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Original question:

CD4046B: Frequency multiplication circuit with at least 100 times magnification

CD4046B: Multiplication circuit to be locked on a range of frequencies from 47 Hz to 63 Hz.

Asad Hameed
Intellectual 685 points
Part Number: CD4046B
Other Parts Discussed in Thread: TMS320F280025, SN74LV4046A

We want to multiply grid frequency 32 times to be fed into TMS320F280025 microcontroller. It is desired that PLL is capable of locking grid frequencies from 47 Hz up to 63 Hz. The application circuit is shown below:

The calculations for R2,R1 and C1 are done as below:

R1 is selected to be 100 k-Ohms and the supply voltage is 5 VDC.

Fmin = 47 * 32 = 1502 Hz.

From the above graph value of C1 comes out to be around 10,000 pF.

Fmax= 63*32= 2016 ==> Fmax/Fmin = 2016/1502= 1.34.

From the below graph R2/R1=0.3. ==> R1=100K/0.3= 330K.

The filter components R3 & C2 are selected as 100 k-Ohms and 0.2 uF respectively as shown by the below calculations:

I am testing the circuit with a signal generator.  It is able to lock between 50 to 61 Hz but does not lock in 47 to 50 Hz range. It is also not locking in 61 Hz to 63 Hz range. It will be nice if some one can validate the calculations and give some guidelines in fixing the frequency capture and locking ranges.

  • 0
    TI__Guru* 96862 points

    Before adjusting any values or devices, I would start by looking at the signals with an oscilloscope. Make sure that you're seeing what's expected at the output of the phase comparator and at the input to the VCO. At the frequencies that don't lock, what's happening at the input / output to the VCO?

    Beyond that, if you are going to operate at 5V, I would recommend switching to SN74LV4046A, which was designed for 5V nominal operation. The CD4k family of logic was built for 15V, so you aren't going to get optimal operation at 5V.

    I would also recommend to expand your frequency range, setting fmin to 1 kHz (or less) and fmax to 2.5 kHz (or greater). Trying to get the minimum at exactly your minimum frequency and the maximum exactly at your maximum frequency sounds like a good idea to maximize range, but it ends up with the device operating at the edges of it's capabilities, while expanding the range will have the operation stay more within the 'nominal' operating range.