App note: VCO enables a hands-free car kit for cell phones

Posted on Sunday, December 1st, 2013 in app notes by DP


Locally broadcast your cell phone through FM band using MAX2606 from Maxim. App note can be found here.

This design idea presents an integrated IF voltage-controlled oscillator (VCO) that can retransmit the audio signal from a cell phone to the FM broadcast band. By placing the cell phone’s speaker near the microphone, the user can use the phone as a hands-free device while driving.

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4 Responses to “App note: VCO enables a hands-free car kit for cell phones”

  1. ee says:

    That’s about one of the most contrived use-cases I’ve seen. The IC is a VCO, i.e. a building block, not a transmitter. It’s a hack. Sure it will work, but so will a spoon when you really need a fork. They must have really been searching the bottom of the barrel when they wrote this application note – apparently by their director of customer applications! What customer would apply it in this manner I wonder.

  2. Drone says:

    I found this little app-note for this VCO part interesting. Have fun with this part. I think that’s what the Maxim app-note implies…

    Personally I would dump the microphone and directly plug the VCO into the phone’s audio jack. The app-note mentions this option but with little detail – not too hard to figure out though.

    Unlike many Maxim parts, this one is not exactly “unobtanium” nor outrageously priced; check Octopart etc.

    I’ve looked at this series of VCO parts before. Know what you’re dealing with in terms of tunable range, stability and (especially) phase noise.

    Here’s a link to a Maxim app-note about the phase noise performance of this series of VCO parts:

    MAX2605/MAX2606/MAX2608/MAX2609 VCO Phase Noise Measurements

    Also pay attention to layout. Maxim has some eval boards (very high price alert), but the board info is useful if you’re rolling your own.

    There are lots of cheap phone audio to FM retransmitters out there. Most are step-locked and (if you are lucky) crystal controlled. So your typical car radio with synth-stepped FM broadcast frequencies may not work well in various countries due to band-plan diffs. And then there’s the instability and inability to find-tune these synthesized cheap phone to FM retransmitters. The Maxim solution opens the door to manually tuning so you’re spot-on in terms of frequency (within stability limits of-course).

    Also note, this app-note describes a monaural solution, so you’ll have to bridge the left-right audio channels with the obvious trade-offs (for music in-particular). But to broadcast the likes of voice (e.g. podcasts/netcasts and maybe audio books), it should be adequate.

    My two-cents worth…

    Regards, Drone

    • vimark says:

      Wow, thanks Drone for the additional info regarding phase noise performance for this devices

      • Drone says:

        @vimark, my pleasure…

        One more note regarding this app-note. There is no mention of FM audio pre-emphasis and I’m not sure if the passive components included in the audio input provide the requisite 50us or 75us (depending where you live, I think the U.S. is 75us) time-constant pre-emphasis. Simple pre-emphasis is achieved with an RC circuit; a 1st order (20dB/decade) high-pass filter with time constant of 75us or 50us. The pre-emphasis network’s series input capacitor does double duty as a DC block. There is plenty more information on this on the Web.

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