Transponder Key Amplifier

[Prisc Chandola]

Acommunications satellite's transponder is the series of interconnected units that form a communications channel between the receiving and the transmitting antennas.[1] It is mainly used in satellite communication to transfer the received signals.

Original analog video only had one channel per transponder, with subcarriers for audio and automatic transmission-identification service ATIS. Non-multiplexed radio stations can also travel in single channel per carrier (SCPC) mode, with multiple carriers (analog or digital) per transponder. This allows each station to transmit directly to the satellite, rather than paying for a whole transponder or using landlines to send it to an Earth station for multiplexing with other stations.

NASA distinguishes between a "transceiver" and "transponder". A transceiver has an independent transmitter and receiver packaged in the same unit. In a transponder the transmit carrier frequency is derived from the received signal. The frequency linkage allows an interrogating ground station to recover the Doppler shift and thus infer range and speed from a communication signal without allocating power to a separate ranging signal.[2]

A transponder equivalent (TPE) is a normalized way to refer to transponder bandwidth. It simply means how many transponders would be used if the same total bandwidths used only 36 MHz transponders.[3][4][5] So, for example, the ARSAT-1 has 24 IEEE Ku band transponders: 12 with a bandwidth of 36 MHz, 8 with 54 MHz, and 4 with 72 MHz, which totals to 1152 MHz, or 32 TPE (i.e., 1152 MHz divided by 36 MHz).[6][7]

I have a simple powered transponder which sends data using binary phase-shift keying (BPSK) on a 5Mhz carrier with a required range of 6-12 inches. It is used to time laps of remote controlled cars and go-karts. It has an inductor on the PCB as the transmitting antenna and looks like the below:

Right now the receiver uses a loop of stranded 24AWG wire. This directly connects to a small transistor amplifier that is powered over coax. The loop has to be tuned using the blue jumpers to adjust the amount of capacitance, making it resonant around 5Mhz. This is dependant on the length of the loop.

Instructions for a professional equivalent show the below, point #3 is a 470 ohm resistor (presumably a virtual ground). The width of the track is "max 10 metres", which could mean a length of anything from a few metres to >20 metres, with no adjustments required by the operator.

I notice that this is very similar to what radio hams call a beverage antenna. These are usually at least a wavelength long, impractical at 5Mhz because one wavelength is 60 metres. However this made me wonder if it would be possible to use the loop more like an RF antenna rather than a fully tuned resonant circuit.

My thoughts were to amplify and band-pass filter what comes off the loop, this is my starting point for removing the powered transistor amplifier. This could hopefully reduce the total cost and will make it easier to operate.

The clue is in the name - transponder. I suspect that the loop not only powers the normally unpowered transponder (fixed to the car) but also acts as receiver. At 5MHz the transmission process will largely be the magnetic part of the EM wave that is used.

Either way, I suspect you need the trackside circuit board and (mag field generator and receiver) still to be trackside for this to work. Trying to convert it to an equivalent of a regular RF antenna is, possibly, missing the point. For it to operate as a beverage antenna it's length needs to be one wavelength and at 5MHz that is 60m long. It is a magnetic antenna not intended for EM waves.

The advancement of optical networks is pivotal in contemporary communication systems, demanding robust and reliable performance. Optical Network Enhancers, namely Erbium-Doped Fiber Amplifier (EDFA), Repeater, and Transponder, play indispensable roles within this framework. This article delves into the intricate workings, applications, and a thorough comparative analysis of these devices, aiming to provide insights into their performance metrics and suitability in diverse scenarios.

EDFAs are primarily designed for optical signal amplification without the need for signal conversion to electrical form. They leverage erbium-doped fibers to amplify signals directly in the optical domain.

Transponders operate as optical-electrical-optical (OEO) conversion devices. They convert optical signals to electrical form and then back to optical signals, facilitating format and wavelength conversion.

Repeaters serve the purpose of amplifying and, if necessary, regenerating optical signals. They are capable of addressing signal attenuation over extended distances, and unlike EDFAs, repeaters may involve signal conversion and regeneration.

Erbium-Doped Fiber Amplifier (EDFA): EDFAs are extensively employed in long-haul optical communication networks, such as transcontinental and undersea fiber optic links. They are strategically placed at amplifier sites along the network to compensate for signal attenuation. Additionally, EDFAs play a vital role in optical backbone networks, enhancing the signal strength without the need for signal conversion.

