PLL Not Locked
A dead SDR, a black waterfall, and the disciplined way to find out it was the dongle and not everything else.
I moved the station to the ham desk and finally put the SDR on a real antenna: a full-length outdoor 2 m / 70 cm vertical, up in the air and clear of the house. I reloaded the occupancy page expecting the band to come alive.
I got a black screen.
The temptation at that moment is to start changing things. New antenna, new desk, new cabling, so blame the antenna and start over. That is how you spend an evening chasing the wrong problem. The better move is to prove what works before you touch what does not.
The symptom
The waterfall was flat. Not quiet with a little texture, flat. The noise floor sat in a narrow band, under a decibel of spread, and nothing moved. The occupancy page renders activity above the baseline, so a spectrum with no activity paints every pixel as quiet, which is to say black. The black screen was not a rendering bug. It was the data.
Ruling things out, one at a time
The software first. The sweeper was advancing, fresh data every few seconds, state running. Not a stall.
Power next. The powered hub had its own supply, both lights on, and the dongle was plugged straight into a hub port. Reseated everything anyway.
The dongle’s own history was a red flag. At boot it had re-enumerated six times and thrown a “cannot set config” error, which is a connection or power complaint, not a dead chip. So I leaned toward the cabling for a while. A reseat made the enumeration clean, but the spectrum stayed flat.
Then the test that actually moved things forward: I keyed a handheld a foot from the dongle. Low power, my own radio, right next to the receiver. Still nothing. A receiver that cannot hear a transmitter in the same room does not have an antenna problem. That one test took the antenna, the feedline, and the adapter off the table.
The tell
With the software, the power, the cable, and the antenna all accounted for, I stopped earshot to free the device and ran the one tool that reads the hardware directly.
Found Rafael Micro R820T tuner
[R82XX] PLL not locked!
There it is. The tuner’s phase-locked loop will not lock. If the PLL does not lock, the tuner never actually tunes to the frequency you asked for. USB enumerates fine, samples stream fine, and the output is flat, dead, and deaf to everything. Every symptom from one root cause.
The confirmation that mattered
One more step before blaming the part: I plugged the dongle into a completely different computer and ran the same tool. Same message. Different host, different USB port, different power supply, and the fault followed the dongle.
That is the line. “PLL not locked” on one machine could be my setup. “PLL not locked” on two machines is the part. You do not get to call a component dead until you have seen it fail somewhere that is not your own bench.
Why it worked before
It did, last week, on a stub antenna. It was already marginal and the relocation finished it. The usual suspect for a tuner that will not lock is the reference crystal giving up. Which points straight at the replacement: a dongle with a TCXO for its reference, so the one part that died here is the one part built to be stable.
The antenna was never the problem. It is up, it is good, and it is waiting. The lesson is older than any of the gear: isolate, do not guess, and prove hardware somewhere other than the station before you blame the station.