Advanced PLL Concepts
Research Assignement Answers
Q:	What are the three different types of fractional compensation methods?
A:	Delay Compensation: Since the root cause is an instantaneous phase error, it seems sensible to try to correct this with an instantaneous phase delay. Current Compensation: A current is injected into the loop filter to try to cancel the fractional spurs. Sigma Delta Compensation: Sigma delta noise shaping is used here. It is based on the concepts of over sampling sigma delta converters. Refer to: "Fractional Compensation" slide in the Fractional N PLL presentation and PLL Performance, Simulation and Design, by Dean Banerjee. (pages 163-165)
Q:	What performance parameters does the loop filter affect?
A:	It influences switching time, loop bandwidth and reference spurs. Refer to: "Loop Filter" slide from PLL Building Blocks presentation
Q:	If I keep the same comparison frequency, but double the N counter value, what is the impact on phase noise inside the loop bandwidth?
A:	Phase noise will increase by 6 dB. The phase noise will increase by a factor of 10*log(N). Refer to: "Table of Transfer Functions" slide of the PLL Performance Presentation, and pg 36 of PLL Performance, Simulation and Design, by Dean Banerjee.
Q:	If a 1 GHz signal and a 900 MHz signal are input to a mixer: What are the frequencies of the two intermodulation products?
A:	1.1 GHz and 800 MHz. See reference: "Interdemodulation Distortion" slide of the Non-PLL RF Basics presentation
Q:	If I have a comparison frequency of 200 KHz, approximately what is the minimum loop bandwidth I can have to change from 889 - 915 MHz to a 1 KHz tolerance in less than 500 uS?
A:	Answers will vary, but will be around 5.5 KHz. Refer to: WEBENCH™ EasyPLL Online Simulation Tools, to do this, set the lock time constraint to 500 uS and design for minimum spur gain.