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Figure 2. Transponders of Es’hail-2


older analog ones, many of them incorporate digital signal processor (DSP) technology after the mixer, at the intermediate frequency level. Some of them are also able to directly sample the whole shortwave portion of the spectrum (dc to 30 MHz). One advantage of the SDRs is that their performances do not degrade with time, since many critical analog components are partially replaced by digital algorithms. Another advantage is that the same performances that require expensive components in analog radios, like mixers or filters, can be obtained in a more cost-effective way by complementing them with different elements like analog- to-digital converters (ADCs) and DSPs. The integration of multiple blocks, such as image rejection mixers, oscillators, and ADCs, in the same silicon device has made feasible new receiver architectures that are very critical


to implement with discrete technology. One example is devices like the AD9363/AD9364 RF agile transceivers that combine all RF front-end, mixed-signal, and digital blocks in a single device for both receiving and transmitting. When paired with an FPGA that manages the digital data flow into and out from the device, the elements remaining to build a complete station are the antennas, the power amplifier, and the software algorithms running on a computer.


ADI offers the ADALM-PLUTO SDR to demonstrate the capabilities of the AD9363, shown in Figure 3. This is a cost-effective hardware tool that can be used by engineers to develop applications where radio is involved based on the new SDR approach. The AD9363 has a receive and transmit bandwidth of 20 MHz and it can easily receive both the narrow and wide downlink transponders of


the Es’hail-2, once they are downconverted externally to its frequency range of 235 MHz to 3.8 GHz. It can transmit on the uplink frequencies without any external upconverter. Another beneficial feature, when compared to devices of the same class and price, is that it has two connectors for receive and transmit, so it supports full-duplex operation. The normal amateur radio interaction is half-duplex (you either talk or listen), but the ability to receive your own transmission in real time allows you to understand whether you are modulating in a clear way, or whether you need to increase/decrease the transmitted power. It also helps to have the ability to point the transmit antenna to the sky once the receive antenna has been adjusted. The ADALM-PLUTO is supported for both transmission and reception by some free software packages, often written by radio amateurs themselves. One example is the SDR Console by Simon Brown (amateur radio callsign G4ELI). This software manages the interaction between the user and the transceiver, and implements demodulation and modulation in software.


An SDR satellite station


Radio amateurs are well known for building their own hardware and repurposing existing equipment to fit their needs. With receive antennas and downconverters, the cheapest alternative is an ordinary satellite dish for commercial satellite television and a low noise block (LNB). The LNB contains the waveguide and the downconverter that translate the incoming downlink signal at 10.450 GHz to less than 1 GHz, which falls inside the receivable band of the SDR. Narrow-band modulation types such as CW (a few tens of Hz) or SSB (less than 3 kHz) mandate highly stable local oscillators to avoid continuous retuning, which is less critical in wideband modulation types such as the ones used by broadcast television (some MHz). In modern digital communications,


compensation for frequency offset and long- term drift due to thermal issues is built into the standards and implemented by everyone. Unfortunately, this is not standardized, or not implemented, for many narrow-band modulation schemes implemented by amateur radio operators, and the expectation is that PLL or sample rate accuracy and drift either in the LNB or baseband signals is perfect. To ensure this assumption is correct, sometimes high precision/low drift reference clocks are used. Since many amateur radio operators are more comfortable swapping a reference clock than implementing complex digital signal processing techniques, many will recommend this easy fix.


Since the uplink frequencies are within the WLAN 2.4 GHz band it is possible for licensed operators to repurpose existing WLAN equipment like power amplifiers and high gain antennae. The ADALM-PLUTO has about 5 dBm power output, which is insufficient to drive a power amplifier that has an output power of a few watts. The CN-0417 reference design, based on the ADL5606 20 dB power amplifier and powered by the LTM8045 SEPIC micromodule converter, yields enough power gain to overcome this limitation. Figure 4 shows how a communication station can be laid out. The station can also be rapidly deployed in the field to support emergency communication.


Summary


In conclusion, we see a shift toward SDR technology in radio communication. This has been possible by integrating multiple analog and mixed-signal blocks in one device. Immediate advantages are cost-effectiveness, improved reliability, and reconfigurability. Quoting the words of Drew Glasbrenner, KO4MA, AMSAT VP Operations, “May the 100th OSCAR satellite be the guide star to future amateur radio satellites and payloads to geostationary orbit and beyond.” analog.com


Figure 3. ADALM-PLUTO and its transceiver AD9363


Figure 4. SDR satellite radio station 34 October 2020 Components in Electronics www.cieonline.co.uk


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