CMOS PLL circuit

Posted on Apr 1, 2012

One of the most interesting projects to work with is that of a synthesizer that can accurately gnerate desired frequencies. This synthesizer uses just three CMOS chips and one PNP transistor. It is a fact that the transistor may be the hardest component to get in some parts of the world!! The synthesizer will generate a 3:1 frequency range anywhere from 300Hz to 4,000,000Hz (4MHz). Using the components shown you can feed the synthesizer with up to 12vDC but with only 9vDC the synthesizer may not achieve more than 3.5MHz. Tye lock-range of this synthesizer is about 10:1 or so.

CMOS PLL circuit
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The CD4060 reference oscillator divides a crystal reference oscillator sucessively by two. From each stage we can select a frequency that is half the value of the previous frequency. The reference frequency of the synthesizer is multiplied by the programmeable divider divide rate to give the final frequency. If our reference frequency is 1KHz then the final output may be programmeable in 1KHz steps. This is quite convenient for a decimal programeable divider. Divide by 1000 = 1000KHz, divide by 1001 = 1001KHz etc. You could also select a crystal that would give you a 100Hz output then you could select frequencies in 100Hz steps. Divide by 1000 = 100KHz, divide by 1001 = 100.1KHz etc. Here are reference frequencies you could use for different oscillator crystal frequencies. Remember that the programable divider will only divide up to 15999 and this limits the maximum frequency you can get out of this synthesizer. With a 100Hz reference frequency you can only get a maximum of 1.5999MHz. With a 1KHz reference frequency you can program up to 15.999MHz which is beyond the range of a CD4046. Incidentally, a 32,768Hz wrist-watch crystal will give you 2Hz so the synthesiser will generate from 6Hz up to 31,998Hz in 2Hz steps. The 4046 has a wide-range Voltage Controlled Oscillator (VCO) which uses C1 and R3 to determine the frequency. C1 is selected by LK2 and I used 33pf, 47pf, 68pf and 100pf. R3 is selected by LK5 and I used 4K7,...

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