Front end protection for SDR Dongle

[Lasse Radio]

It looks like that the front end/LNA of my dongle could have been blown due to strong signal or static. Joanne mentioned that my dongle does not have protection diodes at the front end, so it would be worth the time spent to make one in case that the next dongle/s does not have any. I'm no whiz when it comes to design this, and there are problably a few more beside me that might need help and want to be safe and have an external diod protection.  

So, anyone whish to explain or know a website that explains how to make this and what diods I need. Appreciate all help.

Regards

Lasse

[Pierre F5OPV]

Hi everybody,

to answer Lasse questions, black and cheap chinese sdr model LNA is very fragile.
On many models, U1 which is a double diode protector is missing, for economical reasons.

So U need to add 2 diodes back to back for LNA protection.

The best place to add these two diodes is the base of the mcx connector at the back of the pcb.

These diodes MUST have a low capacitance to prevent UHF RF to go to earth via the diodes and not entering the e4000.
(As U know the capacitive reactance of a cap is Xc=1/Cw where w=2 pi x F that is Xc=1/C x 2 x pi x F)

The best diodes must have a capacitance < 2pF for good work in UHF (especially if F>300MHz)
 
U can use  2shottky diodes as BAT81 (2pf) (my prefered) or every diode with low capacitance < 2pF (pin diode BA595 for example :0.3pF) or BAV99 double diode

1N4148 1N400x are not suitable  because of high capacitance.

Get the soldering iron hot and then remove it from the mains as the LNA doesn't like at all static electricity and is VEEEEEERRRRRRRRY FRAGILE.

Solder diodes and then U will get a static protected RTL SDR Tuner with no risk if thunderstorms or strong Rf signals in input.

Have fun with your RTL SDR Receiver and cheap spectrum analyser

73 de F5OPV
 ----------------------------------------------------------




Date: Tue, 21 Aug 2012 06:38:25 -0700
From: radioactiv...@gmail.com
To: ultra-c...@googlegroups.com
Subject: [ultra-cheap-sdr] Front end protection for SDR Dongle

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[Pierre F5OPV]

other samples of switching diodes usable:

http://www.vishay.com/diodes/band-switching/

---

From: f5...@hotmail.fr
To: ultra-c...@googlegroups.com
Subject: RE: [ultra-cheap-sdr] Front end protection for SDR Dongle
Date: Tue, 21 Aug 2012 16:04:14 +0200

[jdow]

Look back in the articles here (and perhaps the YahooGroups SDRSharp
group) for discussion. The exact diode pair used is available on E-Bay.
I forget the precise part number.

{^_^} Joanne, W6MKU

[jdow]

Pierre, the dongle Lasse has had mounting pads in place for the diodes.
This gives the shortest possible leads for best protection.

{^_^}

On 2012/08/21 07:04, Pierre F5OPV wrote:
> Hi everybody,
>
> to answer Lasse questions, black and cheap chinese sdr model LNA is very fragile.
> On many models, U1 which is a double diode protector is missing, for economical
> reasons.
>
> So U need to add 2 diodes back to back for LNA protection.
>
> The best place to add these two diodes is the base of the mcx connector at the
> back of the pcb.
>
> These diodes MUST have a low capacitance to prevent UHF RF to go to earth via
> the diodes and not entering the e4000.
> (As U know the capacitive reactance of a cap is Xc=1/Cw where w=2 pi x F that is
> Xc=1/C x 2 x pi x F)
>
> The best diodes must have a capacitance < 2pF for good work in UHF (especially
> if F>300MHz)
>

> _*U can use 2shottky diodes as BAT81 (2pf) (my prefered) or every diode with
> low capacitance < 2pF (pin diode BA595 for example :0.3pF) or BAV99 double diode*_
> _*
> 1N4148 1N400x are not suitable because of high capacitance.*_
>
> _*Get the soldering iron hot and then remove it from the mains*_ as the LNA

[Lasse Radio]

It seems like most of the protected dongles have BAV99. Allmost too small to solder without tiny tip, but I'll buy a few BAV99 and test on the damaged dongle before ruining the new one...

Regards

Lasse

[HELMY]

 It seems like most of the protected dongles have BAV99. Allmost too small to solder without tiny tip, but I'll buy a few BAV99 and test on the damaged dongle before ruining the new one...

if you don't have a soldering iron with a small enough tip, and you don't think this is enough reason to buy one, assuming you already have a soldering iron with a large tip say like a 30~40 Watts, you can still  solder the SMD diode by taking a piece of solid strand copper wire about 1 mm and forming it in the shape of an L and winding it around the iron tip, so that the wire is like an extension to the iron tip and the L shape will help you solder the 2 bottom leads of the diode at once, so it should be only as wide as the diode, then use it to solder the diode the top single lead first then solder the bottom 2, if it is not clear how to do so i can make a video or take some pictures if that is really needed

[Lasse Radio]

Thanks Helmy. I do have a smaller tip, although I may need to sharpen it slightly due to wear.  I think I solder the top one first to keep it in place and then the two at the bottom. I probably need to to turn off the iron while soldering while soldering so any static does not ruin the dongle. Luckily I have the damaged dongle to test solder first... :)

Regards

Lasse