projekte:3cmbeacon:start
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projekte:3cmbeacon:start [2023/01/06 15:38] – [RF Design] thasti | projekte:3cmbeacon:start [2023/01/23 19:42] (aktuell) – [Architecture] thasti | ||
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====== Integrated 10 GHz Beacon Transmitter ====== | ====== Integrated 10 GHz Beacon Transmitter ====== | ||
- | After the decline in use of ATV in Germany, an idea of building a 3 cm beacon at [[https:// | + | After the decline in use of ATV in Germany, an idea of building a 3 cm beacon at [[https:// |
The goal of this project was building a 10 GHz beacon transmitter suitable for long-term unsupervised operation from commercially available integrated circuits. To make full use of the transmit power constraints in Germany, an output power of 1-2 W was targeted. Support for contemporary digital beacon transmission modes such as [[https:// | The goal of this project was building a 10 GHz beacon transmitter suitable for long-term unsupervised operation from commercially available integrated circuits. To make full use of the transmit power constraints in Germany, an output power of 1-2 W was targeted. Support for contemporary digital beacon transmission modes such as [[https:// | ||
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+ | **Quick Pointers:** | ||
+ | * **[[https:// | ||
+ | * **[[https:// | ||
+ | * **[[https:// | ||
===== Architecture ===== | ===== Architecture ===== | ||
A simple block diagram of the beacon transmitter is shown in the figure below. | A simple block diagram of the beacon transmitter is shown in the figure below. | ||
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The beacon transmitter accepts an external 100 MHz reference clock provided by a GNSS-disciplined oscillator. GNSS stabilization is the de-facto standard for beacons on 10 GHz and above due to their narrow spacing and small bandwidth. It also enables receiving stations to use the beacon as a frequency reference for aligning their own equipment. | The beacon transmitter accepts an external 100 MHz reference clock provided by a GNSS-disciplined oscillator. GNSS stabilization is the de-facto standard for beacons on 10 GHz and above due to their narrow spacing and small bandwidth. It also enables receiving stations to use the beacon as a frequency reference for aligning their own equipment. | ||
- | A Silicon Labs Si5342 is used as a crystal-driven reference PLL, which takes care of reference clock jitter cleaning and modulation generation. The high-resolution fractional divider allows synthesizing sub-Hz frequency steps of the output RF carrier, which are required for modern modulation formats. Due to the architecture of the synthesizer, | + | {{ : |
- | The HMC952A also has a built-in output RF power detector, which is read out by an on-board | + | A Silicon Labs Si5342 is used as a crystal-driven reference PLL, which takes care of reference clock jitter cleaning and modulation generation. The high-resolution fractional divider allows synthesizing sub-Hz frequency steps of the output RF carrier, which are required for modern modulation formats. Due to the architecture of the synthesizer, |
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+ | The HMC952A also has a built-in output RF power detector, which is read out by an on-board | ||
===== RF Design ===== | ===== RF Design ===== | ||
- | One of the key design features that helps with driving down cost is working with a commercial four-layer FR4 stackup. These have become ubiquitous and cheap to use, so one supplier (JLCPCB from China) was chosen and their PCB process characterized for RF performance. Two dedicated RF test coupons were designed: Based on the results of long simulations, | + | {{ : |
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+ | One of the key design features that helps with driving down cost is working with a commercial four-layer FR4 stackup. These have become ubiquitous and cheap to use, so one supplier (JLCPCB from China) was chosen and their PCB process characterized for RF performance. Two dedicated RF test coupons were designed: Based on the results of long simulations, | ||
The very thin two outer layer pairs can be used for RF traces (outer layer for signal, layer just below as a reference plane). Their biggest advantage is their narrow layer spacing (only about 150 µm), but is also the biggest complication: | The very thin two outer layer pairs can be used for RF traces (outer layer for signal, layer just below as a reference plane). Their biggest advantage is their narrow layer spacing (only about 150 µm), but is also the biggest complication: | ||
- | The 4x multiplier IC (HMC443) was prototyped as a standalone device, since it was found to anyways be a useful piece of equipment for the home laboratory. This design is described [[projekte: | + | The 4x multiplier IC (HMC443) was prototyped as a standalone device, since it was found to anyways be a useful piece of equipment for the home laboratory. This design is described [[projekte: |
Based on these results, the key RF components could be laid out. Since all components are internally matched reasonably well, no on-PCB matching was foreseen, except for an SMD attenuator between the RF PLL and the multiplier. This mostly improves harmonic content of the multiplier output by avoiding excessive overdrive, but also helps with input matching of course. | Based on these results, the key RF components could be laid out. Since all components are internally matched reasonably well, no on-PCB matching was foreseen, except for an SMD attenuator between the RF PLL and the multiplier. This mostly improves harmonic content of the multiplier output by avoiding excessive overdrive, but also helps with input matching of course. | ||
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===== Thermal/ | ===== Thermal/ | ||
+ | Since all components including the power supply and the 1.5 W RF amplifier are to be placed on a single, relatively compact PCB, thermal design and power distribution required some special care. A power budget including power conversion losses etc was created (available [[https:// | ||
+ | The design includes only SMD components, all of which are mounted on the front side of the PCB. The inner layers are preferentially used for routing (where not needed as a well-stitched ground place for RF signals) | ||
