# Inverted spectrum

**URL:** <https://www.ruby-forum.com/t/inverted-spectrum/245761>\
**Category:** GNU Radio\
**Created:** [July 14, 2013, 2:54am UTC](https://www.ruby-forum.com/t/inverted-spectrum/245761 "2013-07-14T02:54:59Z")\
**Posts on this page:** 2\
**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:** [July 14, 2013, 2:54am UTC](https://www.ruby-forum.com/t/inverted-spectrum/245761/1 "2013-07-14T02:54:59Z")

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Hello all,

When using pll\_carriertracking\_cc, the returned spectrum seems to be  
inverted about 0MHz, when looking though the code line 113 of  
gr-analog/lib/pll\_carriertracking\_cc\_impl.cc  
looked odd:

> optr[i] = iptr[i] \* gr\_complex(t\_real, -t\_imag);

could someone explain why the the imaginary component of the NCO is  
inverted?

Thank you all,  
Andrew

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**Author:** ![andrewdon](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/e9bcb4/32.png) [@andrewdon](https://www.ruby-forum.com/u/andrewdon)\
**Post date:** [July 22, 2013, 4:23pm UTC](https://www.ruby-forum.com/t/inverted-spectrum/245761/2 "2013-07-22T16:23:36Z")

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On Sat, Jul 13, 2013 at 8:54 PM, Andrew D. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> Thank you all,  
> Andrew

Andrew,

That block was written so that it would lock to a carrier at a tone  
that is then used to downshift the signal. So you have a signal at fc;  
the PLL would lock onto that and produce a tone at -fc. This is then  
multiplied against the original signal to shift it down to baseband  
directly.

Tom