VCO help

Discussion in 'The Projects Forum' started by kahafeez, Feb 27, 2009.

  1. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
    150
    0
    Hi all. i made an FSK modulator using 74HC4046. i successfully made it. but the problem is that when u see at fsk signal, the amplitude also changes along with the frequency. i've the presentation on tuesday. but how will i jutsify this .... like y is the amplitude level changing too..... can u plz tell me the reason....
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    The outputs of the phasecomparators may not be connected to ground.
    So the pins 1 , 13 and 15 must be open.

    [​IMG]

    Greetings,
    Bertus
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your circuit is not an FSK two frequencies signal.
    It is a phase-locked-loop circuit which is different.
    FSK uses a DC voltage to change the frequency of the VCO into two frequencies.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Since there is no reference signal (SIG IN is grounded), PC1 will be at the same frequency as the VCO. I think this causes the duty cycle of the VCO to be other than 50%, because the control voltage changes at the same frequency as the VCO. The DC control voltage should cause a frequency shift, but I don't see the point of feeding back PC1.
     
  5. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
    150
    0
    @ Bertus

    not grounding the pin 1, 13 and 15pins give the result as in the image....
    i dont know waht is this and why is it occuring.... seems like noise to me.... i just tried grounding the unused pins and it worked. i got rid of this noise

    @ Audioguru

    i know its a PLL. i just used the VCO part of this IC to make an FSK signal.

    @ Ron H

    not feedbacking the PC1 results in nothing. the PLL is a closed loop system and has to be closed in order to get any output.


    does anyone has any idea about the VCO changing its amplitude.... i also tried a simple DC supply at the VCO in.... as i increased the voltage, the frequency of the VCO as well as its DC level, both changed.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    You apparently don't understand PLLs. Take out R3. The VCO will still operate. Feeding back the phase detector input when you have no SIG IN does nothing except change the duty cycle of your output.
    Regarding the shifting amplitude problem: You should be seeing a square wave at the VCO output. What is the bandwidth of your scope? What is the frequency range of your VCO output? Your scope bandwidth and/or probe capacitance, combined with the non-50% duty cycle caused by your "feedback", is probably causing the apparent amplitude shift.
     
  7. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    From the picture of the scope trace in your initial post, it appears that what is happening is not an amplitude change but a DC shift.

    hgmjr
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Look at his photos in the second post. The triangle waveforms are obviously a result of insufficient scope bandwidth, or severe capacitive loading. A frequency change will cause an amplitude change. If the duty cycle is not 50% (hard to tell), a frequency shift will also cause an apparent DC shift.
     
  9. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Ronh,

    I looked at the attached scope photo in kahafeez's second post and it appeared to me that it was the same signal as in the first post's attachment with the scope horizontal sweep rate adjusted to a timebase that obscured the lower frequency DC level modulation.

    Is that not what it is?

    hgmjr
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I think you're correct. That observation doesn't cause me to rethink my conclusions.:)
     
  11. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    It seems that a voltage-to-frequency converter would be a more straightforward means of implementing an FSK generator.

    hgmjr
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    He seems to be wanting frequencies in the Megahertz range. A VFC with a predictable transfer function may not be available in that range.
     
  13. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    That brings up a valid question. What two frequencies are being targeted for this FSK?

    hgmjr
     
  14. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
    150
    0
    actually i've made evrything.... like PCB and all.... adn i have my presentation on this tuesday..... and i just wanted to justify if someone asks me abt it..... thats why i posted.....


    i told u i tried alot of times without R3, the feedback thing..... and nothing worked.... then someone told me that even if u only want to use the VCO, u'll have to close loop it... then i did so and got an fsk signal...... i need an fsk signal as high as possible.... this is why i chose 74HC4046 instead of CD4046 as 74HC4046 has a VCO with max freq of 19MHz whereas the CD4046 has a max freq of 1.2MHz....

    my scope range is 20MHz..... and i just need to give a nice answer to my professor when he asks me why this DC shift is happeneing.... plz help.....
     
  15. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
    150
    0
    i've also done the demodulation..... its perfect..... i did serial communication between two Microcontrollers (89C52)..... the receiver realised the signal also when there was no mod and demod attached and also when they were attached which makes sure that my mod/ demod is perfect.... DC shift is the only answer i require....
     
  16. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
    150
    0
    i should have mentioned this earlier..... the picture in the first post..... the input there is a square wave generated through another IC 7414..... the duty cycle of the input wasnt 50%..... so the FSK signal also changed its duty cycle bcz of the input......

    the picture in the second post shows noise when unused pins were not grounded.....
     
    • srt.PNG
      srt.PNG
      File size:
      13.4 KB
      Views:
      22
  17. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I already told you my thinking. Maybe someone else can come up with another explanation.
     
  18. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
    150
    0
    @ Ron H

    Thanks dude..... :D ..... i hope i do well at the presentation....
     
  19. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I hope you do well also. Good Luck!:)
     
Loading...