Signal chain w/ RTL-SDRBlog Dongle v3 + RTLSDRBlog Wideband LNA & FM Filters?

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StandingWave
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Joined: Fri Jul 27, 2018 1:00 am

Signal chain w/ RTL-SDRBlog Dongle v3 + RTLSDRBlog Wideband LNA & FM Filters?

Post by StandingWave » Tue May 21, 2019 11:12 am

Hi there,

Once again I've realized that I don't know enough, and therefor I'd like to ask this question considering the use of RTL-SDR blog's USB Dongle v3 and the Wideband LNA & FM Broadcast Filters that are sold in the RTLSDRBlog store.

I recently got my RTLSDRBlog Wideband LNA and the FM Broadcast Filter to be used with my RTL-SDR Dongle v3. However, I'd like to ask a question regarding the signal chain before I plug the LNA in and enable the bias tee, let alone do it the wrong way ...

This is from the official RTLSDRBlog Dongle v3's manual page ( https://www.rtl-sdr.com/rtl-sdr-blog-v- ... ser-guide/ ) :
To make things clearer: DC Short Antenna -> LNA -> Coax -> V3(bias tee on) is absolutely fine.

What's not good and makes no sense
anyway is DC Short Antenna -> Coax -> V3(bias tee on).

DC Short Antenna -> Coax -> V3(bias tee off) is fine.
That's a no-brainer. However, I'm using the antenna kit supplied with the v3 USB dongle kit. And so, unless I'm getting it all wrong, the suggestion is to insert the LNA right before the antenna ( = before the coax cable long run that's between the USB receiver and the actual antenna)?

How about the Broadcast FM Band-Stop Filter? Should it be right next to the LNA, or right next to the USB receiver dongle in the signal chain? Any tips on that? Obviously, if in some cases the FM Filter needs to be *before* the LNA, there are no other options than to add the FM trap first in the chain, counting back from the antenna ...

As in, the signal chain being:

Code: Select all

[[ANTENNA]] =>
[[LNA and/or the FM Filter]] =>
[[Coax cable run to the dongle]] =>
[[The antenna input of the USB receiver dongle]]
If I'm reading the guide the right way, at least the LNA should be right next to the antenna, and the coax cable run should be from there on. As I'm using the supplied dipole antenna kit, it should be straight-on compatible with switching of the bias tee ( = DC shorting), right? Or did I get that one wrong, too ...

There is a question in the comments and a reply from an admin considering the question at https://www.rtl-sdr.com/new-products-in ... nclosures/ :
Where is the best place to put the FM Trap filter? In between the antenna and the LNA or in between the LNA and the receiver? I’ve see both answers on the Internet. What’s your point of view?
Thanks for your help.
Regards,
James
The given answer was:
It depends. If your RF environment has extremely strong FM signals that are strong enough to overload the LNA itself, then put the filter in front of the LNA. If it’s not overloading the LNA, put it after to reduce the filter insertion losses.

If you’re not sure, try with the filter after the LNA first, and if you don’t see any overloading issues (signal images etc), then leave it that way. If there are images, you need to put it before the LNA.
However this does not clarify whether i.e. the FM Filter should be in the signal chain right next to the LNA unit, or right next to the RTL-SDR dongle's input (= after the antenna, LNA and the coax cable run).

I'm trying to figure out what would be the best signal chain setup. Should the Broadcast FM filter and the LNA be right next to each other in the receiver signal chain?

I'm trying to use the "logic" here that every centimeter/inch of a [coaxial] cable run means more potentially added RF input ( = interference ... ). In which case, one solution would likely be to get rid of the additional coax cable altogether and try to use as straight-as-possible connection between the USB dongle and the dipole (in other words: to use just the short cable that's attached to the dipole kit's antenna, and ditch the extra coax run altogether).

Also, please raise your voice if the Dipole Antenna Kit that's supplied with the RTLSDRBlog Dongle v3 is not suitable for Bias Tee switching without probing/soldering its interiors separately.

(Sorry for the noob question(s), I'm just trying to play it safe before I try out anything silly.)

All help is kindly appreciated. Thanks.

hotpaw2
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Re: Signal chain w/ RTL-SDRBlog Dongle v3 + RTLSDRBlog Wideband LNA & FM Filters?

Post by hotpaw2 » Tue May 21, 2019 3:15 pm

Actually, there is a trade-off. One solution does not fit all.

A short coax run between the antenna and the LNA or SDR radio means less signal attenuation. But, unless you are on the very top of a hill and off-the-grid, one usually gets better reception and less RF interference by putting the antenna up high, above neighboring (RF absorbing) structures, and as far away from household and industrial electronics (computers, LED lights, networking, motors, etc.) as possible, which might mean a very long coax (tower, mast above the roof, top of a tall tree, etc.) Or even just getting a small antenna off the desk, and closer to the top of an outside window.
So you will have to experiment with what is best for your particular site and frequencies of interest.
In general, an LNA close to the antenna helps if the coax run is long.
A broadcast blocking filter before the LNA may help if you see strong AM/FM stations showing up on very wrong frequencies.

rtlsdrblog
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Re: Signal chain w/ RTL-SDRBlog Dongle v3 + RTLSDRBlog Wideband LNA & FM Filters?

Post by rtlsdrblog » Wed May 22, 2019 2:24 am

However this does not clarify whether i.e. the FM Filter should be in the signal chain right next to the LNA unit, or right next to the RTL-SDR dongle's input (= after the antenna, LNA and the coax cable run).
If you're placing the filter after the LNA, it does not really matter if you put it right after the LNA, or at the dongle end or in the middle of the coax. If it needs to go before the LNA to prevent the LNA from overloading, then it will have to go right next to the LNA, since you still want to have the LNA as close to the antenna as possible.
Also, please raise your voice if the Dipole Antenna Kit that's supplied with the RTLSDRBlog Dongle v3 is not suitable for Bias Tee switching without probing/soldering its interiors separately.
A dipole is a seen as a DC open circuit. So even if you accidentally tried to run DC power through it, nothing would happen as it's an open circuit.

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