TCXO Frequency

The NanoVNA-H uses a Temperature Compensated Crystal Oscillator (TCXO) as its frequency reference. Calibrating this reference ensures accurate frequency measurements across the entire sweep range.

Why TCXO Calibration Matters

The TCXO provides the master clock for:

Si5351 frequency synthesizer (all output frequencies are derived from this reference)
All frequency calculations
Time domain distance measurements

Even small errors (parts per million) can cause significant frequency offsets at higher frequencies.

Understanding PPM

PPM (parts per million) is how frequency accuracy is specified. A 1 ppm error means the oscillator is off by 1 Hz for every 1 MHz:

Measurement Frequency	1 ppm Error	10 ppm Error
7 MHz (40m band)	7 Hz	70 Hz
50 MHz (6m band)	50 Hz	500 Hz
145 MHz (2m band)	145 Hz	1.45 kHz
432 MHz (70cm band)	432 Hz	4.32 kHz
1 GHz	1 kHz	10 kHz

At HF, 10 ppm is barely noticeable. At UHF, 10 ppm can shift a narrowband filter measurement by several kHz — enough to misidentify the center frequency.

The Si5351 PLL Chain

The TCXO frequency enters the Si5351’s PLL, which multiplies it up to a VCO frequency (typically 600–900 MHz), then divides back down to produce the output. Any error in the TCXO propagates through the entire chain:

TCXO (26 MHz) → PLL × N → VCO → ÷ M → Output

A 1 ppm TCXO error produces exactly 1 ppm output error at every frequency. The PLL does not correct or amplify the reference error — it passes through linearly.

Default TCXO Frequency

Model	Nominal TCXO	Typical Range
NanoVNA-H	26 MHz	25.999-26.001 MHz
NanoVNA-H4	26 MHz	25.999-26.001 MHz

Determine Actual TCXO Frequency

Using a Known Reference

Connect a signal source of known frequency (e.g., 10 MHz reference)
Set sweep centered on the reference frequency
Note where the signal appears
Calculate the TCXO error from the offset

Using a Frequency Counter

Connect frequency counter to the NanoVNA-H’s RF output
Set the NanoVNA-H to a CW frequency (e.g., freq 10000000)
Measure the actual output frequency
Calculate the ratio to find actual TCXO frequency

Calculation Example

If you set 10.000000 MHz but measure 10.000385 MHz:

Ratio = 10.000385 / 10.000000 = 1.0000385
Actual TCXO = 26.000000 * 1.0000385 = 26.001001 MHz

Set TCXO Frequency

Go to CONFIG > EXPERT CONFIG
Tap TCXO FREQ
Enter the actual TCXO frequency in Hz
Tap ENTER or a unit key

Via Shell Command

# Set TCXO frequency (in Hz)
tcxo 26001000

# Query current setting (firmware dependent)
tcxo

Save the Setting

After setting the TCXO frequency:

saveconfig

Or via menu: CONFIG > SAVE

Verification

After TCXO calibration:

Set CW frequency to your reference frequency
Verify output matches expected value
Check that known markers appear at correct frequencies

Practical Tips

Alternative Method: Using WWV

If you have a receiver capable of picking up WWV/WWVH time signals:

Set NanoVNA-H to transmit near 5, 10, or 15 MHz
Listen for beat note with WWV
Adjust TCXO until beat note is minimized (zero beat)

Temperature Compensation

Even TCXOs have some temperature coefficient. For highest accuracy:

Note the ambient temperature during calibration
Re-verify if operating at significantly different temperatures
Consider using the NanoVNA-H in a temperature-stable environment for critical measurements

Typical Accuracy

Calibration Method	Expected Accuracy
Factory default	+/- 10 ppm
Calibrated with counter	+/- 1 ppm
Calibrated with GPS reference	+/- 0.1 ppm

At 1 GHz, 1 ppm error equals 1 kHz frequency error.

Shell Command Details

# Set TCXO frequency
tcxo 26000000    # Set to exactly 26 MHz
tcxo 26001234    # Set to 26.001234 MHz

# The value is stored in config._xtal_freq