Add a mode to measure Common Mode Current Chokes

[dkozel]

Work in Progress

A request came up in a forum to support directly measuring and plotting the series resistance and reactance of a Common Mode Current Choke, which can be realistically modeled as a pi network with shunt capacitances and a complex series impedence. With only S11 and S21 available the output capacitance cannot be calculated, but the capacitances are usually similar and someone could physically reverse the NanoVNA to make the complete measurement.

This issue is both a feature request and to gather the information required to implement the measurement mode.

These three values can be easily extracted from the Admittance parameters.

Series Impedence
Z_series = -1 / Y21

Shunt Impedences
Z_shunt,in = 1 / (Y11 + Y21)
Z_shunt,out = 1 / (Y22 + Y21)

Shunt Capacitances
C_shunt,in = Real(1 / (j2 * pi * freq * Z_shunt,in)) * 1e12
C_shunt,out = Real(1 / (j2 * pi * freq * Z_shunt,out)) * 1e12

Math operations

Calculation of Y values

NanoVNA-H/plot.c

Lines 453 to 459 in ba4dcc7

	//**************************************************************************************
	// Y parameters (conductance and susceptance) calculations from complex Z
	// Y = 1 / Z = 1 / (R + jX) = B + jG
	// G = R / (R*R + X*X)
	// B = -X / (R*R + X*X)
	// |Y| = 1 / |Z|
	//**************************************************************************************

Function producing the Admittance Y values for the |Y| mode.

NanoVNA-H/plot.c

Lines 506 to 510 in ba4dcc7

	static float
	mod_y(int i, const float *v)
	{
	return 1.0f / mod_z(i, v); // always >= 0
	}

Plotting Operations

Trace Type:

NanoVNA-H/plot.c

Line 578 in ba4dcc7

const trace_info_t trace_info_list[MAX_TRACE_TYPE] = {

NanoVNA-H/plot.c

Line 594 in ba4dcc7

[TRC_Y] = {"|Y|", "%.3F%s", S_DELTA "%.3F%s", S_SIEMENS, 0, 0.02f, mod_y },

Menu item to select trace type:

NanoVNA-H/ui.c

Line 1641 in ba4dcc7

{ MT_ADV_CALLBACK, TRC_Y, "|Y|", menu_format_acb },