ADF4157 Channel Spacing

Hi,

From what I can  see, the channel spacing can’t be set at desired increments as the denominator is fixed at 33554432. As stated in the data sheet, the resolution is REF/33554432. So, for 62.5KHz channel spacing I guess I have to work out the codes for each channel between 3437.5MHz-3687.4375MHz ? Can you confirm this? Frac N synths in the past I have used have allowed the spacing to be defined.

With reference to page 18 of 24 in the data sheet I was not clear as to how to work out the Frac values in the example. Are there any other examples I could follow?

Regards Allan

  • Hi, Thanks, that has clarified things- I have downloaded the tools.  The reason I looked at the ADF4157 was because it is a PIN for pin replacement for the ADF4106. I am planning to replace the fine and course loop synth  (in an existing design) with one part.

    At present I have a 100MHz reference and I am dividing by 5 and then by 2 to get 10MHz Pdf.  I may well increase this to bring the loop mult factor down.

    Best Regards Allan

  • Well, this part will be worth looking at too the....as I have placed the ADF4157 on my board I will evaluate it anyway.

    I had a look at ADIsimPll yesterday, started a new design with ADF4157 and it only allowed 4/5 dual mod divider whereas the part actually has an 8/9 too-which is the one I need. I am going to have another at this noiw

  • 0
    •  Analog Employees 
    on Apr 19, 2012 1:52 PM over 8 years ago

    Yes, the denominator (modulus) on the ADF4157 is fixed at 2^25. This means the minimum channel spacing is the PFD frequency / 2^25.

    For example, assume your PFD frequency is 10 MHz, then your minimum channel spacing is 0.298 Hz. This means that is your increase your fractional (FRAC) value (in R1 and R0) by 1, the output will increase by 0.298 Hz.

    However, if you want your channel spacing greater than 0.298 Hz, then you increase your FRAC value by more than 1. Increasing FRAC in steps of 100 would set your channel spacing to 29.8 Hz.

    For your channel spacing of 62.5 kHz, you would increase FRAC in steps of 209715. Our evaluation board software will do the calculations this for you. Free download here: http://ez.analog.com/message/38414#38414

    The ADF4157 is designed for its very narrow channel spacing. If you don't need this, I recommend looking at the ADF4153 which will cover your frequency range and has a programmable modulus (definable channel spacing) which may be more suited to your needs.

  • 0
    •  Analog Employees 
    on Apr 19, 2012 2:26 PM over 8 years ago

    The ADF4153 is also pin-compatible with the ADF4106.

    If you can use a higher PFD frequency, you should because this will improve your phase noise. You can use ADIsimPLL to compare the performance of various PFD frequencies. Free download here: www.analog.com/adisimpll