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SN74LS628 VCO : No oscillations

Benjamin Deforge
Intellectual 270 points
Other Parts Discussed in Thread: SN74LS628, TRF7960

Hi, 

I'm currently trying to generate a clock signal for a TRF7960 RFID front-end, using a SN74LS628 VCO, but i can't get any oscillations.

My configuration is the following : a 600R resistor between RX1 and RX2 and a 47pF between CX1 and CX2. The enable input is directly connected to the ground. The RNG freq input is low, and the frequency control input is driven by a Digital to Analog converter, and is usually around 2V - 2.5V.

I can't get any oscillations on Y nor Z output, the only thing I get is a 5V on Z and 0V on Y... I was wondering if the output of the VCO should be loaded with an external capacitor, I tried with a 47pF one but I didn't get better results. 

 

Can anybody help me ?

 

Thanks in advance, 

  • 0
    TI__Mastermind 46480 points

    Hello Benjamin

    I have looked over your set up and I dont see anything wrong with it. For troubleshooting purposes I woud tie both Vcc.s together and both Gnds together. I would also tie frequency control to VCC. This would eliminate any powe loop or gnd loop issues. see if it will oscilate like this. See if it oscilates. If it does not then there are connection problems or the part is not functioning correctly. 

  • 0 in reply to Chris Cockrill
    Intellectual 270 points

    Hi, 

     

    I have just tried to tie frequency control input to Vcc and still no oscillations. My VCO is supplied with 4.9V but I don't think the problem is here because the datasheet specifies a VCCrange from 4.75V to 5.25V. Speeking of, I have seen that the typical characteristics shown in the datasheet are obtained with a "CL = 45pF".

    Is this CL an external capacitor that should be placed between Z/Y output and ground ?

    I have checked the connections on my PCB and everything seems fine, no power loops nor ground loops appeared to me ...

     

    Thank you for your answer, 

  • 0 in reply to Benjamin Deforge
    Intellectual 270 points

    Maybe I could be provided with an updated datasheet (the one I found is from 1988...), or application notes ...

  • 0 in reply to Benjamin Deforge
    Prodigy 10 points

    Hi, 

    I was able to connect this IC as listed above with the VCC's tied together and the Gnd's tied together.  I also set the NC pin to ground, EN to ground, and placed the RNG pin to ground.  I used a 50pf capacitor across the CX pins and a 1K variable resistor across the RX with a range from 400 to 700 ohms.  I was able to get 15MHz to 20MHz output from the Z pin.  I also changed the RNG pin to 3.3v and was able to get around an 8MHz to 10MHz output as well.

    Thanks for the connection setup reference.