
parameters it is unlikely to be the exact same mode of operation that you
need for your project.

Playing with triple stub tuners on PA's has shown me that there are many
combinations of input and output impedances that appear to give similar
results at any one frequency, but give different results at others
frequencies. So it's a matter of finding the input and output impedance that
give you adequate performance over the entire scope of your project.

Thomas

"gwhite" wrote in message
...
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.

"Richard Clark" wrote:
Looking at the "efficiency" side of the equation is simple here too:
Power Consumption (nominal)
(clip)
_________________

Another case of writing without knowing, I see.

The power consumptions you cite are the TOTAL values for those transmitters,
not of the RF power amplifiers alone. The total value includes the exciter,
driver(s), power supply losses, control circuits, and RF combining losses,
as well as power for the internal cooling fans. The PA modules have 80% or
better efficiency, by themselves.

The reason I know is that I was the author of those specs.

RF

If your amp has to operate over a wide frequency range it is not likely that
you can flatten the response just with stubs. Stubs should be looked at as
more single frequency devices than broadband networks. But you can use the
stubs to plot out the appropriate impedance curve on the Smith Chart to
ensure a flat response when you model in the discretes.

I usually just try to get the flatness of the response as close as possible
and rely on a software calibration routine to flatten it off. Saves a lot of
time.

It's my understanding that a triple stub tuner of the right length can reach
anywhere on the Smith Chart where as a double stub can not.

"Richard Clark" wrote in message
...
On Fri, 04 Mar 2005 17:39:04 GMT, "Thomas Magma"
wrote:

No the triple stub tuners are only for development.

Hi Thomas,

I thought 10 seconds was awful quick. How long would it take to
flatten the response when manually adjusting the triple stub tuners?

What merit did you find with triple that could not be found with
double stub tuners?

73's
Richard Clark, KB7QHC

On Fri, 4 Mar 2005 12:42:47 -0600, "Richard Fry"
wrote:

"Richard Clark" wrote:
Looking at the "efficiency" side of the equation is simple here too:
Power Consumption (nominal)
(clip)
_________________

Another case of writing without knowing, I see.

Hi OM,

Yes, I do recall your claims that contradicted Mendenhall's explicit
efficiency computations. So I see no need to pursue undocumented
claims you offer. Unless you can supply specific references from
Harris about this 80% efficiency, then such comments remain as suspect
as before.

The reason I know is that I was the author of those specs.

I am still wondering about the odd entry of:
"Each module is conservatively rated to produce
850W of power into a system VSWR of 1.5:11."

I notice you passed on discussion to this particular point of
accuracy. 11s can be explained by hitting 1 too many times, or 80 by
hitting an errant 0 too many. One of those things that escape the
notice of a spell-checker.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
"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."

Richard must be right. I guess a finger was left too long on the no.1
key and nobody caught it in time.

I admire Gates` scheme of paralleling many relatively low powered
amplifiers. If one fails, you can continue almost as if nothing
happened. Very nice.

Best regards, Richard Harrison, KB5WZI

On Fri, 04 Mar 2005 19:09:20 GMT, "Thomas Magma"
wrote:
If your amp has to operate over a wide frequency range it is not likely that
you can flatten the response just with stubs.

Hi Thomas,

Certainly not as conventional Triple Stubs. However, care to provide
some of the cogent details of that particular project? Any
interesting insights?

73's
Richard Clark, KB7QHC

"Richard Clark" wrote:
Unless you can supply specific references from Harris
about this 80% efficiency, then such comments remain
as suspect as before.

You may take what I wrote as being "from Harris," because I was part of
Harris FM Product Management for those transmitters before my retirement in
1999 (after 19 years there). I was responsible for documenting all
performance features and parameters published for the product line, using
numbers generated and approved by Engineering.

If the PAs alone were as (in)efficient as you imply with your calculations,
power consumption for the entire transmitter would be considerably higher.
Common sense should tell you that PA module efficiency would have to be much
higher than the efficiency calculations you posted in order for total power
consumption to be as stated on the Harris spec sheets.

I am still wondering about the odd entry of:
"Each module is conservatively rated to produce
850W of power into a system VSWR of 1.5:11."

Yes, that is a "typo," as you noted. Very good. It should read
"...VSWR of 1.5:1."

RF