LoRa Preamp shoot-out

 

 

Two-Way Lora Signal Booster Comparisons

As part of the Alpha Cubesat project at Cornell, we want to set up a "TinyGS" ground station to receive telemetry from the flight in December.  There has been a little back and forth about which pre-amp to use for the station.  I tried using a Uputronix 440 mhz amplifier, but didn't have great results with it.  In fact, I had better results with a cheap LNA I bought off eBay years ago.  We heard through the TinyGS community that this other preamp, available on Amazon, works really well for LoRa applications:

 

 

 

I picked one up and ran some tests on the bench.  The spec sheet says it wants a supply voltage of  3.6V-6.0V.  Since I had a USB Breakout board kicking around my parts bin, I used one to make a USB adapter for it.

 

 

 

That done, I hooked it up to my service monitor and ran some tests.  I generated a 433 Mhz signal, and ran it into my Spectrum Analyzer. 

 

 

 

 

 I adjusted the output until it was reading -50dbM.

 

 

 

Then, I inserted the "Two-way Lora Signal Booster" preamp.  With no power applied, no signal was passed.  Once I applied USB Power, I got a gain of about 11-12 db (-50db up to -38db)

 

 

 

Double-checking the spec sheet:   "Receiving gain: 11dB±2dB"

 

It seems to be dead-on what I would expect.

 

Comparison to Uputronix Preamp

I then simply swapped out the LoRa preamp for the Uputronix. 

 

 

 The result was about a 20dB gain (-50db up to -30db).

 

  

The spec sheet shows a bandwidth with a gain of about 19-20db from 427-452 mhz, so LoRa at 433 mhz would fit comfortably inside the bandpass filter, even at maximum (500khz) bandwidth.

 From the Uputronix data sheet.

 

So, the Uputronix, at 20db of gain, seems dead-on spec.

 

Generic eBay preamp

So, finally, I attached the generic "eBay special" preamp I purchased years ago.  It claims 30db of gain.  I powered it with 9 volts.

  

Sure 'nuff, the gain was about 28db (-50db to -22db).

  

Conclusions

The two-way LoRa signal booster performed the most poorly, with only about 11db of gain.  That said, it has a place in applications where you want gain on LoRa output, as it's a bi-directional amplifier.  However, for TinyGS, where we are receive-only, it appears to be the poorest performer.

The Uputronix preamp performed precisely on spec, delivering about 20db of pre-amplification.  That said, I have seen poor performance with it, compared to the generic eBay preamp.  I will note that the uputronix is designed to work with bias-T or USB input power.  When powered by USB power, it back-feeds DC up to the LoRa board.  According to Google, the Semtech SX1276 is sensitive to DC on the RF input, and it should be blocked.  It's quite possible that this is causing the deafness I've observed.

El-cheapo for the win.  The $12 eBay preamp board showed a gain of about 30db.  It is the best-performing on my home TinyGS station.   I'm honestly shocked, as I expected the 30db preamp, with no filtering, to be filling the LoRa board with noise.  Apparently, the LoRa board doesn't mind.

Future Experiments

After the initial tests, I did check to see if any of the preamps were back-feeding DC to the LoRa board.  Only the Uputronix was doing that.  The Googles report that the Semtech SX1276 must not have DC on the RF input.  So, it's quite possible that's why the Uputronix preamp doesn't work with the LoRa board.  There are little DC blocking doodads that I could pick up and try to put inline between the preamp and the LoRa board.  I may try that at some point to see if the Uputronix performance improves.