[ARC5] AM modulating an "ARC-5" transmitter...

Sun Oct 11 01:12:19 EDT 2015

     The assumption is that one is using sine waves. More complex waves 
are different right up to a square wave where peak average and RMS are 
all the same.
     Actually RMS must get in here somewhere. It is the effective 
heating power of an AC waveform and its equivalent to DC.  Most older AC 
voltmeters or amp meters are calibrated in RMS but read something else 
so they read correctly only for a sine wave. The error with other wave 
forms depends on the type of rectifier and the percentage of harmonics 
in the wave.  bridge or full wave rectifiers read average and single 
ended diodes depend on the voltage, for low voltage they approach true 
RMS, for high voltage they approach true peak reading.  More confusion. 
Many digital voltmeters that say they are true RMS really only 
approximate it by calculating and can have significant errors.
     The relationship of the modulation to the carrier is independent of 
the method of modulation, that is, a low level modulated signal, say 
from a grid modulated stage, is the same as for a plate modulated 
system, i.e. same ratio of carrier to sideband power and same variation 
dependance on waveform.
    Also, FWIW, speech waves contain components which are not 
harmonically related so are not directly comparable a sine, sawtooth, 
triangular, or square wave.  Oh, golly, its too hot here to think about 
this.

On 10/10/2015 2:47 PM, DSP3 wrote:
> Genrs,
>
> Ken is correct about, especially, high-level modulation.  The actual 
> power, peak, average, wherever is wholly dependent on the waveform.  
> The difference between the peak and average for two transmitters may 
> well be quite different.  Its always best to "visually" characterize 
> the envelope as well as use other measurement methods.  I recently got 
> into a rather heated exchange with someone who refused to acknowledge 
> the existence of low-level modulation modes.  He insisted that all AM 
> resulted in a peak and average power greater than the un-modulated 
> key-down carrier.  I could not convince him otherwise.  A scope in 
> this fellows shack would have done wonders.
>
> Jeep - K3HVG
>
> On 10/10/2015 5:23 PM, Kenneth G. Gordon wrote:
>> On 10 Oct 2015 at 13:10, Richard Knoppow wrote:
>>
>>> AM can be really confusing.
>> Yes. I am almost constantly confused when dealing with AM...
>>
>>>   The PEAK power is four times carrier
>>> but average is 1.5 times carrier.
>> Well yes, but doesn't that really depend on the waveform of the 
>> modulating
>> signal? I look on that 1.5 times carrier as the average of the 
>> average voice.
>>
>>> PEP works for AM but the FCC
>>> definition does not make sense.
>> No, it certainly doesn't. At least not to me.
>>
>>> In the old days the power of amateur
>>> transmitters was regulated by _input_ power, I think because measuring
>>> RF output can be difficult. It also applied to the carrier only.  So,
>>> the output would depend on the efficiency of the transmitter. Likely
>>> carrier output of a good plate modulated 1 kw transmitter with 
>>> Class-C RF
>>> output stage would be on the order of 700 watts carrier.  The FCC by
>>> memory refers to the "average" PEP.
>> Sounds like "the average of the average" to me...
>>
>>>   So average of a sine wave is about
>>> 0.637 of peak. Now, does the FCC want the _average_ of a 1KW total at
>>> 100% modulation or what?  If they specified peak power at the crest 
>>> of a
>>> 100% modulated wave as no greater than 1KW the carrier could not be 
>>> more
>>> than 250 Watts. If they are specifying the _average of the peak power_
>>> then we have a maximum of 1.57 times this value or about 392 Watts
>>> carrier or about 1.57KW for 100% modulation. Given our assumption of a
>>> 70% efficiency of a Class-C plate modulated stage this would be on the
>>> order of 600 watts input for the carrier.
>> Yes....sigh...
>>
>>>       I am not at all sure this is correct but think the FCC could
>>> certainly write the spec in a less confusing way...
>> if they really cared to...
>>
>>>   They should also have
>>> grandfathered all the 1KW input AM rigs which existed at the time the
>>> new regulations were instituted.
>> Although AM is NOT one of my favorite modes, I most heartily agree with
>> you on this issue.
>>
>> However, I do think that the proper use of AM would be valuable when
>> needed. If one uses the transmitter's VFO, turning off the receiver's 
>> BFO, to
>> zero-beat an SSB station before transmitting, and if one keeps the
>> transmissions very short, it will generally be some time before the SSB
>> station will realize you are on AM. It always worked for me.
>>
>> Ken W7EKB
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>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL