Total Power Heterodyne Receiver
Signal Chain Walkthrough
A total power heterodyne receiver measures the total noise power collected by the antenna, which is proportional to the brightness temperature of the scene being observed. This is a receive-only system with no transmitter.
Antenna (Feed Horn)
Typically a corrugated feed horn with well-characterized beam pattern and low sidelobes. The antenna aperture and beam width determine the spatial resolution of the radiometric measurement.
LNA
The LNA is the most critical component, as its noise figure directly sets the receiver sensitivity (system noise temperature). Cryogenically cooled InP HEMT LNAs achieve noise temperatures below 10K for radio astronomy applications. Room-temperature GaAs LNAs provide noise figures of 0.5-2.0 dB for atmospheric sensing.
Mixer and LO
The mixer downconverts the RF signal to an IF where high-gain, low-noise amplification is more practical. The LO is typically a phase-locked synthesizer for frequency stability. LO phase noise contributes to output noise and must be minimized.
Square-Law Detector
A square-law (power) detector converts the IF signal envelope to a DC voltage proportional to the input noise power. The detector output represents the total power in the IF bandwidth, which is proportional to the antenna temperature.
Integrator / ADC
A low-pass filter (integrator) averages the detector output over the integration time τ to reduce fluctuations. Longer integration times improve temperature sensitivity according to the radiometer equation: ΔT = Tsys / √(B×τ).
Component Specifications
| Component | Parameter | Typical Value |
|---|---|---|
| LNA | Noise Figure | 0.3 - 2.0 dB |
| LNA | Gain | 25 - 40 dB |
| RF BPF | Bandwidth | 1 - 20 GHz |
| Mixer | Conversion Loss | 5 - 8 dB |
| IF Amplifier | Gain | 40 - 60 dB |
| Detector | Sensitivity | 500 - 2000 mV/mW |
| System | Noise Temperature | 100 - 2000 K |
| System | Sensitivity (ΔT) | 0.01 - 1.0 K |