+ Post New Thread
Results 1 to 7 of 7

28th January 2020, 06:31 #1
 Join Date
 Sep 2009
 Posts
 5
 Helped
 0 / 0
 Points
 1,914
 Level
 10
Fractional N Synthesizer  Dual or MultiModulus Divider
Hi folks,
I'm having trouble finding equations or literature on how to design a suitable frequency divider.
Ignoring the sigmadelta stuff (for now), I want to know how to "hit" every channel in a fractionalN PLL synth. Let's say I know the frequency step (channel spacing), reference (comparison) frequency and the VCO output range.
I have the choice of a dualmodulus divider or a multimodulus divider. Apparently the dualmodulus divider might not give me all the channels in the range but I don't know how true this is!
The reference frequency has to be an integer division of whatever the Xtal frequency is and I guess the accumulator size, n, means that the total number of cycles has to be 2^n. These limitations don't make life easier.
So... how can I choose which type of divider will achieve this and also how can I work out which division ratios (N/N+1 etc) will give me every single channel in the output range?
Apologies if I've broken any rules  this is my first (or second) post so please go easy on me!
Cheers!

Advertisement

28th January 2020, 07:53 #2
Awards:
 Join Date
 Apr 2014
 Posts
 16,449
 Helped
 3731 / 3731
 Points
 81,102
 Level
 69
Re: Fractional N Synthesizer  Dual or MultiModulus Divider
Hi,
I can't follow this vague description.
Please post a picture of your topology and give an example with values.
Maybe you are looking for a mathematical solution.
Then often prime factorization is used.
KlausPlease donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.

28th January 2020, 08:34 #3
 Join Date
 Sep 2009
 Posts
 5
 Helped
 0 / 0
 Points
 1,914
 Level
 10
Re: Fractional N Synthesizer  Dual or MultiModulus Divider
Hi Klaus,
Apologies if my description was vague. You're right about finding a general mathematical solution. I've attached a very crude PLL diagram with some arbitrary numbers as an example so please forgive any unrealistic values.
Let's say I want to design a synth whose output, f_VCO, is 690  960 MHz with a 100 kHz channel spacing, f_step. If my reference frequency, f_ref is 10 MHz, then I need a divider which can divide f_VCO by 69 and 96 and all the steps between. So, for a 100 KHz step, this means I need to divide f_VCO in a way that ensures I can reach all 2710 channels in the 690  960 MHz range.
Since it's fractionalN, I can use a dualmodulus or multimodulus divider but I don't know which type of divider would allow me to select all the output frequencies. Once I know what's possible, I'll then need to find out how to program the divider in order to select those frequencies.
I've seen a few descriptions in some theses and papers but no generalised solution.

Advertisement

28th January 2020, 11:27 #4
 Join Date
 Jan 2019
 Location
 Ireland
 Posts
 219
 Helped
 91 / 91
 Points
 1,588
 Level
 9
Re: Fractional N Synthesizer  Dual or MultiModulus Divider
You can use either. The dual modulus divider will introduce more jitter (due to quantization) than a multimodulus divider in general. But you need to make a MATLAB model for it to see its impact and if it affects your performance too much.
Refer to Razavi's RF Microelectronics (fractionalN synthesizer chapter).

28th January 2020, 17:16 #5
 Join Date
 Sep 2009
 Posts
 5
 Helped
 0 / 0
 Points
 1,914
 Level
 10
Re: Fractional N Synthesizer  Dual or MultiModulus Divider
Thanks for your help. I've got the Razavi book stashed away somewhere so will dig it out and take a look.
Also, the jitter/phase noise/spurs issue is something I'll worry about once I've found a generalised solution.
Is there a test I can do to check whether or not the dual modulus divider is sufficient?
From what I've read, in a fractionalN PLL, the channel spacing, f_step, is dictated by f_step = f_ref / (2^K), where K = accumulator size in bits. Now, if f_ref is to be an integer division of the crystal frequency, f_xtal, and (2^K) can be 1, 2, 4, 8 … 32 … 64 ... etc., then for a channel spacing of 100 kHz and f_ref of 10 MHz, K = 13.29. Rounding this to 13 or 14 gives me a channel spacing of 1.22 kHz or 610.4 Hz, respectively. To get the exact 1 kHz frequency step, I'd need to divide the crystal by a noninteger value.
This is before I get to the fun part of finding N/N+1 division ratios which are guaranteed to land exactly on every required frequency point.
(Apologies if this is all wrong  I'm an RF guy by trade and have not had much experience with the digital side of things!)

28th January 2020, 22:24 #6
Awards:
 Join Date
 Jul 2009
 Location
 Aberdyfi, West Wales, UK
 Posts
 13,658
 Helped
 4535 / 4535
 Points
 83,067
 Level
 70
Re: Fractional N Synthesizer  Dual or MultiModulus Divider
As long as your loop filter has long enough time constant you can switch modulus 'on the fly' to produce intermediate division ratios. You don't have to select the exact division ratio for each frequency, you 'jump' each side of the ratio you need and let the loop filter find the middle ground.
Brian.PLEASE  no friends requests or private emails, I simply don't have time to reply to them all.
It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.

Advertisement

1st February 2020, 15:31 #7
 Join Date
 Sep 2009
 Posts
 5
 Helped
 0 / 0
 Points
 1,914
 Level
 10
Re: Fractional N Synthesizer  Dual or MultiModulus Divider
Hi All,
Please ignore my previous reply  I got myself in a pickle and couldn't make up my mind which frequency step to use.
What I need to know more than anything is, what circuits can I use to achieve the integer division? If my reference, f_ref is 10 MHz and my VCO output range is 690  960 MHz, I need a programmable divider, which can divider by 69 and 96 with all integer steps in between.
The fractional steps can of course be achieved by toggling between the two values of N and N+1. As betwixt said, this is averaged by the loop filter.
I just don't know how to make a divider whose value of N can be programmed. Any ideas or even better, some links to a publication or two?
Cheers!
+ Post New Thread
Please login