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VE2HAR > MT63 12.03.05 21:37l 187 Lines 4107 Bytes #-7311 (0) @ WW
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Subj: Re: [MT63] Crest Factors
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Mark Miller wrote:
>Karl,
>
>My setup is a Bird 4431 (model 43 with a sampling port)
>
Sorry I do not know that Bird 4431. What is the bandwidth of the
sampled data? And what is it sampling? Is it sampling the power or the
voltage?
I assume you have a dummy load on your Bird watt meter. I did some
peak power measurements using my Heathkit Antenna. It has a diode that
picks off the voltage across the resistor which you can hook a scope to.
It is quite accurate I found from testing.
With the antenna it would be easy to measure the difference from the
resting power with the transmitter turned on, and then the peak power
with data flow.
> and a Tektronics
>2225 scope. I first calibrate my scope. I use a single tone to set the
>output of my IC-746pro to 40 watts as shown on the Bird, then adjust the
>trace on the scope for some convenient deflection. I ended up with 4
>gradicules peak to peak. Then I fire up MT63. Since crest factor is Peak
>voltage/RMS voltage,
>
Nope. The Crest factor is a Power measurement. And peak voltage is
defined as the peak of the sine wave.
I define peak power as the actual power when MT63 is being modulated
continuously by digital data.
I'm afraid your measuring something else.
>I read the peak voltage from the scope. I then look
>at the average power on the Bird and calculate the RMS voltage by taking
>the square root of P*R where P is the power measured on the Bird and R is
>50 Ohms. I then divide the peak voltage by the RMS voltage to get the
>linear crest factor, I then take 20*log10 of the linear crest factor to get dB.
>
>
>
I see you use the voltage in your calculation of DB. I think you
should stay with power the whole way and forget voltage Mark. Then your
calculation of peak to average power will use 10log.
karl
>My transceiver is linear up to 45 watts, so I typically use 40 W PEP when
>transmitting waveforms that must be linear. That means when transmitting
>MT63, my average power is somewhere around 12 watts.
>
>73,
>
>Mark N5RFX
>
>At 07:23 AM 3/12/2005, karl larsen wrote:
>
>
>
>> Hi Mark, No one I think knows what the crest factor is for MT63, but
>>I am not surprised by your 10DB figure. I run it at about 20 watts out
>>to a 100 watt transmitter so the peaks are not getting out.
>>
>> Please tell us how you made your measurements. Yours might become
>>the "standard"...:-)
>>
>>
>>Mark Miller wrote:
>>
>>
>>
>>>I have just recently measured crest factors (peak to RMS) for a few modes.
>>>
>>>MT63 10 dB
>>>RDFT 8dB
>>>WinDRM 12 dB
>>>
>>>Has anyone else measured crest factors for these modes? I am just
>>>wondering how close I am to the theoretical numbers.
>>>
>>>73,
>>>
>>>Mark N5RFX
>>>
>>>
>>>
>>>
>>>
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