Pointers to DIY instructions for FrontEnd filters, please?

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n3mmr
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Joined: Sat Jul 15, 2017 9:53 am

Pointers to DIY instructions for FrontEnd filters, please?

Post by n3mmr » Mon Jul 17, 2017 10:29 am

The user guide for the rtl-sdr dongle mentions filters:
I gather I should have a minimum of an LP and an HP, selectable, both with cut off at 14.4 MHz or a Spyverter, and an 88-108 MHz bandstop, Otherwise aliasing and FE overload may be a problem.

Are there any good diy filter designs out there, preferably passive designs?

rtlsdrblog
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Re: Pointers to DIY instructions for FrontEnd filters, pleas

Post by rtlsdrblog » Tue Jul 18, 2017 5:13 am

The V3 already has a 24 MHz LPF in place on the direct sampling HF circuit. If you experience broadcast AM overload then you can use an external high pass filter to block them out.

Due to the nature of direct sampling mode signals are mirrored around 14.4 MHz. Usually for most people the mirrors are no problem as most night signals are below 14 MHz, and most day signals above 14 MHz. But if it is for you then a LPF/HPF combo tuned to 14.4 MHz would be useful.

With an upconverter like the SpyVerter you shouldn't need additional filtering as that also already has a LPF built in.

Filters are pretty easy to design and build, just look online for Chebyshev or Butterworth LC filter calculators. Ideally download and use a program like Elsie or RMSIM99.

When purchasing inductors, make sure that the ones you get have a "Self Resonant Frequency" (SRF) above 30 MHz, and a high Q-factor.

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