“TI’s family of aerospace-grade analog and embedded processing products provide compact and low-power solutions for telemetry circuits that enable the measurement accuracy and performance required by the entire system to ensure smooth execution of the entire mission.
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Author: Texas Instruments
Because satellites on space missions are inaccessible once launched, acquiring accurate telemetry data to monitor the health of satellite subsystems can help set a baseline that indicates the system is working properly, while fluctuations can indicate a malfunction. For example, RF power amplifiers and thermoelectric coolers are two sensitive devices that require accurate monitoring of voltage, temperature, and current. In both applications, performance fluctuates with temperature and radiation effects, and the applied voltage and current need to be adjusted to ensure efficient and safe operation. Telemetry circuits monitor critical system power rails and components, as well as collect performance data (which is valuable for current and future satellite designs) and adjust system settings accordingly.
The most important blocks of a telemetry circuit (shown in Figure 1) are the analog front end (AFE) that senses the power rails and temperature, the main processor that analyzes the data, and the output signals needed to adjust different system parameters.
Figure 1. Telemetry circuit block diagram: AFE (green), processing (red), and output signal (blue)
AFE voltage, current and temperature sensing
The AFE monitors three important values of the telemetry circuit: voltage, current, and temperature. To measure and analyze these values, analog-to-digital converters (ADCs) are used to digitize and send these values to a processor. The ADC128S102QML-SP is ideal for an AFE because it has the 12-bit resolution and sampling rates up to 1 MSPS necessary to accurately measure voltage, current, and temperature (the ADC128S102QML-SP has been used in spaceflight for over 10 years), and has Low power consumption from 2.3mW to 10.7mW (using 3V and 5V supplies, respectively). However, there are other necessary components in front of the ADC to ensure proper operation, depending on the three values monitored by the ADC.
Voltages in satellite systems can go up to 40V, including the negative rail. These high voltage rails can damage the detection ADC, so buffering and attenuation stages are used to reduce the voltage and protect the ADC. The resistive divider first divides the voltage so that it is within the input range of the ADC. Designers can then use an op amp such as the LMP7704-SP as a buffer to ensure the signal has sufficient drive strength. The LMP7704-SP also has a good gain bandwidth of 2.5MHz and has rail-to-rail inputs and outputs, which means there is little impact on accuracy, while allowing a greater extension of the ADC’s full-scale range. When using a resistor divider with the LMP7704-SP at the same time, you can take a power rail such as a 40V bias rail and reduce the voltage range. For example, when using a 10:1 resistor divider, a 40V input corresponds to the buffer’s 4V output.
Accurately monitor temperature, voltage and current in aerospace applications
Current sensing can identify faults on any sensitive power rails and flag host processors that need to shut down the power rails to prevent damage to field programmable gate arrays (FPGAs) or data converters. Acquired current data can help determine if devices in the system are drawing more current than expected, which could indicate radiation-induced lock-up. With its wide bandwidth, the INA901-SP current sense amplifier can provide accurate current measurements to help set the baseline of the power rails, while also reacting quickly to sudden faults such as short circuits.
A temperature measurement of a device or circuit board can help determine whether a component is functioning properly. Overheating can be a key indicator of overloading an FPGA or power module, which reduces the runtime of the device. Other devices, such as RF power amplifiers, are sensitive to temperature fluctuations and require proper voltage adjustment to counteract temperature changes. To measure these temperatures, the TMP461-SP digital output temperature sensor can provide local and remote temperature sensing, monitoring the internal integrated temperature diodes of sensitive devices such as FPGAs and high-speed data converters. In addition to detecting these diodes, the TMP461-SP contains an internal sensor that measures the temperature at its location on the board. If the device is placed next to other sensitive devices such as RF power amplifiers, the TMP461-SP can monitor the temperature of these devices.
Processing telemetry data: FPGA vs MCU
To process the AFE data and control the telemetry circuitry in the satellite, two types of devices are used: FPGA or microcontroller (MCU). A typical telemetry circuit uses an FPGA for the communication and processing of telemetry data, operating almost independently, and requiring no communication with other components of the system. The FPGA supports more channels of input to the AFE, fast data conversion, and complex decision-making based on data input. However, most aerospace-grade FPGAs have large package sizes and high power consumption in applications that require ultra-small size and power consumption.
Mixed-signal MCUs such as the MSP430FR5969-SP are a good alternative to help reduce FPGA resources and pins for telemetry circuits, while providing the same functions for telemetry circuits, such as data processing, power sequencing, and pulse width Modulation output. The MSP430FR5969-SP also features integrated ferroelectric random access memory (FRAM), which tends to be more resistant to radiation effects than traditional memory types such as double data rate.
System recalibration
After reading measurements from the system, the telemetry circuitry can adjust various system functions to maintain optimal operation through system recalibration.
If the main processor determines that there is a fault, the main processor can shut down the element or switch its operating mode. For applications requiring more precise control and drive strength, external digital-to-analog converters such as the DAC121S101QML-SP can precisely adjust the bias of system components such as RF power amplifiers.
Epilogue
TI’s family of aerospace-grade analog and embedded processing products provide compact and low-power solutions for telemetry circuits that enable the measurement accuracy and performance required by the entire system to ensure smooth execution of the entire mission.
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