Measurement parameters and low power device

Microwave measurement parameters are the quantities used to describe signals and components at microwave frequencies, where power and wave patterns are measured instead of voltage and current.

Major microwave measurement parameters

• Power: average or peak power delivered to a matched load (W, dBm).
• Frequency / wavelength: source frequency f and guide wavelength λg.
• VSWR: S = V_max / V_min on the line; shows mismatch.
• Reflection coefficient: Γ = (Z_L − Z0)/(Z_L + Z0); |Γ| = (S − 1)/(S + 1).
• Impedance: load impedance found from VSWR and position of minimum.
• Attenuation / insertion loss: power loss through a component in dB.
• S-parameters: reflection and transmission ratios of a network.
• Q factor: stored energy / energy lost per cycle in a cavity.
• Noise figure: degradation of SNR by a device.

Low power measurement device: the bolometer bridge

Low power (below about 10 mW) is measured with a bolometer – a temperature-sensitive resistor – in a bridge.

Types of bolometer:

• Barretter: thin platinum wire; positive temperature coefficient; sensitive but burns out easily.
• Thermistor: semiconductor bead; negative temperature coefficient; robust, commonly used.

          +---R1---+---R2---+
          |        |        |
 DC bias -+        G        +-
          |        |        |
          +---R3---+--Rb----+
                       ^
                       | microwave power
                  bolometer mount

Working principle (DC substitution):

1. With no RF, DC bias current I1 through the bolometer Rb heats it until Rb equals the value needed for balance (R1/R2 = R3/Rb); the galvanometer G reads zero.
2. The microwave power is applied to the bolometer mount. It heats the bolometer further, its resistance changes, and the bridge unbalances.
3. The DC bias is reduced to I2 until balance returns. Now Rb is again at the same value and temperature, so the drop in DC power equals the RF power:

P_RF = (I1² − I2²) · Rb

Example: Rb = 200 Ω, I1 = 8 mA, I2 = 5 mA → P_RF = (64 − 25)×10⁻⁶ × 200 = 7.8 mW.

To remove ambient temperature drift, practical meters use a self-balancing double bridge with a second compensating thermistor: P = (V_c² − V_rf²)/(4R).

Other low-power sensors: thermocouple sensors (about 1 µW–100 mW) and Schottky diode sensors (down to about 100 pW, square law below about −20 dBm).