Measurement Techniques

Signal to Noise Ratio

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  • A receiver produces a noise power of 200mW without signal, as signal is applied, the output level becomes 5W.

Noise Figure

  • A figure of merit to measure the degradation of SNR of a system
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  • For a passive device with G=1/L and in thermal equilibrium at the temperature T, N0 = kTB = Ni , So =GSi
  • An amplifier with input signal 100µW and the noise power is 1µW. The amplified signal is 1W with noise power 30mW.
  • An amplifier with NF 6dB has an input SNR=40dB

NF of a cascaded circuit

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Frequency measurements

  • Two approaches: using frequency counter to measure frequency directly, and using probe to measure the wavelength in a transmission line.
    Frequency counter approach
    (1) Basic principle: direct counting <500MHz
  • Using frequency down-conversion techniques for microwave signals
    (2) Pre-scaling: divider circuit <2GHz
    Transfer oscillator down-conversion: use PLL to relate the harmonic relationship between the low frequency oscillator and the input microwave signal > 40GHz

Wavelength measurement approach

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  • Distance between two adjacent minima is 1.9cm in a WR-90 waveguide.

Wavemeter method

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Operating principle

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Detection devices

  • Power detector: bolometer (thermistor and barretter), thermocouple voltage detector: crystal detector, Schottky barrier diode, GaAs barrier diode
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  • Thermistor: a metallic- component with a negative temperature oxide coefficient of resistance
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  • Thermocouple: a pair of dissimilar metal (Sb-Bi) wires joined at one end (sensing end) and terminated at the other end (reference end). The difference in temperature produces a proportional voltage. Crystal detector: use the diode square-law to convert input microwave power to detector output voltage
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  • Schottky barrier or GaAs barrier diode: high sensitivity noise equivalent power (NEP): the required input power to produce, in 1Hz bandwidth, an output SNR = 1 tangential sensitivity (TSS): the lowest detectable microwave signal power
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Power Measurements

  • Difficulty in measuring voltage or current at microwave frequencies → power measurement simpler and more precise
    Power range: low power <0dBm, medium power 0dBm~40dBm, high power >40dBm
    power detector sensitivity: diode ~-70dm, thermistor ~-20dBm
    Thermistor power meter
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Power measurement arrangement

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  • Medium power case: use directional coupler or attenuator at the DUT (device under test) output
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  • High power case: use directional coupler in reverse direction Power Meter
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Attenuation Measurements

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  • If T: DUT reflection coefficient and T: DUT transmission coefficient,
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  • P4 → Insertion loss is the characteristics of DUT itself. As input port and output ports are matched, IL= attenuation.

VSWR measurements

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  • If E probe penetrates too far into the slotted line, → disturb the field distribution and detected signal too strong to drive the detector out of its square-law region.

S- Parameters

  • Problems to use Z- , Y- or H- parameters in microwave circuits
  • ► Difficult in defining voltage and current for non-TEM lines
  • ► No equipment available to measure voltage and current in complex value as oscilloscope
  • ► Difficult to make open and short circuits over broadband
  • ► Active devices not stable as terminated with open or short circuit.
    • S-parameters of a two-port network
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  • Reasons to use S-matrix in microwave circuit
  • ► matched load available in broadband application
  • ► measurable quantity in terms of incident, reflected and transmitted waves
  • ► termination with Z0 causes no oscillation
  • ► convenient to use in the microwave network analysis

Microwave analyzers

  • Spectrum analyzer
  • Purpose: measure microwave signal spectrum, can also be used to measure frequency, rms voltage, power, distortion, noise power, amplitude modulation, frequency modulation, spectral purity,...
  • Operating principle
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Network analyzer

  • Purpose: measure two-port S-parameter of a microwave device or network, can also be used to measure VSWR, return loss, group delay, input impedance, antenna pattern, dielectric constant,….
  • Operating principle
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  • Scalar network analayzers measures the magnitutude of S-parameters
  • Hp8510 vector network analyzer