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Block
Diagram Shown below is a block diagram of the components that make up the Pico Beacon system. The PIC Microcontroller is the heart of the system and controls all onboard devices. A 19.2MHz TCXO provide a clock to the PIC controller and DDS. The GPS power is controlled with an LDO switch. GPS information is transfer through a UART operating at 9600 baud. The 1M x 8 flash memory and AD9954 are controlled through an SPI interface. The temperature is read through an I2C interface. The DDS clock frequency is programmed to 364.8MHz (TCXO 19.2 MHz0 * 19). The DDS is a sampling system. So it not only generates the main tone, but also images that are related to the clock frequency. For example, if the output is programmed for 80MHz, you'll get the main tone at 80MHz. But you'll also get tones at Clock Frequency +/- 80MHz. So there is a tone at 284.8MHz (364.8 - 80) and 444.8 MHz (364.8 + 80). The amplitude rolls off pretty quickly and follows sin(x)/x curve. So the image tones will be 13dB lower. Note with most DDS designs, there is always a low pass filter on the output to get rid of the higher images. The trick in the Pico Beacon is to band pass filter the second image tone and use that signal as the main carrier. Two band pass filters and amplifiers that follow the DDS are setup for 410-460 MHz. As with any filter, there is roll off towards the edges, so 420-450 is the usable range. With the DDS, the main tone is at -6dBm and the second image tone is at -20dBm.  Pico Beacon Block Diagram. |
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