[Hackrf-dev] Low pass filter lesson 1

[Matteo Terzi]

Hi Anon,
thanks for this explanation.
Just a thing: so if I understand well, my "new" 0 is 97.5mhz.  Why if I'm
filtering at 75khz (cutoff freq) and so 97.5mhz+75khz, I can see the other
higher/lower frequencies on the fft sink?
a filter should brutally cut the frequencies after the "cutoff freq"
thanks a lot

Matteo

2018-05-09 12:32 GMT+02:00 Anon Lister <listeranon at gmail.com>:

> At that point they do not.
>
> Hackrf's job is to take energy at a desired frequency, say a 200khz
> broadcast radio station at 97.5mhz and down convert it to a 200khz wide
> station spanning -100khz,100khz.
>
> However, the hackrf doesn't work best when running at exactly 200khz
> sampling rate. It likes higher sampling rates. So you sample at say
> 8Msample, and bring down 97.5-4, 97.5+4.
>
> But now you have a problem, the energy you are listening to (centered at
> 0,) will have many higher frequency components that got pulled in with that
> 8M span the hackrf brought down. So a low pass filter will remove that
> unwanted extra stuff from both sides.
>
> At this point the data can be resampled, and demodded.
>
> On Tue, May 8, 2018, 09:00 Matteo Terzi <matteo.terzi97 at gmail.com> wrote:
>
>> Hi all,
>> I'd like to know why does Micheal Ossmann use a Low Pass Filter in the
>> Lesson 1 (https://greatscottgadgets.com/sdr/1/ --> minute 22:00).
>> He says that in that way just frequencies near to the zero Hz can pass
>> but it doesn't make sense....how radio frequencies can pass if they have a
>> value of MHz??
>> Thanks
>>
>> Matteo
>>
>> --
>> Matteo TERZI
>> Google Gmail Member
>> _______________________________________________
>> HackRF-dev mailing list
>> HackRF-dev at greatscottgadgets.com
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>>
>


-- 
Matteo TERZI
Google Gmail Member
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