E4B

Measurement accuracy, S parameters, VNAs

Explain what limits meter and counter accuracy, read S parameters, calibrate a VNA, find Q, and test SSB for intermodulation.

11 pool questions1 on the exam

Syllabus: Measurement technique and limitations: instrument accuracy and performance limitations; probes; techniques to minimize errors; measurement of Q; instrument calibration; S parameters; vector network analyzers; RF signals

A reading is only as good as the instrument's reference and loading

A frequency counter counts cycles over a timed gate, so its accuracy comes from its time base (the internal oscillator that sets the gate).

A voltmeter's ohms-per-volt rating gives its input impedance: full-scale reading × ohms per volt. A low value loads the circuit under test and the reading sags.

Input impedance = full-scale reading × ohms-per-volt rating. A low value loads the circuit and the reading sags.

RememberCounter: time base. Voltmeter: full scale × Ω/V = input impedance.

S parameters: two digits name two ports

The two digits are ports. Read S21 as: signal in at port 1, measured at port 2.

The subscripts name the port or ports where measurements are made. S21 (in at port 1, measured at port 2) is forward gain. S11 (in and measured at port 1) is input return loss, the reflection coefficient (equivalent to VSWR).

RememberS21 = forward gain. S11 = input reflection.

A vector network analyzer: calibrate with three loads, then measure a device's response

Three known references let the analyzer remove its own cables and connectors from the measurement.

Calibrate with a short, an open and a 50 Ω load.

A VNA measures	A VNA does not measure
Filter frequency response	Phase noise
Input impedance	Pulse rise time
Output impedance	Forward power
Reflection coefficient

RememberShort, open, 50 Ω. Then response, impedance and reflection.

The load absorbs forward minus reflected power

Power absorbed = forward power − reflected power.

Reflected power25 W

Load absorbs75 W

Absorbed power100 W forward − 25 W reflected = 75 W absorbed

RememberWhat is reflected is not absorbed.

Q comes from the bandwidth of the frequency response

Measure Q from the response: find the −3 dB bandwidth, then Q = centre frequency ÷ bandwidth.

Q of circuit20

Bandwidth355 kHz

Sweep a tuned circuit and read the width of its response between the half-power points. Narrow means high Q.

RememberQ = centre frequency ÷ bandwidth.

Test SSB distortion with two audio tones and a spectrum analyzer

Two AF tones in, spectrum analyzer on the RF out. Extra spikes beside the two tones are the intermodulation distortion.

Feed two AF tones that are not harmonically related into the transmitter. Intermodulation shows up as extra signals on the RF output, seen with a spectrum analyzer. A single tone cannot show intermodulation.

RememberTwo AF tones in, spectrum analyzer on the RF out.

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Compendium articles on the ideas in this lesson, beyond what the exam asks.

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Practise the 11 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

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