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Building a device for RF transmission/detection

  1. Apr 18, 2014 #1
    Hi Physics Forums,

    I'm a biochemist whose electronics experience is limited to resonant RLC circuits. I have been tasked with building a device that essentially acts as an RF antenna. In transmit mode, this device must create a sine wave of specific frequency (current plan is 40.05 MHz), convert it to analog, and transmit it to a resonant RLC circuit with sub-ms timing resolution. The device must then use the RLC circuit to listen to an exponentially-decaying sinusoid response signal of similar frequency, amplify the signal, convert to digital, split the signal into quadrature phase waveforms, and output the data to a computer.

    So far, I have determined that I will need a DDS to create the initial waveform, DAC to transmit to the RLC circuit, LNA, ADC, quadrature circuit (directional coupler, mixers connected to DDS) to separate the phases, and I/O board to a computer.

    Question #1: Given that the device should operate using a preprogrammed sequence so that users can just push 'start' to use, which controller would be most appropriate for the device? I have seen similar devices use FPGAs, but would a microcontroller be easier to use? Do you have a favorite brand or product that you would recommend?

    Question #2: As I look for commercial products of the above components, what compatibility issues should I keep in mind? I'm not entirely familiar with the ramifications of choosing particular clock frequencies, operating voltages, or memory.

    Question #3: Which tutorials/references would you recommend as being particularly useful in this application? Google has been super helpful with generally understanding the components alone, but I would really like to learn how they behave when connected.

    Thank you very much for your help!
  2. jcsd
  3. Apr 18, 2014 #2


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    For the receiver you might want to look at USB dongle-type receivers, the Rafael Micro R820T chip can output a raw I/Q data digital data to a computer over the 24 - 1766 MHz range. GNU Radio can be used (or a custom program with the GNU Radio libraries) to decode almost any RF signal.


    Receiver application "gqrx" output from a USB dongle "rtl-sdr" at 27mhz.
  4. Apr 18, 2014 #3


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    Science Advisor

    This sounds like a very complex project for someone without a lot of experience.

    Can you repurpose a commercial radio? A lot of RC toys operate at 40 MHz so you might be able to buy a device to do the heavy lifting for you.

    Maybe you can start along these lines: http://www.nemhobby.com/xs-6-pro-sport-syn-40-mhz-p31858.html
  5. Apr 18, 2014 #4


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    Staff: Mentor

    Can you say more about the application? Is it for an NMR/MRI device?

    Will the EM energy be contained within a shielded room or lab? If not, is 40.05MHz a licensed band? The FCC (for the U.S.) has regulations that govern what energy gets launched in the RF spectrum -- it's very important to understant and abide by those rules, since it's a very bad thing to create unwanted interference with others who are correctly using the RF bands.
  6. Apr 18, 2014 #5
    @nsaspook Thanks for introducing me to the RTL-SDR. That's a really cool product. Here's an idea for signal throughput in 'detect mode':

    RF damped sine wive inductively detected with loop -> LNA -> fixed-frequency transmitter (crystal oscillator?) -> RTL-SDR -> computer

    @analogdesign I agree that this will take a lot of work, but my deadline is rather relaxed (about 1.5 years to protoype), and I am very motivated to learn more about RF electronics. Repurposing other devices is an optional, but I want to try piecing it together myself first.

    @berkeman Yes! I'm so happy that my description was at least accurate enough for someone to guess that it is an NMR spectrometer. My project is to build a benchtop spectrometer for performance of biochemical assays on low-volume, time-sensitive samples. The design emphasizes convenience and time resolution, but some sensitivity issues are to be expected with low-field NMR.

    Edit: And while it will be benchtop operated, I do plan to have the device properly shielded.
    Last edited: Apr 18, 2014
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