# The Microwave Temperature and Humidity Profiler: Description and Preliminary Results
## 2 Instrument Description
### 60 GHz Receiver  
The 60 GHz MTHP channel digital high-resolution spectrometer update represents a significant advancement over the earlier MTP version [18]. With the increasing demand for higher resolution mapping of the 60 GHz band for conducting PBL studies, an upgrade was necessary for the receiver back-end. The need to measure a large number of channels around 50-60 GHz was raised to increase the resolution and accuracy of thermal profiles [19]. To meet these requirements, the back-end channel was enhanced with a state-of-the-art Commercial-off-the-Shelf (COTS) high-resolution digital spectrometer. This digital spectrometer, known as the P19800B, is sourced from Pacific Microchip Corporation (PMCC) [20]. The digital spectrometer is capable of providing integrated power spectra of up to 4 GHz of instantaneous bandwidth resolved in 512 to 8192 channels and at user-configurable integration times. The spectrometer is implemented in a low-power application-specific integrated circuit (ASIC) and can be controlled by a Serial Parallel Interface (SPI). Figure 3 provides a detailed block diagram and a general overview of the complete front-end and back-end of the 60 GHz channel receiver. The signal is collected by the antenna and amplified by a first-stage low-noise amplifier (LNA) designed and manufactured at JPL. The signal is down-converted using the 2 nd subharmonic of a 23.5 GHz oscillator (i.e., the central frequency is 47 GHz). The upper sideband of the down-conversion is amplified and filtered. The total gain of this receiver chain is ~70 dB. The MTHP's goal is to cover as much band as possible in the 60 GHz band. Hence, the entire Federal Communications Commission (FCC) reserved band for Earth Exploration--Satellite (passive) [21] comprised between 51 and 59 GHz is measured. To measure up to an 8 GHz bandwidth using the PMCC chip, a time-division dual channel strategy is followed. This dual-channel strategy is depicted after the amplification blocks in Figure 3. First, the signal is split into two paths using a 3-dB splitter, whereby the lower path is filtered using a 4-8 GHz COTS band-pass filter (RF-Lambda RBPF33G04G08) and the upper path by an 8-12 GHz COTS band-pass filter (RF-Lambda RBPF33G08G12). The output of the filters  
Figure 2: **(Left)** MTHP Antenna and servo assembly block diagram, (right) MTHP top view highlighting horn external connection, antenna assembly, and servo motor. is directed to a single-pole dual throw (SP2T) switch that switches between the channels. The output of this SP2T is then connected to a second SP2T that is used to switch between the measurement chain (i.e., the previous SP2T) and a calibration signal required for the nominal operation of the PMCC digital spectrometer. The calibration signal is a 500 MHz tone, and it calibrates the gain and offset of the different Analog-to-Digital Converters (ADC) of the PMCC spectrometer. Additionally, a second clock is required for the PMCC ADC to work, selected at 1 GHz. Finally, a Field-Programmable Gate Array (FPGA) board with an embedded dual-core ARM is used to control the PMCC and the calibration/channel selection. The board used is the RedPitaya STEMLab 125, which acts as a Command and Data Handling (CDH) interface with the external system. The digital spectrometer produces a 1024-point power spectral measurement every 10 ms (i.e., 10 ms integration time). Each spectral channel is thus 3.9 MHz wide. Furthermore, the measurement software switches periodically between the upper and lower channels. This strategy virtually produces 2048 3.9 MHz--with points between 51 GHz and 59 GHz. The switch between the upper and lower channels occurs every two rotations. The rotor positions are read by the CDH and synchronized with the channel selector switch. The measurement scheme is detailed in Figure 4. This scheme produces 162 measurements per rotation, with an angular resolution of 2.25\({}^{\circ}\) per measurement. Figure 4: Detailed measurement time-sequence scheme for the MTHP 60 GHz channel.