CW RF generator mode in NanoVNA.

Operation in the CW RF generator mode and measurement of parameters of radio components using NanoVNA.

The NanoVNA can function as a stable, fixed-frequency Radio Frequency (RF) signal generator (CW Mode or Zero Span). The generator's signal is output exclusively from port CH0.

Activating the Generator via the Menu

1. Open the main menu by tapping the screen.
2. Navigate to the STIMULUS section.
3. Tap the CW FREQ button.
4. Using the virtual keypad, enter the desired frequency and select the multiplier (e.g., M for MHz). The device will immediately switch to emitting a continuous carrier wave at the specified frequency, and the frequency sweep will stop.

Generator Output Signal Characteristics

• Nominal output power: Approximately -13 dBm into a 50-ohm load.
• Waveform: Since the generator is based on the Si5351 synthesizer chip, the output signal is a square wave. This means that in addition to the fundamental frequency, the output spectrum contains a large number of odd harmonics (3rd, 5th, etc.). This crucial feature must be taken into account when injecting the signal into radio receiver paths.

Activating the CW FREQ continuous wave mode on the NanoVNA.

Measuring Radio Components and Circuits in the Lab

Thanks to its high-precision measurement capabilities, the NanoVNA can successfully replace several traditional lab instruments, such as RLC meters, Q-meters, Capacitance/Inductance meters, and Grid Dip Oscillators (GDO).

1. Testing Crystal Resonators

Crystal resonators are measured in transmission mode (S21 parameter, CH1 Through). The crystal is connected in series between the center pins of the CH0 and CH1 ports. For highly accurate measurements, it is recommended to use a test fixture with a Pi-Network attenuator to eliminate the influence of the device's 50-ohm port impedance. Calibration must be performed in THRU mode.

On the LOGMAG S21 graph, the crystal's series resonance appears as a sharp transmission peak (minimum attenuation). The parallel resonance (anti-resonance) is located slightly higher in frequency and is characterized by a deep dip in the frequency response. When matching crystals for a crystal filter, use the marker to record the series resonance frequency of several units, selecting crystals with a frequency spread of no more than 10–50 Hz.

2. Measuring LC Resonant Circuits (Grid Dip Oscillator Mode)

For contactless measurement of the resonant frequency of LC circuits, a single-turn copper wire coupling loop is connected to port CH0. The path is calibrated (OPEN/SHORT/LOAD) with the loop already attached. The coupling loop is then brought coaxially near the inductor of the circuit under test at a distance of 5–20 mm. At the resonant frequency, the circuit will absorb energy, which will appear on the LOGMAG S11 graph as a distinct dip. The Quality factor (Q) can be calculated using the formula: Q = f₀ / Δf, where the bandwidth (Δf) is determined at the -3 dB level from the peak.

3. Measuring Capacitance (C) and Inductance (L)

Measurements are performed on port CH0. The component is connected via an SMA adapter with alligator clips. To compensate for the parasitic inductance and capacitance of the adapter probes, it is mandatory to set an ELECTRICAL DELAY in the SCALE menu. For measuring capacitors, set the sweep range between 50 kHz and 2 MHz. For inductors, set the sweep around 200 kHz. In SMITH chart mode, the marker will automatically calculate and display the equivalent capacitance or inductance of the component.

4. Measuring Capacitor ESR

The Equivalent Series Resistance (ESR) of a capacitor is measured at its self-series-resonant frequency, where the capacitive and inductive reactances cancel each other out. On the impedance magnitude graph |Z| (RESISTANCE format on NanoVNA), find the absolute lowest point of the U-shaped curve where the reactance X = 0 Ohms. The active resistance value (R) at this exact point is the capacitor's ESR.

5. Measuring the Frequency Limit of Capacitors

As frequency increases, due to the Equivalent Series Inductance (ESL) of the leads, the capacitor passes through its resonant point f₀ (where X=0 and impedance equals ESR) and begins to act like an inductor above this frequency. Using the NanoVNA and the complex impedance graph, you can find this f₀ frequency—this is the maximum usable frequency limit for this specific capacitor in filtering and decoupling circuits.

6. Measuring Magnetic Permeability of Ferrite Toroids

Measure the physical dimensions of the toroid core (outer diameter D, inner diameter d, and height h). Wind a test coil of N = 10 turns and measure its inductance (L) at a low frequency of around 200 kHz using the NanoVNA. The initial magnetic permeability (μ_r) is calculated using the practical formula:

μ_r = (2500 × L × (D + d)) / (N² × h × (D - d))

Note: In this formula, the inductance L is substituted in microhenries (μH), and the geometric dimensions are in millimeters (mm).

Setup for measuring the inductance of a test winding on a ferrite toroid.

7. Testing RF Transformers (Broadband Transformers and Baluns)

The input of the transformer is connected to CH0, and the secondary winding is terminated with a non-inductive resistor equal to the design output impedance (e.g., 200 Ohms for a 1:4 transformer). In the operating frequency band (e.g., 1–30 MHz), the input SWR graph should be a flat line at a level of < 1.1–1.2. A rise in SWR at low frequencies indicates insufficient inductance, while a rise at high frequencies points to the influence of inter-turn capacitance or core losses.

8. Evaluating EMF, Mechanical, and Crystal Filters

These measurements are taken in transmission mode (S21). Since the VNA has 50-ohm ports, and the filters are designed for higher impedances (hundreds of Ohms or kilo-ohms), the filter's input and output must be impedance-matched using series non-inductive resistors. Perform a THRU calibration with these matching resistors installed (using a wire jumper in place of the filter). Once the filter is connected, you can evaluate its amplitude-frequency response (LOGMAG S21): the -3 dB bandwidth, passband ripple (difference between peaks and dips), insertion loss, and the shape factor at the -60 dB level.

9. Cold Tuning of Power Amplifier Input Circuits

The input matching networks of a tube or transistor power amplifier (PA) are tuned without applying power supply voltages. The input impedance of the amplification stage is simulated using a non-inductive resistor connected to the grid or base. Connect the device's CH0 port to the circuit's input, and adjust the trimmer capacitors or coils to achieve an SWR close to 1.0, an active input resistance of R = 50 Ohms, and a reactance of X = 0 Ohms at the operating frequencies.

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