Repeater: Repeaters are crucial components in optical networks designed for ultra-long-haul and submarine communication systems. Placed at regular intervals, Repeaters amplify and regenerate optical signals, mitigating the cumulative effects of fiber attenuation. In scenarios where optical signals traverse vast distances, repeaters ensure signal integrity and prevent degradation.

Transponder: Transponders find essential roles in wavelength-division multiplexing (WDM) networks and high-capacity data transmission systems. In WDM systems, Transponders enable wavelength conversion, allowing for flexible allocation and reallocation of optical channels. Transponders support the dynamic management of optical signals by facilitating conversion between different wavelengths and formats in optical cross-connect, metropolitan area network (MAN) and data center applications.

The E62 is a transponder module for status monitoring, ingress switch remote control, RF power measurement and automatic alignment / ALSC. It is installed into the RIS module slot of Teleste E3 amplifier or E8 node.

In tropical regions, satellite communication links at Ku-band frequencies face excess up- and downlink paths loss due to rainfall. The uplink rain attenuation causes a decrease in the signal level received at the satellite receiver, which leads to a decrease of the satellite transmitter power. This signal level is further reduced by the downlink rain attenuation. This may cause the signal level to fall below the sensitivity threshold level of the ground receiver a for specific BER performance. Adaptive power control systems in the satellite transmitter and ground receiver can solve this problem. This article describes the design, development and characterization of a Ku-band channel amplifier with an automatic level control (ALC) system for spacecraft applications to control the input of the final power amplifier (TWTA or SSPA) according to the signal level arriving at the channel amplifier input. Thus, the ALC system also protects the final power amplifier against any accidental high power from the uplink.1 This channel amplifier can operate in an ALC mode as well as in a fixed gain mode (FGM). In the fixed gain mode, the gain is 44 dB and in the ALC mode, the gain varies automatically from 39 to 59 dB depending upon the input power level. This amplifier has an adjustable gain control (22 dB) system to operate the final power amplifier in different back-off conditions in both modes of operation.

The required gain of the amplifier is achieved by using three amplifier modules using PHEMT devices (CFY67-08). In the ALC mode, the channel amplifier operates as a closed loop feedback system.1 The amplifier contains a Schottky diode detector to detect the sampled RF power. This detected voltage is amplified by a differential DC amplifier and applied to the control input of a variable PIN diode attenuator. The attenuation of the attenuator varies according to the input power level so as to maintain the output power level of the channel amplifier constant. Another PIN diode attenuator is used for the adjustable step gain control.

The gain of the microwave amplifier modules, the attenuation of the PIN diode attenuators and the detected power level of the Schottky diode detector are all functions of temperature. Thus, suitable compensation circuits are required to compensate for the temperature variation of the channel amplifier performance over the specified temperature range for satellite transponders.

The schematic circuit diagram of the channel amplifier with the different temperature dependent control signals is shown in Figure 2. The temperature-controlled reference voltage VR1 is generated using a thermistor to compensate the gain in FGM operation. The temperature-controlled reference voltage VR2 is generated using P-N junction diodes to compensate the output power level variation in ALC mode operation.1 To achieve a temperature invariant and accurate step attenuation, an improved control circuit is used for the PIN diode attenuator.4

The amplifier modules A1, A2 and A3 are used to achieve the required channel amplifier gain. A1 and A2 are three-stage amplifiers and A3 is a two-stage amplifier. All the amplifier modules are made using PHEMT devices (CFY67-08) for the Ku-band downlink frequency of 11.45 to 11.70 GHz. The MIC assembly drawing of the three-stage amplifier is shown in Figure 3. All the matching networks are fabricated on a 25 mil thick alumina (Al2O3) substrate (?r = 9.9), 0.25" 0.5", and the required bias resistors are also accommodated on the alumina substrates in the RF tray. The two-stage amplifier A3 is realized with the same MIC cards used in the three-stage amplifier by eliminating one inter-stage matching network. These circuits were simulated and analyzed with the HP (EEsof) circuit simulator, series IV. To adjust the overall gain of the channel amplifier the resistors RD1, RD2 and RD3 are used in the +V1 supply lines of the amplifier A1, A2 and A3, respectively.

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