Re: Measuring SSB RMS Power

Posted by Jim Brown-10 on Jun 20, 2019; 6:15pm
URL: http://elecraft.85.s1.nabble.com/Measuring-SSB-RMS-Power-tp7653233p7653243.html

Bob is correct to the extent that Pink Noise approximates the SPECTRUM
of speech, but John's is on target that speech is not continuous, but
highly variable in amplitude. For uncompressed speech, the average value
is typically as much as 20 dB below the peak value; compression can
reduce that to 10 dB or less.  Highly sophisticated audio processing
that has been used by radio broadcasters for at least 40 years reduces
that ratio to 6 dB or less. "If the modulation meter moves below 100%,
it isn't loud enough!"

73, Jim K9YC

On 6/20/2019 10:13 AM, John Oppenheimer wrote:

> Hi Bob,
>
> Viewing pink noise with an oscilloscope, it is continuous, where as
> speech has breaks.
>
> The web page is evaluating both expected power to a load and compression
> gain. Using a little over six second recorded speech. Recording contains
> fast spoken words, mostly numbers and a few "this is a test" etc, for
> maximum power over time. Evaluating scope shoot, about 25 words during
> six second recording. Not even close to pink noise.
>
> John KN5L
>
> On 6/20/19 11:27 AM, Bob McGraw K4TAX wrote:
>> To evaluate speech use a pink noise source.  That is the standard source signal for acoustic measurements.
>>
>> Bob, K4TAX
>>
>>
>> Sent from my iPhone
>>
>>> On Jun 20, 2019, at 10:34 AM, John Oppenheimer <[hidden email]> wrote:
>>>
>>> Hi Don,
>>>
>>>> On 6/20/19 9:21 AM, Don Wilhelm wrote:
>>>> John,
>>>>
>>>> That connection is OK if you realize that your scope is reading the DC
>>>> voltage developed by rectifying the RF from 1/2 of the load.  You still
>>>> have to use the power formula that is in the DL1 manual.
>>> Well yes, power references are DCV^2 / 25
>>>
>>> DdB ratios computed using voltage ratios in the form of
>>> 20 * Log(v1/v2). For example, 0 compression Pep/Pav dB =
>>> 20 * log(15.3/5.24) = 9.3. I do need to flip the table title to Pav/Pep
>>> for proper negative dB values, Done.
>>>
>>>> I prefer to allow the 'scope to read the full peak RF voltage.  Connect
>>>> the 'scope probe to the side of one resistor which is connected to the
>>>> BNC jack (one of the leads closest to the BNC), and the 'scope ground
>>>> probe to one or the resistor leads furthermost away from from the BNC jack.
>>> Using detected RF envelope for oscilloscope average RF measurement at
>>> each digitized point during long, six second, trace. This will result
>>> with increased accuracy when measuring modulated speech.
>>>
>>> John KN5L
>>>
>>>>> On 6/20/2019 9:38 AM, John Oppenheimer wrote:
>>>>> Using DL1 with Oscilloscope to measure KX3 SSB RMS power:
>>>>>
>>>>> https://www.kn5l.net/ssbpower/
>>>>>
>>>>> Question, is this technique correct?
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