[hpsdr] Level calculation error in Two Tone Test generation routine

[Warren C. Pratt]

Hi Scott,

You are correct that I usually think in terms of "voltage" as that's the 
way the code has to be written.  However, the -6dB level of each tone 
relative to peak, as displayed on the panadapter, is correct no matter 
whether you think of "voltage" or "power".  Note that the panadapter 
display is labelled in dBm (Power) however, it is actually generated by 
taking the envelope [sqrt(x^2+y^2)] of the FFT output of the complex I-Q 
receive "voltage".

Say we start with a voltage at a level of 1.0.  If we reduce it by a 
factor of two, we have 0.5.  The corresponding dB reduction is 
20*log10(0.5/1.0) ~= -6dB.  Now consider that P=V^2/R.  So, the power 
was reduced by a factor of 0.5^2=0.25.  The corresponding dB reduction 
is 10*log10(0.25) ~= -6dB.

The key point in my previous message was that the DAC and amplifier are 
being driven to the same peak envelope level no matter whether you have 
a TUN signal, an SSB signal, or a two-tone with the generator level set 
at 0.0dB.

73,

Warren  NR0V


On 11/24/2016 6:44 AM, Scott Traurig wrote:

I am not surprised that you think more naturally in terms of voltage, 
Warren, as one of your primary goals is to obtain a particular set of 
voltage conditions on a DAC.

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