# Gradual loss of amplitude as flowgraph runs

**URL:** <https://www.ruby-forum.com/t/gradual-loss-of-amplitude-as-flowgraph-runs/243015>\
**Category:** GNU Radio\
**Created:** [August 14, 2015, 10:58pm UTC](https://www.ruby-forum.com/t/gradual-loss-of-amplitude-as-flowgraph-runs/243015 "2015-08-14T22:58:41Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Rafa\_F](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/958977/32.png) [@Rafa\_F](https://www.ruby-forum.com/u/Rafa_F)\
**Post date:** [August 14, 2015, 10:58pm UTC](https://www.ruby-forum.com/t/gradual-loss-of-amplitude-as-flowgraph-runs/243015/1 "2015-08-14T22:58:41Z")

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Still working with the polyphase channelizer program. While everything  
“works”, there is something very strange: the output amplitude slowly  
drops the longer the program runs. As near I can tell, this happens in  
or following the frequency translating FFT block.

To test, I stripped the flowgraph down to the bare minimum and put a  
frequency display at the output of the UHD block, and another at the  
output of the fx fft block.

By using the “max hold” feature of the display, I can easily monitor  
amplitude drop during the program run. I’m using a signal generator to  
feed the USRP a known signal level (162.475 MHz at -70dBm).

In the attached screenshot, the left display is the UHD output, and the  
right is the fx fft output. The flowgraph had been running for about 5  
minutes. The signal out of the fx fft has dropped 20+ dB while the UHD  
signal level remains the same. The longer the program runs, the further  
the fx fft output drops.

I’m not seeing any error messages on the console to indicate overrun,  
underrun, or other issues. Due to size limits, I attach only the fft  
screenshot, but here are all the relevant files:

[http://www.febo.com/pages/gr-projects/amplitude\_loss\_test.grc](http://www.febo.com/pages/gr-projects/amplitude_loss_test.grc)  
[http://www.febo.com/pages/gr-projects/amplitude\_loss\_test.py](http://www.febo.com/pages/gr-projects/amplitude_loss_test.py)  
[![](https://www.febo.com/pages/gr-projects/amplitude_loss_test_fft.png) ](https://www.febo.com/pages/gr-projects/amplitude_loss_test_fft.png)  
[![](https://www.febo.com/pages/gr-projects/amplitude_loss_test_flowgraph.png) ](https://www.febo.com/pages/gr-projects/amplitude_loss_test_flowgraph.png)

Any idea what could lead to this kind of problem? It seems like some  
sort of accumulating error, but I’m lost as to what.

Thanks,  
John

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<div class="post-metadata">

**Author:** ![John\_Ackermann\_N8UR](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/cdc98d/32.png) [@John\_Ackermann\_N8UR](https://www.ruby-forum.com/u/John_Ackermann_N8UR)\
**Post date:** [August 17, 2015, 10:17pm UTC](https://www.ruby-forum.com/t/gradual-loss-of-amplitude-as-flowgraph-runs/243015/2 "2015-08-17T22:17:43Z")

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Take a look at the phase rotator in the frequency translating FIR  
filter.  
Is it done with sine/cos lookups or is it a rotating phasor? Rotating  
phasors can lose amplitude due to finite precision effects.

Matt

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<div class="post-metadata">

**Author:** ![John\_Ackermann\_N8UR](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/cdc98d/32.png) [@John\_Ackermann\_N8UR](https://www.ruby-forum.com/u/John_Ackermann_N8UR)\
**Post date:** [August 18, 2015, 12:51am UTC](https://www.ruby-forum.com/t/gradual-loss-of-amplitude-as-flowgraph-runs/243015/3 "2015-08-18T00:51:57Z")

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On Fri, Aug 14, 2015 at 4:57 PM, John Ackermann N8UR [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> amplitude drop during the program run. I’m using a signal generator to  
> screenshot, but here are all the relevant files:  
> John

Hi John,

I’m not sure what to say about this. I ran your example overnight and dd  
not see any power loss.

Tom