PPM Frequency Error Calculator
Check oscillator drift against manufacturer spec — tailored to your TX mode.
Frequency
Manufacturer spec
TX mode
PPM error itself comes from the reference oscillator and doesn't change with TX mode — but how much it matters does. Narrowband and digital modes have far less margin than wideband analog FM. These tolerances are practical rules of thumb, not a formal standard.
About this calculator
Every radio has a reference oscillator, and no oscillator is perfect. The difference between the frequency you asked for and the frequency you actually get is the frequency error, and it's usually given in parts per million (ppm). This calculator turns a measurement into ppm, and then tells you whether that error actually matters for the way you operate.
The sums behind it
The ppm error is the difference between the measured and nominal frequency, divided by the nominal frequency, times a million. If you already know the ppm figure, it works the other way round and gives you the frequency you should expect to measure. It also shows the error in hertz, because that's the number you'll hear and see on the air.
The same ppm is a bigger number of hertz on a higher frequency. A 1 ppm error on a 10 MHz oscillator is 10 Hz, but on 145 MHz the same 1 ppm is 145 Hz. For example, a 2.5 ppm error on 145 MHz works out at about 362 Hz.
Why the mode matters
The oscillator doesn't care what you're transmitting, but your mode certainly does. Wide FM is very forgiving. FT8 is not, because its decoder only listens across a narrow window. These are the tolerances the calculator uses:
| Mode | Practical tolerance |
|---|---|
| Wideband FM (25 kHz) | ±3 kHz |
| Narrowband FM (12.5 kHz) | ±1.5 kHz |
| SSB / CW | ±100 Hz |
| FT8 / PSK31 / JT65 | ±20 Hz |
| DMR / P25 / NXDN | ±250 Hz |
These are practical rules of thumb, not a formal standard, so use them as a guide.
How to use it
- Enter the nominal frequency in MHz.
- Choose whether you are entering a measured frequency or a known ppm error.
- If you have the manufacturer's rated tolerance in ppm, add it and the calculator will say whether your radio is within spec.
- Pick your transmit mode to see whether the error is acceptable for it.
To get a measured frequency, use a frequency counter, or zero-beat the radio against a trusted reference such as a GPS-disciplined source. Everything runs in your browser and nothing is sent to a server.