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

Richard Harrison wrote:

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.

The question seems to be: If an FM transmitter's output signal
is not a reasonably pure sine wave, is a low-pass filter used
between the transmitter and antenna to reduce the harmonics?
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:
Richard Harrison wrote:

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.


The question seems to be: If an FM transmitter's output signal
is not a reasonably pure sine wave, is a low-pass filter used
between the transmitter and antenna to reduce the harmonics?

How can a filter filter correctly when its input is terminated in
an indeterminate impedance?
73,
Tom Donaly, KA6RUH

Richard Fry wrote:
"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)."

Well, Richard Fry didn`t say there were no out-of-band filters or traps.

One could have a low-pass filter that cut-off above 108 MHz, but below
176 MHz, and no harmonic would get through the filter.

80% or better efficiency isn`t coming from a Class A amplifier, so maybe
it comes from a Class B amplifier.

One fly in the ointment is found on page 354 of Terman`s 1955 edition:
"The theoretical maximum possible plate efficiency that can be realized
in a Class B amplifier is pi/4 or 78.5 per cent;---."

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote

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.
___________________

It may sound just fine to you, but carefully made performance measurements
of a received FM signal having high AM show otherwise. AM on FM is far from
irrelevant.

RF

"Cecil Moore" wrote:
The question seems to be: If an FM transmitter's output signal
is not a reasonably pure sine wave, is a low-pass filter used
between the transmitter and antenna to reduce the harmonics?
___________________

Yes. This question was answered in my post in this thread of 00:24UTC
today, which I will paste below:

"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

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 Harrison"
80% or better efficiency isn`t coming from a Class A amplifier, so maybe
it comes from a Class B amplifier.

One fly in the ointment is found on page 354 of Terman`s 1955 edition:
"The theoretical maximum possible plate efficiency that can be realized
in a Class B amplifier is pi/4 or 78.5 per cent;---."
____________

No fly to those who really know this subject. They are neither Class A or B.

Those wanting to comment on modern solid-state VHF amplifier designs and
performance parameters really should research them before doing so.

We are 60 years beyond the date of this citation from Terman.

RF

Thomas Magma wrote:


I've often questioned why manufactures put small signal parameters on their
datasheets? Makes no sense to me.

They might be of some use for specific cases. For example, if the PA is class
A, is used well backed off because of high PEP-to-avg ratios of the signal, and
you've managed to get the output load dialed in, s-params can be useful for a
first cut at the amplifier *input* match. I've always still had to do some
tweeking though. Also, with some work and considering the load-line match, they
can give you an idea of what gain can be accomplished. This might already be in
the data sheet though, as you mention.

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.

One of the large signal parameters I like best is how much power the device can
dissipate. ;-) Voltage breakdowns and Imax are nice too. ;-) ;-)

"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