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Telemetering Beacon For VUSAT
Prof. T.K. Mani (VU2ITI)

 

The beacon controller is intended to be operated for telemetering data available on board of the VUSAT in Morse code so that the ham radio operator can decode the info without any costly and complex equipment. The design strictly follows the KISS concept. It uses low cost, easily available components through out the design. A block diagram of the beacon controller is as shown:

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Figure 1. Block diagram of the beacon controller

The controller can accept 16 single ended data channels of which the dc level represents the parameter to be telemetered. These channels are selected one by one (multiplexed) by using 2 number of CD 4051 (analog MUX). Voltage from selected channel is given to the ADC chip for converting this voltage to its digital equivalent. A channel counter (4-bit binary counter) controls the channel selection. This counter counts every clock pulse given to it and thus gives a 4 bit binary output. This binary is given to the analog mux as address of the channel to be selected. For every clock pulse to the counter, successive channels are selected. The channel select clock is derived from the end of message signal (EOM), which will be explained later. ADC chip used is AD7574. This chip is an eight bit Successive Approximation type ADC made by 'Analog Devices'. This ADC starts converting the analogue voltage input when the RD! signal goes low and the digital value is available at the output pins approximately 15 microseconds later. RD! signal is derived from the "EOM" signal available from the EPROM. Eight-bit Binary output from ADC is given as the address of 8 most significant bits of the EPROM through two nos. of quad 2 input multiplexer chips. This two chips form an 8 bit 2 input multiplexer, one input is being the 8 bit binary from ADC and the other being the channel address (4 LSBs the upper nibble are made LOW). This digital MUX selects either the value of the input channel (binary from ADC) or the channel number (4 bit address from channel counter). The selection is also controlled by the control logic that derives the control from the EOM signal from EPROM. EPROM with 8-bit output stores the Morse code data in look up table form. Only one bit output is used to generate the Morse code and one more bit is used to generate EOM. The memory cell inside the EPROM is divided into two tables. The first table is the "Channel value " table that has 256 rows. Each row represents a message in Morse coded with binary bits. Each row is addressed by using 8 bit binary numbers. A key up condition is coded as '0' and key down as '1'. One can encode Morse code like this and a dash can now be represented as 1110 and a dot can represent as 10 where as a letter space as 000. For example the first entry in this table represents the number "zero zero zero" and to be coded as "111011101110111011100"

Table 1. Coding scheme for Morse equivalent of digits

Similarly table 2 contains a 16 rows, each row represents the channel number/ name encoded with bits as explained above. If we want to transmit the Morse code for an input channel number/name, then table 2 is selected and the channel address from 7493 is given as the address for table 2 to select the wanted row of messages. Now the string of bits from this row is to be given out one by one sequentially. The counter IC CD4060 does this. This IC contains a multistage counter and an integrated oscillator. This counter is used as a 6 bit binary counter (counts from 0 to 63). The oscillator time period is selected is equal to the dot period (about 120 HZ). So when this counter is enabled, it starts from its reset state of 000000 and counts upwards until it is reset. This Morse counter output form the least 6 bits of address to the EPROM. Seventh address bit of the EPROM is used to select the either the table 1 (when this bit is zero) or the table 2 (this bit is 1). The EPROM memory is thus segmented to form two tables with 256 entries in the first and 16 entries in the second as shown below:

Table 2. Memory organisation for the EPROM

EOM: An end of message (EOM) is available from the D1 bit of EPROM. The EPROM Bit D1 is programmed in such a way that this bit goes 'HIGH' at the next clock of Morse counter after the end of every message. This bit is used to read the ADC and to select the next message to be played. Circuit Operation: The Power on reset circuit resets the Channel counter and the Morse counter upon power on. IC No U13A is a monostable multivibrator generates a pulse to reset IC 7493 and IC 4060 at the time of power on. Mono stable chip U13B also resets the Morse counter for few milliseconds so that the Morse counter now starts from '000000'. The JK flip-flop (U2A) wired as a toggle F/F will also be initialized upon power up. The function of this F/F is to alternately select the Channel and its value to address the Morse through the8 bit multiplexer circuit (U9 & U10). The message bits in Morse corresponding to the channel selected will be send out bit by bit for every increase in counter IC 4060. This will be available at the D0 pin of EPROM. An EOM generated at the end of message will then toggles the F/F and thus selecting the ADC output as address of the next message to be played. The flip-flop operation also triggers the conversion of ADC channel that was selected by the channel counter. EOM causes to trigger the monostable U13Band it in turn resets the Morse counter for a period of approximately equal to the word space. This action introduces a space of about one word space before sending the next message. After this word space the Morse counter restarts and sends out the Channel value read by the ADC. Again at the end of message, the channel counter gets a pulse advancing the channel counter to give address to select the next channel Data (voltage) for conversion. Analog multiplexing of the 16 such channels are achieved with the use of Analog mux chips U4 and U6. The channel increment pulse also resets the Morse for a period of about 4-word space duration with the aid of another monostable circuit using IC U5B. The process repeats in cyclic mode enabling to send all the channels one by one until the stop button is pressed. Message format is in the following form:

<channel name><word space> <channel value><channel name> < word space> < channel value > <4 world space> < next channel name > <world space > < channel value > <next channel name><word space> <channel value > < 4 word space >.. and so on.

The circuit was wired and tested and is working fine with excellent Morse quality. The speed set is around 10 words per minute. It is possible to modify the Morse table so that we can change the channel name according to the actual parameter name to be telemetered. It is proposed by the AMSAT organizers that an identifying Morse signal must be added and the same can by easily incorporated by sacrificing the first channel. Any one who knows Morse code can easily decode and understand the parameters easily without the use of extensive equipment. Modulator: It is also proposed to use a special modulator so that the Morse signal can be copied using an FM receiver as well as SSB receiver. Circuit diagram of such a modulator is as shown in the figure2

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Figure 2. Modulator circuit

The 555 timer IC is configured as an astable oscillator giving square wave of 800 Hz. Using the frequency control pot meter P3 can vary this frequency. This oscillator is keyed by the digital signal available from the EPROM (Morse Out). The square wave output is given to an audio coupling transformer and output is taken through a level control pot meter P1. Suitable wave shaping of the square wave to achieve good tonal quality is also done by the transformer with the aid of C2 and C1. When the Morse Out goes HIGH, the 555 oscillator is enabled and it generates the audio tone. At the same time a potential divider arrangement using P2 and R2 applies a DC voltage derived from the logic high level is applied to the modulator. P2 is adjusted in such a way that the frequency shift due to this DC level applied across the modulating vari-cap diode is around 800 HZ. Now, if an SSB receiver is tuned to zero beat with the carrier, the keying causes frequency shift of 800 Hz and a beat note is heard in the SSB receiver. So the message is readable in an SSB receiver. Where as the tone voltage presented to the vari-cap also causes frequency modulation and the same can be demodulated using an FM receiver. A prototype of the beacon controller was homebrewed and tested OK with excellent performance. Carrier generation and transmitter will be done by VU2POP and then only we could test the modulator and transmitter circuit.

prototype

Figure 3. Prototype homebrewed by VU3WIH

I was able to finish the project only because of the help and encouragement given to me by the AMSAT-INDIA organizers, especially by OM Nagesh, VU2NUD. I am also indebted to OB Vinay, VU3WIH who helped me in assembling and testing the circuit.

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Fig 4. Circuit Diagram of Beacon

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Fig 5. Circuit Diagram of Beacon using PIC

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