FM Transmitter IC with ultrasonic and sleep mode?

Discussion in 'Wireless & RF Design' started by Zazoo, Aug 26, 2011.

  1. Zazoo

    Thread Starter Member

    Jul 27, 2011
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    Hello, I'm hoping someone with more experience than me (i.e. everyone) can offer some advice.

    A project I'm working on requires the wireless transmission of ultrasonic sensor data (around 40kHz) over a very short distance (< 3 meters).
    We are currently looking to utilize commercially available FM TX/RX IC's to implement this function. As I'm very new to wireless/RF I've been having trouble wading through the wealth of IC offerings on sites like DigiKey.

    Transmission only needs to occur when the sensors register an event in the 40kHz range, so it would be ideal if the FM IC featured some form of sleep/wake function (to minimize power consumption as these devices will be battery operated.)

    Does anyone know off-hand of a manufacturer who makes a chip with these features (i.e. accepts ultrasonic modulating frequencies and has a sleep function)?

    Thanks
     
  2. radiohead

    Active Member

    May 28, 2009
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  3. upand_at_them

    Active Member

    May 15, 2010
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    The XBee suggestion is premature. What format is the sensor event in? Is it data conforming to some serial protocol? Or is it something like a PWM pulse?

    If the sensor event is the former, then XBee could work. If the latter, however, you'll need a custom device such as a microcontroller that can read the event and then pass it to the wireless device, which could still be an XBee, or a $5 wireless device from SparkFun (4800baud).
     
  4. Zazoo

    Thread Starter Member

    Jul 27, 2011
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    The sensor is a MEMS vibration sensor developed in house. The output from the sensor is just a raw differential comb capacitance (160fF range). This is currently attached to an Analog Devices AD7747 capacitance-to-digital IC (24-bit I2C output), but this is only sufficient for low-frequency testing.

    We looked at XBee, the issue here was bandwidth (we need the 40kHz signal to be transmitted to the terminal for processing.)

    We were hoping to couple the variable comb capacitance directly to the FM circuit, and do the A/D conversion on the terminal end.

    This may be a bit more ambitious than we thought though, given our limited experience with FM.
     
  5. upand_at_them

    Active Member

    May 15, 2010
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    The XBee is capable of the data rate you need, it goes up to 250kbps, I think. The problem is that you need to convert the sensor value and hand it over to the XBee rather quickly, and it looks like the AD7747 is not up to that task. So you either need a faster cap-to-I2C device or a custom device.

    You might be able to couple the cap directly to an FM circuit - it would be part of the transmitter circuit - but again, that would be another custom device.

    Or, instead of coupling it, you could make the cap part of a resonant circuit, which some device then reads the instantaneous frequency of and transmits it (possibly through an XBee).
     
    Last edited: Sep 3, 2011
  6. Zazoo

    Thread Starter Member

    Jul 27, 2011
    114
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    Assuming we sample at 80kHz that only gives us 3-bits to encode the data samples for real-time transmission without data loss, right?
     
  7. upand_at_them

    Active Member

    May 15, 2010
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    Okay, I've been confused a bit in this thread. But, if you have a 24-bit value to send 80,000 times per second...that's 1,920,000 bits per second. So I think we've finally concluded that you can't use XBee at all. It looks like you need someone to build you a custom solution.
     
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