Re: [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 :

> 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  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 sensehow radio frequencies can pass if they have a
>> value of MHz??
>> Thanks
>>
>> Matteo
>>
>> --
>> Matteo TERZI
>> Google Gmail Member
>> ___
>> HackRF-dev mailing list
>> HackRF-dev@greatscottgadgets.com
>> https://pairlist9.pair.net/mailman/listinfo/hackrf-dev
>>
>


-- 
Matteo TERZI
Google Gmail Member
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Re: [Hackrf-dev] Low pass filter lesson 1

[Chuck McManis]

Hi Matteo,

It is the difference between "baseband" and "RF". If you take an RF signal
that is at 100Mhz and has a bandwidth of 2.5Khz,  you can mix it with
another signal at 100Mhz and that will produce two outputs, one at 200Mhz,
and one at 0Mhz. It will still have a 2.5Khz bandwidth so on the low end it
will be between 0 and 2.5kHz so a low pass filter is needed.

This is the fundamental principle behind radios.

--Chuck


On Tue, May 8, 2018 at 5:59 AM, Matteo Terzi 
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 sensehow radio frequencies can pass if they have a
> value of MHz??
> Thanks
>
> Matteo
>
> --
> Matteo TERZI
> Google Gmail Member
>
> ___
> HackRF-dev mailing list
> HackRF-dev@greatscottgadgets.com
> https://pairlist9.pair.net/mailman/listinfo/hackrf-dev
>
>
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[Hackrf-dev] Low pass filter lesson 1

[Matteo Terzi]

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 sensehow radio frequencies can pass if they have a
value of MHz??
Thanks

Matteo

-- 
Matteo TERZI
Google Gmail Member
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