Re: Collins 32V-3 HF Transmitter NICE!!!

Good explanantion!


"Gary Schafer" <gaschafer@xxxxxxxxxxx> wrote in message
news:ej7ft11qo9ncn2cc8ct05k6hf9t4ckbtqg@xxxxxxxxxx
>
> Let's review some definitions to start:
>
> AVERAGE POWER
>
> Average power is found by squaring RMS voltage and dividing by
> resistance. Or RMS voltage times RMS current.
>
>
> PEP
>
> It helps to fully understand exactly what PEP is in an SSB
> transmitter. Then it is easier to see in an AM transmitter.
>
> Peak envelope power is important because that is how the FCC defines
> how much power we can run.
>
> Let's look at the FCC definition of Peak Envelope Power:
>
> "Peak envelope power output of a transmitter is the AVERAGE power at
> the crest of the modulation envelope over at least one rf cycle."
>
> NOT TO BE CONFUSED WITH AVERAGE POWER READ ON A METER as it swings
> around!
>
> If you think about what that is saying it will make sense. If you
> transmit just a carrier with an SSB transmitter of say 100 watts. That
> is 100 watts average power output. It is also 100 watts PEP output.
> (in this case the envelope is infinitly long) If you were to key it on
> and off in the CW mode the power relationship would be the same. 100
> watts PEP on each CW dash or dot sent.
>
> If you now switch to the SSB mode and modulate the transmitter so that
> the peaks on the scope looking at that signal reach the same height,
> the transmitter will be putting out 100 watts PEP.
>
> If you were to modulate that same SSB transmitter with 2 equal
> amplitude tones you would get a scope pattern that looks similar to an
> AM signal modulated with a single tone. The crest (or peak) of the
> waveform represents 100 watts PEP same as with voice but with the
> tones it is easier to see as the waveform will be stable.
>
> If you were to increase the speed of the time base on the scope and
> spread the waveform out you would see that each crest of the audio
> wave form has within it many cycles of the RF frequency. These many
> cycles of RF are the AVERAGE power contained in the signal.
>
> You will note that the maximum AVERAGE power is only reached for
> several RF cycles at the crest of each audio cycle.
>
> This is what is known as PEAK ENVELOPE POWER. (see definition above
> again)
>
>
> PEP WATTMETERS
>
> A true PEP reading watt meter will show the peak envelope power of the
> above signals as described.
> There are a lot of so called PEP watt meters on the market. Not all
> are able to properly read. Even the Bird meters have problems with
> some types of wave forms.
>
>
> S METER READINGS
>
> S meter readings will vary according the particular receiver being
> used but most all S meters are peak responding circuits. Most will
> read pretty close to the peak values, depending on the decay time of
> the circuit some may not hang up there like others do. If you think
> about it if you have ever been plagued with pulse noise like ignition
> noise it only takes a very narrow pulse occurring at a rather slow
> rate to hold the S meter up high. Increasing the rate will not
> increase the meter reading.
>
> Likewise with an SSB signal, once the station is transmitting his peak
> power on a regular basis, increasing mike gain or increasing
> compression will not raise the S meter reading maximum.
>
> The AVC circuit in the receiver must respond to the peaks or the
> receiver would overload the detector if the gain was not cut back when
> a peak was received. The S meter reads AVC voltage.
>
>
> AM TRANSMITTER
>
> It is best to try and understand the output signal of the AM
> transmitter before trying to coralate it with what goes on at the
> input side. Swapping back and forth can be confusing.
>
> Take our 100 watt carrier output transmitter again. Measuring the
> output voltage of the RF we find that it is 70.7 volts RMS across our
> 50 ohm load. P = I squared / R so 70.7 x 70.7 = 5000. 5000/50 = 100
> watts.
>
> Let's modulate 100% with a single audio tone. We get out of it a 100
> watt carrier and two 25 watt side bands. 3 distinct signals. As you
> stated before the carrier always remains constant.
>
> If we look at the output signal on our scope we will see that it looks
> similar to the SSB signal that was modulated with 2 tones. We see the
> modulation envelope. We can again expand the scope's time base and
> look at the RF cycles within each modulation peak. Same as with the
