REMOTE WIRELESS SYSTEM USING AMPLITUDE SHIFT KEYING MODULATION 1
Syahrim Azhan bin Ibrahim, 2Nurliza binti Salim, 3Hafizah binti Mustapha.
1,2,3
Operations and Space System Division, Malaysia Space Centre, Banting, Selangor, Malaysia e-mail:
[email protected],
[email protected],
[email protected]
Abstract. CubeSats are normally piggybacked onto a large rocket platform and deployed in numbers. They will be orbiting in close proximity with each other and it might take some time to match the North American Aerospace Defense Command (NORAD) radar observations to individual satellites. The on-board transceiver can be used to transmit signal once a CubeSat is deployed but it consumes much power and therefore is not suitable to transmit power in long period. Therefore a beacon signal transmitter with RF output power of 100mW, hence using much lower power consumption than the transceiver is normally used to characterize individual CubeSat. This paper presents design and development of a remote wireless system using Amplitude Shift Keying (ASK), digital amplitude modulation of 433.92MHz carrier frequency, which might be used as a base in designing the low power beacon transmitter. Keywords: Amplitude Shift Keying, beacon transmitter
1. INTRODUCTION The space communication system mounted on a CubeSat composed primarily of a transceiver, a terminal node controller (TNC) and antennas for telemetry and command systems. Whilst our main interest may be science data from the CubeSat, we must have a beacon signal otherwise we will be unable to locate our satellite and characterize the orbit. Unlike ISIS and Alinco made transceivers to name a few, the Microhard’s MHX425 that is chosen as our transceiver does not have its own beacon transmitter. Using the transceiver to characterize our CubeSat will consume much power, hence it is not suitable to transmit signal in long period. Therefore a beacon signal transmitter with low power consumption is normally used. A beacon transmitter typically uses on-off keying modulation to transmit Morse code over radio frequencies (referred to as CW (continuous wave) operation). In general this type of modulation is less prone to interference since it is less used around our environment. For this reason a low power signal is generally adequate for communicating. 1.1 Amplitude Shift Keying Modulation ASK modulation is a form of modulation that represents digital data as variations in the amplitude of a carrier wave. The amplitude of an analog carrier signal varies in accordance with the bit stream (modulating signal), keeping frequency and phase constant. The level of amplitude can be used to represent binary logic 0s and 1s. We can think of a carrier signal as an ON or OFF switch. In the modulated signal, logic 0 is represented by the absence of a carrier, thus giving OFF/ON keying
operation and hence the name given. Both ASK modulation and demodulation processes are relatively inexpensive. The ASK technique is also commonly used to transmit digital data over optical fiber. Figure 1 shows ASK modulation.
Figure 1: Amplitude Shift Keying Modulation
1.2 Beacon Transmitter Beacon transmitter is normally used to transmit ID call-sign of a CubeSat and telemetry data like battery status and temperature of the satellite. The amount of data is small in comparison to the transceiver data rate transmission. Therefore we can afford to use CW operation. The block diagram of the beacon transmitter design is shown in Figure 2.
Figure 2: Block Diagram of a Beacon Transmitter
The telemetry data from various subsystem will be converted into digital form by the microcontroller (Mcu) IC. Mcu will arrange the data according to Morse code arrangement, attach the data together with identification number and send them to ASK Modulator and Transmitter IC. This IC will modulate the data using ASK modulation system and transmit them to RF amplifier IC. The RF amplifier will subsequently amplify the data into a level suitable for transmission. By the way this paper only cover the ASK modulator and transmitter as well as RF amplifier design.
2. DESIGN GOAL The aim of this transmitter design is to achieve the following: 1. 2.
Mode of communication: Simplex; Digital modulation technique: Amplitude Shift Keying (ASK); 3. Carrier frequency: UHF 433.92MHz and 4. Transmitter power output: 100mW (i.e. 20dBm). Simplex communication refers to communication that occurs in one direction only. In our case it is a transmitter on the CubeSat and receiver at Earth station. Digital modulation technique will be achieved by the use of an UHF ASK transmitter IC made by Micrel Semiconductor and will be explained in detailed later. UHF falls into allocated radio amateur band. The main reason to have an UHF transmitter on board of a satellite is that with a limited power and size of the antenna, the downlink signal from Low Earth Orbit range (i.e. 600-700 km) where the CubeSat is aimed to orbit, capable to reach Earth station. Smaller antennas also guarantee less mishap on antenna deployment. Additionally, for the ground station, receiving on UHF avoids the higher noise levels and amateur activity on the 2m band (i.e. VHF). As mentioned, amount of data to be transmitted is small and for this reason a transmitter power output of 100mW is targeted. The value is also based on survey from detailed satellite information which can be obtained from the radio satellite amateur corporation (AMSAT) website.
