Reaching outside of the RTL_SDR v3's capabilities with harmonics
Published: 2025-04-09
Disclaimer, this is some real nerd stuff.
So I'm in the middle of figuring out first-kid life, and we needed a baby monitor. We were avoiding it, but
we really coudnl't hear him cry from our room. $20 and a Walmart trip later, we had it, and on the way home
my wife read the box, "Encrypted voice". I was like, really... Let's check that.
We got home and I pulled out the RTL SDR v3 with some coax and an antenna tuned for 1090MHz (ADS-B
project!).
Should work.
Popping Gqrx open, I scanned the 900MHz band, and didn't find anything. I searched in the portion right
above the 70cm band, and still nothing. Looking for the FCC ID came up with nothing (is it valid? Wasn't in
the DB!), so I searched VTech DM1111 frequency us on Kagi. Nothing in the relevent Amazon.com
listing, nothing on the Vtech website, but then device.report came up with a promising result:
Seems to be exactly what I needed. The only problem was that the RTL SDR wouldn't go up to the frequency range and the waterfall plot was just noise. Hmm. Another Kagi search revealed that the RTL SDR v3 only goes up to 1.7GHz.
How can you get around that? Frequency Harmonics. Just like in singing, all waves have harmonics, or other frequencies that regularly match the original. This time we'll have to move backwards, so we need to find a frequency that has 1921.536MHz as a harmonic. Kagging for "frequency harmonics calculator" brought me to Mustcalculate.com - Harmonics calculator. I started messing around with the values at 146.52MHz, the national 2m calling frequency, and found that the 13th harmonic was close at 1.90476GHz. Continuing to mess around with the values, I brought it up to 147.81MHz, resonating at 1.92153GHz. Pretty darn close to 1.92156GHz.
I then made some noise in the baby end of the monitor - and a couple spikes appeared approximately 2MHz apart, exactly what I'd expect for a FH digital mode. Perfect!
Not too sure if I did it correctly, but when I yelled into the microphone there were changes in the waterfall and in the spikes. The GIF doesn't do it justice.