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Re: Can I shift center frequency twice with Xlating FIR

From: Fabian Schwartau
Subject: Re: Can I shift center frequency twice with Xlating FIR
Date: Fri, 9 Jul 2021 10:04:24 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

Hi Phil,
I read your post like four times and could not figure out what you are
doing and what you are trying to achieve. Where is your target signal
originally located? At fc? Is fc = 100k? Why do you mix 100k down with
the Frequency Xlating FIR and then define a different center frequency
for your Frequency sink? An example graph would be helping.
Nevertheless, it seems like you just want to shift the frequency around
(i.e., mix the signal) and low pass filter it. You can do this as many
times in parallel as you want. And maybe using the Low Pass Filter
component will make things easier, as you don't have to define taps
manually. Mixing can be done with a complex signal source combined with
a multiplier or you use the Frequency Xlating FIR with [1] as taps. Note
that a positive center frequency for the Frequency Xlating FIR will
actually mix the signal down (negative frequency).

Hope that helps a bit,

Am 08.07.21 um 21:32 schrieb Phil Wiggum:
> I Have this setup
> SDR(fc)
>  -> 'Frequency Xlating FIR Filter' (Low pass, Center Freq: 100k)
>   -> Resampler( 5M/100k)
>    -> 'QT GUI Frequency Sink' (Center: fc+100k, Bw 100k)
> And parallel with QT GUI FSink,
>    -> 'Log Power FFT' -> Python (Peak detektor)
> My problem I have is the read out from 'Log Power FFT' starts at 'fc+100k'.
> I would like it to start 'fc+50k'.
> Can I use 'Frequency Xlating FIR Filter' again and lower my frequency 
> starting point?
> Like this parallel with QT GUI FSink,
>    -> 'Frequency Xlating FIR Filter' (????)
>     -> 'Log Power FFT' -> Python (Peak detektor)
> What kind on taps do I need if this is possible?
> /Phil

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