FM Modulation

Discussion in 'Wireless & RF Design' started by PRS, Jun 20, 2014.

  1. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
    I've been trying to make an FM modulator, using reactance techniques and varactor diodes and the 566 VCO. Nothing has worked until today when I got two working 566 ICs in the mail.

    About a month ago I ordered a 566 from EBay and when I got it I set it up according to a book but it didn't work. Never having fussed with the chip before I didn't know if my book was at fault or the IC was bad. Today I discovered it was a bad IC by the fact that the book's implementation worked with the two ICs that just arrived in the mail.

    However, I'm not getting the frequency I'm supposed to be getting. The calculated fc is about 7 kHz but I'm getting 3.33 MHz with one of the new chips and 11.6 kHz with the other. Even so, I hooked up a low frequency signal to pin 5 and sure enough the fc moves back and forth on the scope just as one would expect. At 1 Hz I can watch it in slow motion.

    Does anyone have any idea why the actual frequency is different than the formula's prediction?
    Last edited: Jun 20, 2014
  2. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    Must be a parasitic oscillation as the chip typically won't run above 1mhz. Do you have a 1nf cap between pins 5&6 per the datasheet?
  3. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
    I have a .001 uF capacitor between pins 5 and 6. And as I said above, one of the 566s I just bought has a frequency of 11.6 kHz, which is closer to the expected frequency than the other 566 which has a frequency of 3.33 MHz. I'm wondering if this was a typical problem with the 566. Something inside that chip is screwy.

    I'm wondering if the modern chips made in China are reliable. And I will say this: both of my new chips do in fact create FM, albeit at different frequencies from each other and from the theoretically calculable rate.

    I think this mystery has a solution that lies in the funky procedures of Chinese fabrication technique. I have a vice in my shop with the Made in China logo upside down. I have name brand pants now made in China that don't fit like they used to.

    I will photograph my circuit design tomorrow.
    Last edited: Jun 20, 2014
  4. dougc314


    Dec 20, 2013
    What varactor diode are you using? The National Semiconductor data sheet shows that the typical cap value for 10 KHz to be 5000nF, pretty big compared to most varactor diodes. Also I bet the the varactor needs to be DC isolated from the 566, could you post your circuit? Using a small cap and a big R might cause problems, I've never used this part, so I can't say for certain. Are you using a plugin breadboard, or hard wiring your circuit? Oscillator's are one circuit that require extra care in construction practice when developing.