74HC4046 - Setting VCO frequency

Alvin P. Schmitt

I am having difficulty setting the VCO frequency for the 74HC4046. I want to set its frequency by a varactor diode in the C part of the circuit. The main capacitor is a capacitive sensor that is 63.55 pf. I want to add to it a varactor diode whose capacitance is controlled by phase detector. For my first test I have my capacitive sensor as C It is stable and set to 63.55 pf. R2 (pin 12) is infinite (open) and R1 (pin 11) is a pot of 5k to zero. VCOIN (pin 9) is grounded. I want to set the VCO to 13.56 MHz. It adjusts up to 12 MHz but then jumps sometimes to 17 MHz and also 24 MHz. Maybe I should make my own VCO but I don't have the circuit for the VCO in the 4046. Any ideas you have would be greatly appreciated. Thank you. Alvin...

over 9 years ago

0 ShreyasRao over 9 years ago

TI__Guru 61950 points

Hi Alvin ,

Do you have any plots and schematics of your setup ? It could be helpful for us to see and understand the setup.

0 Alvin P. Schmitt over 9 years ago in reply to ShreyasRao

Intellectual 940 points

it will be patented can you assure me no one will see it?

0 Alvin P. Schmitt over 9 years ago in reply to ShreyasRao

Intellectual 940 points

Can you reveal the VCO circuit to me? is it digital or analog? How do you achieve oscillation with just one R and one C?

0 Clemens Ladisch over 9 years ago in reply to Alvin P. Schmitt

Guru 316470 points

it will be patented can you assure me no one will see it?

Just show the part that you've already described above.

0 Alvin P. Schmitt over 9 years ago in reply to Clemens Ladisch

Intellectual 940 points

I will have to redraw the part and the connections I am talking about on eagle later today. Thanks

0 ShreyasRao over 9 years ago in reply to Alvin P. Schmitt

TI__Guru 61950 points

Alvin ,

Since this is an open forum , the schematic will be accessible for all . You can send me a private msg with the schematics and I can discuss with the colleagues and give you more feedback .
If the PLL freq is jumping from 12 to 17Mhz , could be something with the external components and phase detector locking , im not sure .

0 Alvin P. Schmitt over 9 years ago in reply to ShreyasRao

Intellectual 940 points

Attached is the 74hc4046 schematic of the VCO connections

0 Ron Michallick over 9 years ago

TI__Guru**** 164546 points

Alvin,

Always use a series resistor is series with the potentiometer otherwise a short can happen on R1 pin.

Data sheet minimum for R1 is 3k ohms, so use that as series resistor.

To get the frequency needed, you will need to reduce the C1 capacitance.

0 Alvin P. Schmitt over 9 years ago in reply to Ron Michallick

Intellectual 940 points

Lowering C1 got me to 13.56 MHz which is where I want to be so Thank you for your help. I noticed that zero volts at VCOIN causes the VCO to stop oscillating. I guess that means I cannot set the VCO's frequency by setting VCOIN to zero and adjusting C1 or R1. Maybe by setting VCOIN to 1 volt it would allow me to adjust VCO frequency to 13.56 MHz . I will have to try it. Can TI release the VCO schematic or is it proprietary?

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74HC4046 - Setting VCO frequency