# RF Signal Chain
The radiometer's RF section (also known as the front end of the receiver) comprises a 50-Ohm termination, a low-loss RF switch, a low-noise amplifier (LNA), a bandpass filter, and a 2nd stage amplifier. As the receiver captures both polarizations, the signal chain is replicated for both hands of polarization.
A 50-Ohm termination (load) is used for calibration purposes, similar to a Dicke receiver. As the radiometer is not kept in a temperature-controlled environment, the physical temperature of the calibration load may vary, affecting its noise figure. To account for this, a temperature sensor is placed near the load to measure and adjust for any temperature changes. The temperature is measured every 10 seconds.
Schematic of the dual-amplifier chain.
The RF switch, one for each polarization, is used to switch between the antenna and the calibration load approximately every 350 ms. The RF switch is followed by the LNA, filter, and 2nd stage amplifier. The bandpass filter has a passband from 1290 to 1515 MHz. The combined gain of the LNA and 2nd stage amplifier is 40 dB. From here, the RF signals go into the A/D converter and digital correlator.
The following components are used:
* RF switch: Analog HMC545
* LNA: NXP USA Inc. BGU8052X
* Bandpass filter: Mini-Circuits CBP-1400BD+
* 2nd stage amp: Analog Devices Inc. HMC741ST89E
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