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+ | A solid (10 mm thick) copper block was attached to the PCB via a "gap pad" for optimal heat transfer. This heat spreader was kindly milled, drilled and tapped by David F1URI. | ||
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+ | A final consideration is RF shielding, mostly due to co-use of the beacon site with commercial operators: Cross modulation from GSM/ | ||
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+ | ===== PCB Revisions ===== | ||
+ | Two PCB revisions were fabricated. Revision 1 still used an on-board OCXO and can be operated completely standalone (without requiring an external reference clock). This was used as an opportunity to evaluate the tiny Connor-Winfield DOCAT series of OCXOs. A 50 MHz oscillator proved highly stable and fully suitable for example in a SSB transverter application. Warmup time was short, power consumption very reasonable - however in the end for a stationary beacon application GNSS was chosen as be the preferred solution. | ||
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+ | The schematics and PCB design files (in KiCad format) can be found in the following repository: https:// | ||
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+ | ===== Software ===== | ||
+ | A brief overview of the software functionality in the beacon was already provided above. In summary, the software running on the on-board MCU has the following functionality: | ||
+ | * Initialization of all on-board hardware (power monitoring ICs, PLL ICs via I2C and SPI) | ||
+ | * Measurement of temperature and output power using on-chip ADC | ||
+ | * Measurement of supply voltages and currents (readout via I2C) | ||
+ | * Modulation of CW and PI-4 on 10 GHz carrier using the PLL fractional dividers | ||
+ | * Telemetry transmission to indoor monitoring and control computer | ||
+ | * Fan control (two-point temperature controller) | ||
+ | * Monitoring of PLL registers to detect loss-of-lock | ||
+ | * Listen for serial commands | ||
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+ | The serial (RS-422) interface is, during nominal operation, mostly used for time synchronization of the beacon transmitter and for telemetry monitoring (health of the electronics and environmental conditions). | ||
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+ | The requirement of full remote operation necessitated some interesting features for the software: The TX can be disabled through software, or switched to a continuous unmodulated carrier for measurements by regulatory authorities. Even remote firmware updates are possible thanks to the built-in bootloader of the STM32G-series devices. The bootloader is started through a special command sent to the beacon, followed by the upload and verification of a new firmware. | ||
===== Testing ===== | ===== Testing ===== | ||
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+ | Before going into continuous operation at DB0HDF, the transmitter hard- and software were extensively tested on the lab workbench. The output power, phase noise and spurious performance was first evaluated, with a particular emphasis on varying input voltage conditions. Long supply wires could possibly cause regulator instabilities in final installations, | ||
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+ | A crucial discovery (made already on revision 1 of the PCB) was the presence of EMI issues due to the specific mode chosen for the input SMPS regulator. The LTC3624 supports both " | ||
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+ | After this modification, | ||
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+ | Various failure conditions were stimulated to make sure the monitoring logic was sound and successfully brought the transmitter to a safe state (TX+PA off) when issues were detected. I2C and SPI communication failures were tested. | ||
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+ | Finally, a pre-integration environmental test was performed. The transmitter was installed in an environmental chamber and multiple ramps between -20°C and +55°C were exercised. The transmitter output power, power consumption, | ||
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+ | Across the full temperature range, the output power was found to vary by about 2.5 dB. This is not untypical for a fixed gate voltage design without temperature compensation. The negative temperature coefficient ensures there is no thermal runaway at high temperatures, | ||
===== Transmitter Integration at DB0HDF ===== | ===== Transmitter Integration at DB0HDF ===== | ||
+ | With the PCB battle-tested, | ||
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+ | The transmitter and its slot antenna were finally mounted on a large heat sink and placed on a base plate. A 10 GHz circulator was added in-line to avoid damaging the TX in case the return loss of the antenna suddenly degrades due to e.g. temporary water intrusion. An outdoor GNSS antenna was mounted adjacent to the transmitter, | ||
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+ | A corresponding indoor unit (19" rack mount) was manufactured by Rolf DL2ARH. It houses a mains power supply, the Raspberry Pi plus RS-422/ | ||
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+ | ===== Installation and Operations ====== | ||
+ | The hardware was transferred to DB0HDF by Stefan DK3SB and Rolf DL2ARH in January 2022. At first, the ODU was operated from inside the equipment room due to bad weather not permitting installation on the roof. The transmitter was mounted on the roof of DB0HDF in April 2022 (tnx Rolf DL2ARH es Ilona DG1ASK). | ||
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- | ===== Operations ====== | + | The beacon has since been operating without issues, being very realible in operation. A total of three PLL unlock events have been recorded over 8 months of operation, all of which the software automatically recovered from without manual intervention. Currents and voltages remained in the expected range and RF output power shows the expected temperature dependency. In the outdoor enclosure, PCB temperatures always remain at least 10 degrees above ambient air temperatures, |
- | ===== References and Resources | + | ===== Acknowledgements |
+ | Many thanks to contributors to all project contributors: | ||
+ | * Rolf, DL2ARH | ||
+ | * Ilona, DG1ASK | ||
+ | * Severin, DK1SEV | ||
+ | * Sebastian, DL3YC | ||
+ | * David, F1URI |
projekte/3cmbeacon/start.1673019518.txt.gz · Zuletzt geändert: 2023/01/06 15:38 von thasti