RTL LNA Power

Question for @SoNic67 about external power on RTL 1090 LNA, I have seen 2 closed posts about externally powering the LNA, 1 with an LDO and 1 that looks like it has a USB lead directly soldered, now the question is where would I solder the 5v+ & 5v- to on the LNA to power from USB as I don’t know if anything else needs to be desoldered on it, I can handle the soldering job, I just can’t read pcb electronic diagrams.

You are referring the the RTLSDR Blog filter/amp?

There is no -5V on USB, just 0V and +5V


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Hi @geckoVN these are the posts I’m referring to

And

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Ok, just checking we were talking about the same thing.

Just remove the 0 Ohm resistor or you’ll feed 5V back down the antenna feeder.
Then use its pad for the 5V

It was this way I was looking at, as I can make out where the +5v goes as its the hole pad on the PCB. But just struggling to see where the 0v wire goes to and also it looks like the 0ohm resistor it bridged to the pad next to it but not sure, I just want to be 100% on what to do as I really don’t want to fry the LNA as I can’t afford to get a new one.

This post shows how I powered RTL-SDR Tripple Filtered LNA using a low cost bias-t and a low cost 5v dc PSU, both from Chinese sellers on ebay.

(1) $4.39/ea, Free shipping

 

image

 

(2) $3.66, Free shipping

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That shouldn’t work, but it looks like you have a different PCB revision.
Did you add a solder bridge to the 0R resistor?

What receiver are you using?
As you have it, you’ll be sending 5V down the antenna cable to the receiver.

I have not done anything yet to it, I want to be sure what I’m doing before even attempting it, that pic is not my LNA

The safest way is to purchase a $4 Bias Tee from Ebay and use it, like I did.

Using an external Bias Tee has advantage that there is no soldering to Amplifier pcb, hence there are no mistakes in soldering at wrong points of pcb of Amplifier.

Also no soldering is needed to the pcb of Bias Tee, as dc 5V wires are connected to screw terminals.

However one has to take care of following:

(1) +ve & ground connection to be according to marking on screw terminal.

(2) DC+RF marked SMA is connected to Amplifier OUT, and RF marked SMA is connected to dongle

The issue is that the PCB accepts BIAS-T, but is not stabilized. For short coax cable runs it doesn’t matter, it is good as it is.
For long coax cable runs, you will have an unknown DC voltage drop. Stabilizer IC allows to raise the voltage at Bias-T a bit, to compensate (like 7.5V).
The circuit board has the space for a 5V stabilizer.

I understand from your sentence that the stabilizer is not provided, only a space is provided so that user can add it if he wants, and he has enough knowledge and skill to do it.

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Yes to each of those questions.

  • The OP says he has the soldering skills, so that’s covered.
  • The pic above show the pads where the LDO can be fitted
  • the link to @SoNic67 previous post shows the mod to reposition the 0R resistor

for under C$3, you can buy practice boards if you are not confident

Yes is true for @geckoVN, @SoNic67, @abcd567, @Jonseyt23, and many more users, but is definitely NOT true for each and every user. My comments were for general hobbyists who do not posses enough knowledge and skills to do the job properly and safely.

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@Jonseyt23 asked the question and the answers are in the context of this thread.

Anyone else is welcome read the comments and judge for themselves if they can make use of the information.

Even if someone doesn’t have the skills/ equipment, many will know someone who would be willing to do the job for the price of a beer.

Not all Bias-T’s are created equal. Some are suitable for HF, but not higher, so there is plenty of potential for distorting or attenuating the signal only because of poor/uninformed component selection.

I understand your point and you are correct that not everyone is capable of carrying out this mod, but not every thread needs to be dumbed down to the level of the least capable, least equipped user who wouldn’t do the mod anyway.

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Exactly. Might not even be needed, if the power supply is in the 5.2V range, and coax is short. Yes, I have a few “5V” packs that are labeled to output that. Amazon Kindle Fire for example.

But then… you almost ask why would you need a LNA.

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