PLL design - guidance required.

[mhtplsh]

mc145151 design

I am designing FM transmitter 88~108Mhz with PLL.
This will hv .5Mhz steps.
Using MC145151-2. & MC 12073P single modulus prescaler.
Datasheet of MC145151-2 attached herewith.
I got this old IC's from a friend.
Using 4 Mhz crystal at pin no. 26-27 then devide by 512 with reference address input. Hence the reference freq. will be 7.8125Khz.
Prescaler will be devide by 64-MC 12073P.
The supply voltage will be 5V DC fully stabilised.
Now i am stucked at designing low pass filter design. I am going to use opamp as on page 18 as 4.1C.
This is very important part of design. Hence guidance required in the matter.
This is my first PLL project.

what type of VCO will be better.
I am thinking of using varactor based vco.

 

[RFDave]

mc145151 108 mhz

Before you dive into filter design, you will need to figure out how you are planning on doing the FM modulation, as the various approaches may impact your loop design.

Why did you choose 7.8 kHz as the comparison frequency? You may be better served by using 500 kHz as the reference frequency. This will put reference spurs outside the modulation range.

Dave

 

[mhtplsh]

I want to hv .5mhz steps. In the refernce address code divide values r 8, 128, 256 512 & so on. If i select 8 then the reference frequency will be 500khz.
Then i am going to use divide by 64 prescaler. My minimum frequency is 88 mhz. so devide by 64 end up with 1.375 & then the n counter will come into place. dividing 1.375mhz by .5mhz will be 2.75 count. how to do this as minimum counter is 1.
so next step is selecting proper refernce addrese divide. Hence i selected 512. Here the refernce frequency is 7.8125 with 4 mhz as crytal frequency.
Now dividing 88 mhz~108Mhz with with 7.8125 the count becomes 11264~ 13824. Now dividing with 64 prescaler remains 176 ~ 216. This means each count will be for 500khz step. easy to do.

 

[biff44]

You have to be carefull with this sort of thing. The +/- instantaneous frequency deviation, after dividing down by N, has to be <<2Π. Otherwise the loop goes nuts. You probably have to divide by something like 2000 to meet that criteria.