Article By : Albert Lo
Accurate telemetry data used to monitor the satellite health can help set a baseline for nominal operation.
Because satellites are inaccessible once launched, acquiring accurate telemetry data to monitor the health of the satellite subsystems can help set a baseline to indicate a working system, while fluctuations can indicate failures.
Two examples of sensitive components would require accurate monitoring of voltage, temperature and current are the radio-frequency power amplifiers and thermoelectric coolers (TEC). In both applications, performance fluctuates with temperature and radiation effects, and requires adjustments to the applied voltage and current in order to ensure efficient and safe operation.
A telemetry circuit monitors critical system power rails and components, gathers performance data (which has value both now and for future satellite designs) and adjusts system settings accordingly.
The most significant blocks of a telemetry circuit (See Figure 1) are the analog front-end (AFE) that senses the power supply rails and temperature, the main processor that analyzes the data and the output signals needed to adjust different system parameters.
An AFE monitors three important values for a telemetry circuit: voltage, current and temperature. In order to measure and interpret these values, an analog-to-digital converter is used to digitize these values and send them to the processor. A radiation-hardened (rad-hard) ADC with higher channel count, higher resolution and sampling speeds up to 10 MSPS are necessary for accurate measurement of voltage, current and temperature all while having low power consumption.
Furthermore, simultaneous sampling ADCs enable a non-phase shifted readings that lends itself towards motor drive feedback measurement. There are, however, other necessary components in front of the ADC to ensure proper operation, depending on which three values the ADC is monitoring.
Voltages in a satellite system can reach up to 40V and include negative voltage rails. These high voltage rails can damage the sensing ADCs, thus buffer and attenuation stages are used to scale down the voltage and protect the ADC. A resistor divider will first divide the voltage down to fall within the input range of the ADC. Designers can then use an operational amplifier as a buffer to ensure that the signal has enough drive strength. The operational amplifier should have good gain bandwidth, of around 2.5MHz, and rail-to-rail on both input and output; which means minimum impact on accuracy while maximizing the full-scale range of the ADC.
By pairing the resistor divider and an operational amplifier, a rail such as a 40-V bias rail can be used to translate that voltage range to a smaller scale. For example, using a 10-to-1 resistor divider, a 40-V input, would correspond to 4-V output from the buffer.
Current sensing can determine faults on any sensitive power rails and flag the main processor of the power rails that need to be shut off to prevent damaging FPGAs or data converters. The current data collected can help determine whether devices in the system are drawing more current than expected, indicating a possible radiation-induced latch-up.
A current-sense amplifier with wide bandwidth can provide accurate current measurements to help set a baseline for the power rails, while also reacting quickly enough to protect mission-critical components like the FPGA and the RF power amplifier in the event of a sudden failure such as a short circuit.
Temperature measurement of devices or the circuit board also can help determine if components are working properly. Overheating can be a key indicator of an FPGA or power module being over stressed, reducing device runtime.
To measure these temperatures, a digital-output temperature sensor, like the TMP461-SP from Texas instruments, can provide both local and remote temperature without the need of an additional ADC. The result is a 3X reduction in the board area (See Figure 2).
The remote temperature sensor monitors the integrated internal temperature diodes of sensitive devices such as FPGAs and high-speed data converters.
Another example is providing accurate temperature feedback, using remote sensing, to a TEC system so it can properly cool or heat systems like a laser communication system or star-tracker system. In addition to the sensing these diodes, a radiation hardness-assured temperature sensor also includes an internal sensor that can measure the temperature at its location on the board such as near the power supply.
To process AFE data and control the telemetry circuit in a satellite, there are two options available: an FPGA or a microcontroller. A typical telemetry circuit uses FPGAs to handle the communication and processing of telemetry data. It can almost stand alone, requiring little communication with other parts of the system. FPGAs enable more channel inputs from the AFE, fast data translation and complex decision-making based on the data input. Most space-grade FPGAs have a large package footprint, however, and high-power consumption in an application where minimal size and power are desired.
A mixed-signal MCU is a good alternative to help reduce FPGA resources and pins used for telemetry circuits, while providing the same functionality for telemetry circuits such as integrating the ADC and other analog components, processing the data, power sequencing and pulse-width modulation outputs for constant current sources.
System recalibration
After reading the measurements from the system, a telemetry circuit can adjust the different system functions to maintain optimal operation through system recalibration.
If the main processor determines there is a fault, it can shut off components or toggle their operating mode. For applications that require more precise controls and drive strength, an external digital-to-analog converter can accurately adjust the bias of system components like RF power amplifiers.
In conclusion, there are a variety of space-grade analog and embedded processing products providing integrated and low-power solutions for telemetry circuits, with the measurement accuracy and performance needed throughout the system to ensure proper operation for entire mission.
This article was originally published on EE Times.
Albert Lo is a systems engineer at Texas Instruments.
