HMC983 and HMC3716 PLL design ?

Question

Hi all, 
 Our customer will design the PLL circuits with HMC983 and HMC3716. 
 They evaluated the phase noise performance with their custom boards. 
 Comparing the in-band phase noise characteristics of integer mode and fractional mode with the nearly same frequency setting, fractional mode deteriorates by about 5~10 dB. 
 At this time, if "00 = MASH1" ,"01 = MASH11" of "Reg 0Eh Sigma Delta Modulator Configuration Register [1: 0] SD Modulator Type" is set, the phase noise characteristics will change in the direction of improvement.. But these two register are "- Reserved" . 
 Table 22. Reg 0Eh Sigma Delta Modulator Configuration Register [1:0] SD Modulator Type DSM Type. 00 = MASH1 - Reserved 01 = MASH11 - Reserved 10 = MASH111 - Delta Sigma Modulator Mode B 11 - Delta Sigma Modulator Mode A 
 Q1 : Reg 0Eh [1:0] Is it possible to set and use register 0Eh [1:0] 00 and 01? If possible, what is the difference between 00 and 01 features? 
 Q2：Reg 0Eh [1:0] What is the difference between the functions of register 0Eh [1:0] 10 and 11? There seems to be no description of the difference between MASH1,11,111 and Delta Sigma Modulator Mode A and B in the data sheet. 
 Q3: 
 Can you give us some advice on points and precautions when configuring a PLL using the HMC983 and HMC3716? Also, can you provide reference designs, application notes, etc.? 
 
 Best regards 
 sss

aandrews · Accepted Answer

Hello, 
 For a general description of MASH I would direct you to this paper 
 To summarise: 
 The number after MASH (e.g. MASH1, MASH11, MASH111) refers to the order of modulator. The SDM mode A is equal to fourth order (MASH1111). 
 I do not know why the decision was made to reserve the two lower order MASH modes, the relevant team members that made the call are no longer with the company. Perhaps in our characterisation they were not deemed necessary for most applications. I think it is okay to use 1st or 2nd order settings if the customer is happy with performance. 
 How wide is the customer's LBW? It seems that overall, higher order SDM modulators are preferred since they have easier to filter spurs out of band. But if the LBW is very wide then the higher order SDM modulators may have the possibility degrade the in-band performance slightly, which could be what is happening here. 
 I don't have any app notes or reference designs on interfacing HMC3716 with HMC983. Since the HMC983 was designed to be used as a pair with HMC984, I would direct you to below link on a note about using it standalone (below). But if customer is already able to take measurements closed loop then I think their design is sound. https://ez.analog.com/rf/f/q-a/72955/hmc983lp5e-as-a-frequency-divider 
 Regards, 
 Alex)

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HMC983 and HMC3716 PLL design ?