Cecil, W5DXP wrote:
"What is the linear source impedance of a class-C amp?"

A conjugate match is necessary for maximum power transfer.

A Class C amplifier is not inherently linear. That is disasterous for an
already modulated AM signal but it is of no importance to an FM signal.
As Richard Fry points out, no tank circuit is required. A low-pass
filter to suppress all harmonics is all that is needed for a clean
signal. No tuning is required of a tank at the operating frequency.

A tank circuit is not selective enough to prevent intermod anyway. When
I worked the morning shift at 790 KHz in Houston, and I fired up the
transmitter, I would hear 740 KHz`audio coming out of the monitor
speaker. They started programming earlier than we did. They were 15
miles away. Our transmitting antenna made a dandy receiving antenna. The
received 740 KHz modulated our final amplifier and we rebroadcast it
although at a level much lower than our own modulation. Any one
listening to 790 KHz who turned up the volume to hear the 740 KHz audio
got their ears knocked off when our modulation started. We had high
level plate modulation of the final amplifier for our own signal. For
the 740 KHz signal the level of modulation was much lower, millivolts
not kilovolts. Either program was cleanly modulated on our carrier. The
only difference was the enormous difference in modulation levels.

Best regards, Richard Harrison, KB5WZI

Owen Duffy wrote:
"Most supply authorities would not allow you to connect a capacitive
load (a leading PF load),-----."

Incorrect. Overexcited synchronous machines are commonly used to correct
the power factor causing reduced line current and lower power loss on
the a-c power transmission line.

Best regards, Richard Harrison, KB5WZI

(Richard Harrison) wrote in
:

Owen Duffy wrote:
"Most supply authorities would not allow you to connect a capacitive
load (a leading PF load),-----."

Incorrect. Overexcited synchronous machines are commonly used to
correct the power factor causing reduced line current and lower power
loss on the a-c power transmission line.

In this part of the world, connection of an overall leading PF load
usually requires approval of the supply authority. Most electricians
would be aware of the restriction.

The reason being is that it can cause an increase in supply voltage
beyond spec in the local area.

Yes, large factories for example may use static or rotary power factor
correction to improve their PF for tariff reaons, but if an installation
reaches an overall leading PF it usually breaches rules in this part of
the world, and commonly permission is required to attach individual
'devices' with capability for leading PF. Some tariffs are based on
maximum demand, and often maximum apparent power (ie VA), not maxiumum
real power (ie W), hence the economic interest in PF correction.

Today, active static power factor correction that can dynamically track
a varying load at varying PF is a lot more attractive than rotating
machines.


Owen

(Richard Harrison) wrote in news:20731-4850859D-
:

I wrote:
"In Class C, the plate current pulses last less than a half cycle.
Practical efficiencies are in the range of 60 to 80 per cent."

Owen Duffy wrote:
"Again, not rectangular pulses, not nearly."

A pulse does not need to be rectangular. According to my electronics
dictionary: ...

Yes Richard, pulse can mean all things to all people... but you did say
"The PA is a switch. Almost no voltage across it when it is closed and no
current through it when it is open." That implies a rectangular current
pulse... and I think we are now agree that is not the case for Class B or
Class C (which were the stated scope of the OP's question).



Owen

"Roy Lewallen" wrote
No mysterious "reverse r-f energy" is needed to explain this well-known
and well-understood phenomenon.
__________

The quote above reads as though the existence of reverse (reflected)
r-f energy is being denied.

For skeptics, the link below leads to a field report showing a measurement
of the reflection of a narrowband r-f pulse by an analog broadcast TV
antenna, back toward the source. The H.A.D. of the sin² pulse used
represents the shortest transition time that can be accommodated in a ~ 4
MHz transmission channel.

Note that the reflected pulse appears some 6.2 µs after the incident pulse,
which corresponds to a round trip through the ~1,525 feet of 6" OD, 75 ohm,
air pressurized transmission line leading to the antenna in this system.

The return pulse amplitude indicates a far-end match (elbow complex +
antenna) of about 1.05 VSWR (2.3% reflection).

http://i62.photobucket.com/albums/h8...easurement.gif

RF (RCA Broadcast Field Engineer 1965-1980)

Richard Harrison wrote:
Cecil, W5DXP wrote:
"What is the linear source impedance of a class-C amp?"

A conjugate match is necessary for maximum power transfer.

Is the class-C amp conjugately matched during
the 75% of the cycle when it is off? Is there
any such thing as an instantaneous conjugate
match? Don't we have to move downstream from
non-linear sources for our linear math models
to start working?
--
73, Cecil http://www.w5dxp.com

Richard Fry wrote:
"Roy Lewallen" wrote
No mysterious "reverse r-f energy" is needed to explain this
well-known and well-understood phenomenon.
__________

The quote above reads as though the existence of reverse (reflected)
r-f energy is being denied.

Especially strange since the "reverse r-f energy"
is the cause of the impedance responsible for the
mismatch problem. If there were no "reverse r-f energy",
the impedance causing the mismatch wouldn't even exist.
Why is the cause of the offending impedance of no importance?
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote in message
...
Richard Harrison wrote:
Cecil, W5DXP wrote:
"What is the linear source impedance of a class-C amp?"

A conjugate match is necessary for maximum power transfer.

Is the class-C amp conjugately matched during
the 75% of the cycle when it is off? Is there
any such thing as an instantaneous conjugate
match? Don't we have to move downstream from
non-linear sources for our linear math models
to start working?
--
73, Cecil http://www.w5dxp.com

Richard, its a common myth that Class C amps are non-linear, but the truth of
the matter is that although the condition at the input of the pi-network is
decidedly non-linear, the energy storage in the pi-network tank circuit isolates
the input from the output and the result is a totally linear condition at the
output of the pi-network. Evidence proving this is true is that the output of an
unmodulated signal at the output of the network is an almost pure sine wave.
With a Q of at least 12 the difference between a pure sine wave from a signal
generator and that from the pi-network output can not be seen on a dual trace
scope with the traces overlapping.

I don't know about the energy storage in the filters you mention, but I would
assume that if the filter output is a sine wave then the energy storage required
to produce a linear output is sufficient.

Walt, W2DU

Richard Harrison wrote:
Cecil, W5DXP wrote:
"What is the linear source impedance of a class-C amp?"

A conjugate match is necessary for maximum power transfer.

*in a linear system*

Jim Lux wrote:
"in a linear system"

It produces no significant harmonics, so the system is linear.

Best regards, Richard Harrison, KB5WZI