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

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

"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

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:
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

On Fri, 4 Mar 2005 13:41:50 -0600, "Richard Fry"
wrote:

If the PAs alone were as (in)efficient as you imply with your calculations,
power consumption for the entire transmitter would be considerably higher.

The implication is drawn by and from your inertia.

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.

Hi OM,

It is tedious to have to carry your water for you. I had to chase
down your Mendenhall references, this seems to be a consistent trait.
Claims are generous in this group and heavily discounted due to the
paucity of facts. Such facts as may be drawn out, but could have had
been as easily offered by you:

"For even greater reliability, any
PA module can be used as an IPA module,
with absolutely no modification."

It is quite obvious that as an IPA, that in the lower wattage systems
it represents overkill at 845W to generate drive to final PAs to 2.2
KW output. Hence the lower total efficiency.

On the other hand, an IPA driving 845W to generate 22KW obviously
makes better efficiency sense and is found in the overall 64.5%
figure.

NOW, if the PA finals, accounting for 22KW are 80% efficient, that
must mean that they only consume 27.5KW of power to do so, and that
with a power input rating of 31KW then leaves the IPA (an identical
80% efficiency module) and control circuitry to absorb 3.5KW to
deliver the drive of .845KW.

It follows that for an 80% efficient IPA, it accounts for 1KW power
consumption. This remainder is easily attributable to power supply
losses (if we simply assign an industrial average efficiency of 95%
for power conversion) otherwise the system TTL circuits and LCD meters
suck down 2.5KW on their own.

This, as you put it (but fail to evidence), would quickly subdue
suspicion. And an equal treatment to more conventional, retail
Amateur Radio Transmitters also reveals efficiencies through the same
exercise. It is quite evident that such transmitters are no where
near these vaunted examples - but few dare venture into these
dissections.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote regarding Harris "Z" FM broadcast transmitters:
"For even greater reliability, any
PA module can be used as an IPA module,
with absolutely no modification."

It is quite obvious that as an IPA, that in the lower wattage systems
it represents overkill at 845W to generate drive to final PAs to 2.2
KW output. Hence the lower total efficiency. On the other hand,
an IPA driving 845W to generate 22KW obviously makes better
efficiency sense and is found in the overall 64.5% figure.

Yet another case where you write with guesswork, not knowing the facts.

Obviously you do not understand the architecture of this line of
transmitters, even though what I am about to write is available on the
Harris website. The PA and IPA modules are the same, and consist of two,
independent amps--each amp capable of 425W output. Their actual output
power depends on the tx they are installed in, and the power level required
from it. The only thing they have in common is a heat sink. An IPA at any
power level uses only one of these amps per 5kW (or less) block of PA amps.
The other amp of the IPA remains unpowered and in reserve, and autoswitches
on line if the active one fails.

The lower AC input to RF output efficiency of the lower powered transmitters
arises from the fixed overhead in all units for losses OTHER than in the RF
amplifiers, i.e., power supply losses, exciter and controller power, RF
combiner and harmonic filter losses, and cooling power--the AC consumption
for which in low power units is a larger proportion of the total.

NOW, if the PA finals, accounting for 22KW are 80% efficient, that
must mean that they only consume 27.5KW of power to do so, and that
with a power input rating of 31KW then leaves the IPA (an identical
80% efficiency module) and control circuitry to absorb 3.5KW to
deliver the drive of .845KW.
It follows that for an 80% efficient IPA, it accounts for 1KW power
consumption. This remainder is easily attributable to power supply
losses (if we simply assign an industrial average efficiency of 95%
for power conversion) otherwise the system TTL circuits and LCD meters
suck down 2.5KW on their own.

Your analytical skills are seriously wanting. Please re-read my response
above.

It is quite evident that such transmitters are no where
near these vaunted examples - but few dare venture into these
dissections.

It is "evident" only to those who don't understand the subject. Others have
not dared to venture into these dissections probably because THEY know
better.

RF

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

"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

Richard Fry wrote:
"Not so. You confuse receivers and transmitters."

FM transmitters often use Class C amplifiers and frequency multipliers
on the modulated signal. An AM signal can not be amplified by a Class C
amplifier because of severe distortion of the modulated signal. In FM,
amplitude distortion is irrelevant no matter where it occurs, receiver
or transmitter. The recovered audio will sound just fine. It`s one of
the many advantages of FM.

Best regards, Richard Harrison, KB5WZI