Re: [Discuss-gnuradio] Capturing maximum complex value

[Marcus D. Leech]

On 03/17/2019 11:09 AM, John Ackermann N8UR wrote:
> I'm working on a flowgraph that generates an output waveform about 375 
> kHz wide with very high peak-to-average power -- at least 20 dB.  This 
> is sent into a USRP1 with BasicTX board to generate an output at 29 
> MHz.  I'd like to show in the flowgraph GUI the peak instantaneous 
> output level, to help in setting the gain.
>
> It's painfully clear on a spectrum analyzer when the gain is too high. 
> The attached shows (a) single CW carrier at max output (orange); (b) 
> output without clipping (yellow); and (c) output with occasional 
> clipping (blue).  These are max-hold plots so the clipping is caught 
> even though it happens only very infrequently.
>
> Is there a probe or other function that can capture and hold the 
> maximum instantaneous value of a complex stream?  If not, is there a 
> way to create that capability out of other blocks?
>
> And, am I barking up the wrong tree -- is the distortion likely caused 
> by the complex output being driven into digital clipping when enough 
> vectors add up, or is this a function of the analog conversion in the 
> USRP?  i.e., is the problem likely to be on the input or output side 
> of the DAC?
>
> Thanks,
> John
ANother approach is to simply multiply your signal by, let's say, 0.5 
before being presented to the radio.   Crank that value up until you get
  distortion, then back off by sqrt(2).