3. PARTS DESCRIPTION The main ICs used in this design are commercial of the shelf (COTS) products. There are ASK modulator cum transmitter IC; MICRF112 of Micrel Semiconductor and a broadband high linearity
amplifier; MMG3003NT1 Semiconductor.
of
Freescale
3.1 MICRF112 MICRF112 is a single chip ASK Transmitter IC for remote wireless applications in the 300 to 450MHz frequency band. It is chosen because of its high performance in three areas: power delivery, operating voltage, and operating temperature. In terms of power, it is capable of delivering +10dBm into a 50Ω load with operating voltage from 1.8V to 3.6V. Hence it is tailored to CubeSat’s power supply of 3.3V and 5V. Most transmitter modules available in the market require much higher operating voltage (i.e. up to 15V) to deliver such power. In terms of operating temperature the IC operates from -40°C to +125°C. This wide operating temperature range makes MICRF112 an ideal candidate for the demanding space application. The functional block diagram of MICRF112 is shown in Figure 3.
Figure 3: Functional Block Diagram MICRF112
The MICRF112 can be best described as a phase lock loop transmitter. For ASK application, the system can be partitioned into five functional blocks; crystal oscillator, PLLx32, power amplifier, enable control and under voltage detect. 3.1.1 Crystal Oscillator The reference oscillator is crystal-based Pierce configuration. It is designed to accept crystals with frequency from 9.735MHz to 14.0625MHz. The circuit can be implemented using a minimum of components: The low manufacturing cost of this circuit, combined with the outstanding frequency stability of the quartz crystal, give it an advantage over other designs in many consumer electronics. 3.1.2 PLL x32 The function of PLLx32 is to provide a stable carrier frequency for transmission. It multiplies the frequency from crystal oscillator and modulate the data input from ASK pin 3.1.3 Power Amplifier The power amplifier serves two purposes: 1) to buffer the Voltage Controlled Oscillator (VCO) from
external elements and 2) to amplify the phase locked signal. The power amplifier can produce +10dBm at 3V (typical).
4. DESIGN DESCRIPTION 4.1 Schematic Design
3.1.4 Enable Control Enable control gates the ASK data. It only allows transmission when Lock, Amplitude and Under Voltage Detect conditions are valid. 3.1.5 Under Voltage Detect "Under voltage detect" block senses operating voltage. If the operating voltage falls below 1.6V, "under voltage detect" block will send a signal to "enable control" block to disable the Power Amplifier. 3.2 MMG3003NT1 The MMG3003NT1 is a General Purpose Amplifier
that is internally input matched and internally output pre-matched which is often necessary to provide the maximum possible transfer of power between a source and its load. According to the manufacturer, MMG3003NT1 is designed for a broad range of Class A, small signal, high linearity, general purpose applications and suitable for application with frequencies from 40 to 3600 MHz. Other feature worth mentioning is its typical small-signal gain of 20dB at frequency 900MHz. Moreover its 1 dB gain compression measurement (P1dB) is 24dBm at 900MHz. An amplifier maintains a constant gain for low-level input signals. However, at higher input levels, the amplifier goes into saturation and its gain decreases. The P1dB indicates the power level that causes the gain to drop by 1 dB from its small signal value.
Figure 5: Schematic Diagram for MICRF112
The MICRF112 only needs a few additional external parts to create a complete versatile transmitter. It is well suited to drive a 50 ohms source, monopole or a loop antenna. Schematic and component values shown in Figure 5 above are based on recommendation from the manufacturer. Capacitor C3, capacitor C4 and inductor L1 are impedance matching circuit which is often necessary in the design of RF circuitry to provide the maximum possible transfer of power between a source and its load. Part of the matching network is to attenuate the second and third harmonics. When matching to a transmit frequency, care must be taken not only to optimize for maximum output power but to attenuate unwanted harmonics.
Figure 6: Schematic Diagram for MMG3003NT1
Figure 4: Functional Block Diagram MMG3003NT1
Schematic diagram for MMG3003NT1 shown in Figure 6 is a 50 Ohm application circuit for frequency range 40-800MHz recommended by the manufacturer. All Z parts above are microstrip transmission line used to form impedances. Output from the MICRF112 will be the input of MMG3003NT1. 4.2 Printed Circuit Board (PCB) Design
With all of the features above, we expect that it can amplify the RF output from MIRF112 to the 20dBm level required. Figure 4 shows functional block diagram of MMG3003NT1.
The PCB is used to mechanically support and electrically connect electronic components using conductive pathways, or traces, that are etched from
copper sheets laminated onto a nonconductive substrate. In this work, PCB etching is done manually on a copper clad laminate FR-4 material. Therefore there is a slight difficulty in placing the components closed to each other. Figure 7 shows PCB for MICRF112 circuitry while Figure 8 shows PCB for MMG3003NT1.
by the manufacturer to minimize Return Loss (RL) at both input and output stages.
