Re: [Discuss-gnuradio] Inverted spectrum

[Tom Rondeau]

On Sat, Jul 13, 2013 at 8:54 PM, Andrew Davis <address@hidden> wrote:
> 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

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