I don’t know if anyone is interested, but I’ll post this regardless.
I have used for a while the RTL-SDR LNA. First was fed via Bias-T from my Airspy R2, but I didn’t feel “safe” in case of a short on the cable.
Then I fed it with 4.5V via a Bias-T power injector. The power supply is a cheap wall wart, and I could not regulate the output voltage in stable and definite steps. 4.5V
This is the LNA: New Product: RTL-SDR Blog 1090 MHz ADS-B LNA
Well, due to my long (and mismatched impedance with the LNA/Airspy) cable (150ft of RG6), I wanted to have the maximum gain from the above LNA. They claim 27dB at 1090MHz, but we all know that the gain on those devices is variable with the supply voltage. So I wanted to provide the max 5V to the chip, accounting for any DC voltage drop on the cable, in the safest way. Well, the hint is on their page too:
Finally, if you are confident with soldering SMT components, then there are also pads and a 0 Ohm resistor slot on the PCB to install an LDO and power the LNA directly.
So I have ordered a few LM1117-5.0 LDO regulators from eBay. I wanted 3 of them, but by mistake I click on a 5 pack, so now I have 15 of those buggers. Oh, well.
Now I had to remember my SMT soldering skills and I was surprised that I kind of lost my steady touch. And my eyesight - needed to use two pair of reading glasses one on top of another. Gosh, aging, where are my 20 year old self hands and eyes!
The chip is soldered a bit crooked, but it’s in place and working well. I had to unsolder and resolder the 0 ohm resistor to the alternate pad and bridge the power pad to the input pin of the LDO chip. I am feeding this from a bias-t injector in my garage, with approx 7.5V.
Those are the “before” and “after” pics.
The original config, before the mod. The central pin voltage gets DC blocked, filtered and applied directly to the MGA-13116 chip:
The modded config, with the center pin power rerouted trough the LDO:
It’ll be interesting to see the results with a more stable power source.