5. RESULTS AND DISCUSSION Figure 9 shows ASK Transmitter Test Setup. 12V rechargeable battery is used as the main power supply. A voltage regulator circuit converts battery voltage into 3.3V for used by MICRF112 circuit. The RF amplifier, MMG3003NT1 obtains voltage supply direct from the battery. Output from the RF amplifier is led to a spectrum analyzer.
Figure 7: PCB Diagram for MICRF112
Figure 9: ASK Transmitter Test Setup
Figure 8: PCB Diagram for MMG3003NT1
PCB Layout is of primary concern to achieve optimum performance and consistent manufacturing results. Care must be used on orientation of components to ensure they do not couple or decouple the RF signal. In particular, inductors should not be placed in parallel to each other as mutual inductance will occur and change their respective values. PCB trace length should be short to minimize parasitic inductance. For example, depending on inductance values, a 0.5 inch trace can change the inductance by as much as 10%. To reduce parasitic inductance, the use of wide trace and a ground plane under signal traces is recommended. Vias with low value inductance should be used for components requiring a connection-to-ground. In both PCBs above, ground planes can be found on the other side of the circuit traces to ensure stability of the power output from each circuit. In addition components with leads must be avoided especially for matching circuit. This is because the leads will have inductance that can throw your matching way out. Therefore all parts for the circuits use Surface Mount Technology (SMT) inductors and capacitors. For MMG3003NT1, combination of schematic and PCB design above are recommended
Figure 10 shows output power level of 5.256dBm from MICRF112 circuit for continuous 3.3V input. It is 4.744dBm less than the desired 10dBm level. Cable loss and connector loss contributed about 1dB. The other losses might be caused by return loss affected by the inefficient matching circuit, parts positioning or the PCB itself.
Figure 10: Output power level from MICRF112 for continues 3.3V input.
Although the output from MICRF112 is lower than the targeted value, MMG3003NT1 should counter this since as mentioned before the IC
has a typical gain of 20dB. Figure 11 shows RF amplifier’s output power level of 20dBm for continuous 3.3V input at MICRF112.
Meanwhile Figure 12 shows modulated and amplified output signal of the circuit when serial UART at 1200 bit per second (bps) is sent to the input of MICRF112. Figure 13 is the actual form of the data after amplified and modulated. There is about 48dB different between high and low input levels. However further checking on higher frequency spectrum reveals obvious second and third harmonics from the overall circuit as shown in Figure 14. There is about 18dB different between fundamental frequency and the second harmonic. Previous checking on MICRF112 shows very low level of harmonics. Therefore the harmonics definitely come up at the RF amplifier circuit; MMG3003NT1.
Figure 11: Output power level from MMG3003NT1 for continues 3.3V input at MICRF112.
Figure 14: RF Spectrum second and third harmonics from the circuit. Fundamental is at 433.92MHz.
6. CONCLUSION Figure 12: Output power level from MMG3003NT1 for 1200bps input data at MICRF112.
The combination circuits duly achieved RF output of 20dBm or 100mW. However there are issues that need to be look at especially at the second and third harmonic levels. Additional filter may eliminate them but may come at a cost of reducing output level. Else a study needs to be conducted on how the harmonics may affect the beacon transmitter system itself and other subsystems when all are integrated in the CubeSat. Next is to do further checking on power consumption to measure efficiency of the circuit. A comparison study is required with the main transceiver while it transmits 100mW RF output. A lower power consumption should be obtained otherwise it defeats the purpose of having the beacon transmitter. Additional testing on overall performance of the circuit is also obligatory when combined with Microcontroller such as on receiving quality, power level and harmonic noise.
Figure 13: 1200bps amplification.
data
after
modulation
and
Circuit wise, the RF amplifier MMG3003NT1 requires input voltage of around 79V for the desired output of 20dBm. Therefore it is desirable to find an RF amplifier IC which can operate at 5V input for comparable output power to MMG3003NT1. There should be reduction in power consumption as well as avoiding usage of unregulated power supply from Electronic Power System of the CubeSat. A reduction of size for overall circuit is also necessary to increase efficiency of power transfer as well as allowing ease of installation at later stage when the CubeSat begins assembly. 7. REFERENCES Bowick, C., Blyler J. and Ajluni, C. 2008. RF Circuit Design 2nc ed. Newnes Press, Oxford. Micrel Semiconductor, Inc. 2008. MICRF112 Datasheet. Freescale Semiconductor, Inc. 2008. MMMG3003NT1 Datasheet Rev 7. http://en.wikipedia.org/wiki/Amplitude-shift_keying. National Instruments Corporation Tutorial. http://zone.ni.com/devzone/cda/tut/p/id/2952.
2010
