Band expansion and frequency accuracy (TCXO): what’s possible, the risks and the law

Two eternal topics swirl around DMR radios. The first — “how do I unlock it so it transmits anywhere.” The second — “the frequency drifts, the link falls apart, I need to replace the TCXO.” They’re about different things, but both come down to one point: exactly where and how precisely your transmitter sits on frequency. Let’s sort out what is actually limited by firmware in a radio, why a stable reference matters, when a TCXO is justified — and why straying outside your own bands is never acceptable under any circumstances.

Why a radio is locked and what “expansion” means

The manufacturer programs a transmit range into the radio, bounded by the synthesizer and the filters. Imported units often arrive locked to consumer/amateur bands for a specific country’s market. Baofeng (e.g. the UV-5R) for the US market trims the TX bands down to the amateur 144–148 and 420–450 MHz, whereas the “unlocked” versions transmit wider — 136–174 and 400–520 MHz. With TYT (MD-380 and the like) the factory front end is designed for 136–174 / 400–480 MHz, while the exact TX limits depend on the regional firmware. “Band expansion” means removing exactly this software lock: by patching the firmware, a service codeplug, or third-party firmware.

It’s important to understand the layers:

• Software lock — the limits in the firmware/CPS. Removed by software; they don’t change the hardware’s physics.
• Hardware band — what the synthesizer, output stage (PA) and matching/band-pass filters are actually designed for. Firmware does not “expand” this.

Removing the software lock is easy. But if you go outside the band the hardware was designed for, the radio will work badly and not for long — more on that below.

The boundaries of what’s allowed: operate only in your own bands

In Russia the main amateur VHF/UHF band is 70 cm (430–440 MHz), where transmitting is permitted for radio amateurs of categories 1–4. And even within it there are hard caveats:

The practical takeaway: even with an “unlocked” radio, you plan your frequencies around your category and region, not “wherever it reaches.” The RF frequency plan for Russia is published by the SRR and local plans — check against those, not the baked-in factory presets “for China.”

Where frequency accuracy comes from

The radio forms the carrier starting from a reference oscillator — a crystal or a TCXO. The reference’s stability is measured in ppm (parts per million). At 433 MHz, 1 ppm is about 433 Hz of drift. Ballpark figures:

• The TYT MD-380 is rated at around ±1.0…±2.5 ppm — that’s up to ~0.4–1.1 kHz on 70 cm.
• The ADF7021 RF chip (the heart of many MMDVM modems) requires a reference no worse than 2.5 ppm, and on 800–900 MHz even more stable.
• A cheap Chinese “TCXO” on bargain boards is sometimes 20 ppm instead of the spec’d 2.5 — that’s already up to ~8.6 kHz of drift, off the channel.

The main curse of a cheap reference is temperature drift. A cold start gives one frequency; five minutes into warm-up the board wanders off: on budget MMDVM modems people have observed drift of up to 1 kHz as it warms up. A simple one-shot “cold” calibration won’t fix this — the correction for the cold and hot states is different.

TCXO: what it is and when you really need one

A TCXO is a temperature-compensated crystal oscillator: inside is a circuit that tunes the frequency against temperature, so the drift is many times smaller than a plain crystal. A VCTCXO is the same but with a voltage control input: it’s convenient to calibrate by trimming after assembly, removing the initial spread.

Where a TCXO is justified, and where it isn’t:

Calibration instead of a soldering iron

In 90% of cases the “drifted” frequency problem is solved in software, without opening anything:

1. Run an echo/Parrot or talk into a nearby radio and watch the BER.
2. Run a sweep over the offset: step RXOffset in 100–200 Hz increments across a window of roughly −1500…+1500 Hz.
3. Find where BER drops to zero — that is the receiver “center.” Do the same for TXOffset.
4. For a precise measurement — the MMDVMCal utility (G4KLX) with a reference receiver/analyzer.

# example: a rough starting point for MMDVM frequency tweaks
RXFrequency=433000000
TXFrequency=433000000
RXOffset=0      # Hz, tuned for minimum BER
TXOffset=0      # Hz

And only if the reference is genuinely “junk” and drifts with heat — then it’s time to talk about replacing it with a proper TCXO. Calibration first; the soldering iron is the last resort.

Risk summary

• The law. TX outside the bands authorized to you is a violation; interfering with services is a separate, serious charge.
• Output stage (PA). Outside the “hardware” band the filters/matching are mismatched — the SWR and reflected power rise, the PA heats up and can burn out. A software unlock doesn’t move the physics of the filters.
• Signal purity. Beyond the front end, harmonics and out-of-band emissions creep out — you’re trashing your neighbors on the air.
• Decoding. Replacing the reference in a DMR radio breaks digital voice — people stop understanding you.
• Warranty. Reflashing/opening — say goodbye to factory support.