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  • Jay June 2015

Software Defined Radio

JayJay
edited May 2015 in RADAR Discussions
Software Defined Radio (SDR) is a relatively new technology available to the common enthusiast at decreasing cost over time. This discussion will attempt to show how low cost Hardware and free Software may be adapted to perform as RADAR Receivers.

Comments

  • These inexpensive TV Tuner Dongles that have recently become available below $20, are not appropriate for RADAR Receivers as is. However, it appears their hardware may be hacked to become a useful "Front end" Tuner for RADAR reception.
  • JayJay
    edited June 2015
    I successfully tied 2 Dongles together, one clock driving the other after carefully removing its crystal. The trick to a stable and reliable sharing of clocks from one dongle to the other is to add a very small RF capacitor to the source dongle's crystal output to capacitively couple its output to the crystal input of the other dongle. The capacitor transfers the clock signal without loading the source oscillator. However, it is very important to select the appropriate value of coupling capacitor, too large, will load the oscillator, too small the signal integrity to the slave tuner is to low. 
    In this image the capacitor is seen on the right dongle under the coax on the right crystal pin, on the left dongle the coax is connected to the left crystal pin (after the crystal is removed). The choice of coax is also important, as seen here, Teflon low C is the one to use. Stray capacitance from the wrong coax causes phase instability. Sub-picofarad stray capacitance causes phase shift. Also, the dongles should be close but not too close, and physically parallel, not opposite each other. The source clock coax should be grounded close to the crystal, both ends of the coax are grounded, do not leave the sink dongle's coax shield floating.
    image
  • JayJay
    edited June 2015
    In this image the 2 dongles are mounted in a water-tight box with f-connectors to the outside of the box. Not shown, this box is then mounted in a Faraday box to shield the dongles from unwanted EMI.
    image
  • JayJay
    edited June 2015
    If you do not know how Phase-Locked-Loops (PLL) work, very simply, they are complex non-linear circuits comprising a low pass filter (sometimes the filter is dynamic), a 2 input comparator, one or more programmable counters, and a Voltage Controlled/Numerically Controlled Oscillator (may be dynamically adaptable). The PLL is not a simple thing, mostly because it never works flawlessly requiring multiple engineering inputs from the environment to keep it under control and is the source of much experimentation over the past 50 years; is the consummate circuit and foundation technology of modern (Digital) Radios.The PLL is not something any novice can get right as a beginner. To forgo a tedious circuit description, the 1PPS from the GPS constellation is used to check the numerical accuracy of the Master PLL Counter timebase frequency once per second. If the count is too high or low, the Counter is programmed to adjust the frequency/phase of the oscillator, reducing the oscillator’s drift (but never perfectly). Multiple Oscillators of different frequencies can be synched to the 1PPS at the same time by sharing the local Oscillator in a Master SDR or Timebase. This is how SDR (RADAR and Radio Astronomy) works to sync to multiple timebases employing many receivers, transmitters; both or either. What is not usually mentioned is that these SDRs must be physically close together (less than 2 meters). This is why it takes a FPGA-controlled timebase in every unit to keep multiple timebases phase locked on a single 1PPS timebase, a Command and Control nightmare otherwise.
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