> SSB signal, at the crest of the modulation envelope is the peak
> envelope power of the composite signal.
>
> Now let's get back to measuring that PEP. We know that the carrier
> alone had a power of 100 watts which produced 70.7 volts across a 50
> ohm load. If we look at the scope with and without modulation we see
> that the voltage output doubles with modulation so it will be 141.4
> volts RMS at the crest of the modulation wave form.
> Again P = E squared /R. 141.4 x 141.4 = 20000. 20000 / 50 = 400 watts
> PEP.
>
> If we were to measure this with a good PEP wattmeter we would see the
> meter also indicate 400 watts PEP.
> Again some so called PEP meters do not do well on this type of wave
> form. The carrier tends to confuse the meter as it causes an offset
> in the voltage being read by the meter and the meter tries to average
> it so the net result is some gets subtracted from the reading. It is
> due to the way in which the particular meter circuit operates.
>
>
> CONVERTING RMS TO PEAK
>
> It would seem at first glance that you could find the peak power of
> the 25 watt side bands and add things together but that doesn't work.
> You can not convert power. THERE IS NO RMS IN POWER!
>
> There is a wide misconception that there is something called RMS
> power. There is no such thing! There is only AVERAGE power and PEAK
> power. (note the FCC definition of PEP)
>
> You find average power by using RMS voltage. But once you multiply or
> divide, RMS term, the RMS goes away.
> So once you have power you can not multiply it by 1.414 to find peak
> power.
>
> To find peak power you must first add the voltages together or find
> the peak voltage of an rms voltage by multiplying by 1.414 then
> finding power.
>
>
> AM LINEAR
>
> Operating an SSB transmitter and amplifier in the AM mode, if properly
> set up, will produce exactly the same looking output signal as a plate
> modulated AM transmitter.
>
> If we have an SSB amplifier that will put out 1000 watts PEP on SSB it
> will also put out 1000 watts PEP on AM.
> But in order to do so the carrier output must be limited to 250 watts
> output. It must be tuned up in the CW mode for 1000 watts output. Then
> when switching to AM the carrier is reduced to 250 watts output
> without touching any tuning controls. The amplifier must still be
> tuned to be able to produce the 1000 watt peak envelope output.
>
> When we modulate the 250 watt carrier with AM the peak envelope power
> at 100% modulation will reach 1000 watts pep (or 4 times the carrier)
> just as it did with the plate modulated AM transmitter. Looking at the
> output with a scope we will see the voltage double with modulation
> verses just the carrier.
>
>
> POWER IN SIDE BANDS
>
> As a note is seems that having 2 side bands with the same information
> in an AM signal is useless but it is not.
> In the detector of the receiver the energy in both side bands combine
> and add together. So rather than using only 1 side band of 25 watts
> you are really using 50 watts of side band energy. So a 3 db addition.
> There is also another 3 db gained in the detector because of the
> voltage doubling with the side bands being coherent in the detector.
> So the carrier is really the only thing wasted.
>
>
> PLATE CURRENT AND VOLTAGE DOUBLING
>
> It is easiest to see with a triode tube that is plate modulated.
> Doubling the plate voltage will cause the plate current to also
> double. That is if the tube is capable of providing enough emission.
> This must be a linear function in order to avoid distortion when
> modulating.
>
> Tubes that are weak may not be able to provide this. That is one
> reason that PEP may not fully reach 4 times the carrier power with
> 100% modulation.
>
> Screen grid tubes are not linear in this respect. Plate current is
> somewhat independent of plate voltage. That is why you must also
> partly modulate the screen along with the plate when using a screen
> grid tube in the final. You want to have a linear plate voltage to
> plate current relationship.
>
> This is also why a lot of broadcast transmitters use triodes in the
> final. Easier to maintain linear modulation.
>
>
> HANDBOOK
>
> All this can be found in the AM section in some of the older
> handbooks. The newer ones do not cover AM very well.
>
> 73
> Gary K4FMX
>
>
>


.

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