74HCT4046 range problems

[barrylorimer]

Hi all,

I am trying to design a PLL circuit with a 4046. I'm looking for a centre frequency of 1MHz and a capture range from as near to 0 as possible upto 2MHz. I've had a play with the circuit and it doesn't seem possible. I've also found this online calculator and this seems to back up my theory..........

ww.changpuak.ch/electronics/calc_03.php

Any help would be greatly appreciated!!!

 

[FvM]

According to the datasheet, an fmax/fmin ratio of near to 100 can be achieved. So depending on how you understand "near to 0" it should be possible.

 

[barrylorimer]

Thanks for you reply! My fmin would be around 8Khz, I should maybe have made that clearer. The closest I can get with an Fmax of 2Mhz is down to around 400Khz.

Not good!

 

[FvM]

The closest I can get with an Fmax of 2Mhz is down to around 400Khz.

How did you determine this? It's not a limitation according to the datasheet.

I thought to remember, that the offset frequency can be set to effectively zero by omitting R2, but it's not suggested in the datasheet. Maybe, it has been possible with CD4046 only. Suitable loop filter dimensioning and stable PLL operation with near "zero" frequency is a different point, of course.

 

[barrylorimer]

Sorry, what I mean is by manipulating R1 & C1, this is the closest I can get to my preferred o/p. I have omitted R2 to reduce the offset but even this doesn;t seem to be enough.

 

[FvM]

Are you referring to a real measurement? If so, what's the respective VCO control voltage.

P.S.: I checked an existing HC4046A circuit, and found that the VCO frequency goes down to zero with R2 omitted.

 

[barrylorimer]

With a max of 2.3Mhz and min of 400KHz, my VCO control voltage varies from 0.82vdc up to 4.2vdc. Vcc is 5vdc.

I can get the frequency range to start very low, by changing the values of R1 and C1 but this has the effect of drastically reducing the range. ie, 8Khz - 100Khz.

Obviously the vco range is governed by the values of R1 and C1 (R2=open) however the frequency range is determined by the sum of R1C1. This would seem to be my problem.

 

[FvM]

the frequency range is determined by the sum of R1C1

I don't understand how to sum a capacitance and a resistance. Accorcding to my experience, the VCO frequency can be controlled down to 0, VCOin must be possibly slightly lower than 0.82V. But the datasheets don't have guaranteed values for this operation, it may be also a problem of exemplar variations.

 

[barrylorimer]

Thanks for your input and effort. I think however I shall have to attack this from a different direction.

Thanks again, Barry.