Design Porting Considerations for the MAX2870

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Abstract

This application note compares the MAX2870 ultra-wideband synthesizer/voltage controller oscillator (VCO) to the Analog Devices ADF4350 synthesizer. Registers and loop filter design are discussed in detail.

The MAX2870 ultra-wideband phase-locked loop (PLL) and voltage control oscillator (VCO) can operate in both integer-N and fractional-N modes, similar to the Analog Devices ADF4350 wideband synthesizer. This application note compares the MAX2870 and ADF4350 registers and loop filter design in detail. Users who already familiar with ADF4350 can use this application note as a quick design reference.

Register Table Comparison

Register 0


Bit Position MAX2870 ADF4350 Difference
Bit ID Name/Description Bit ID Name/Description
31 INT Int-N or frac-N mode control — Reserved The MAX2870 provides an additional method to switch between int and frac modes (Note 1).
30–15 N[15:0] Integer division value N16– N1 Integer value Both are 16-bit resolution, but the MAX2870 offers a wider usable range. See the MAX2870 data sheet for details.
14–3 FRAC[11:0] Fractional division value F12– F1 Fractional value N/A
2–0 ADDR[2:0] Register address C3– C1 Register address N/A

Register 1


Bit Position MAX2870 ADF4350 Difference
Bit ID Name/Description Bit ID Name/Description
31 CPOC CP output clamp: set to 1 for int mode; set to 0 for frac mode — Reserved The MAX2870 provides programmability on the CP output clamp to achieve better noise performance in int-N mode.
30–29 CPL[1:0] CP linearity: set to 00 for int mode; set to 01 for frac mode — Reserved The MAX2870 provides programmability for CP linearity to achieve better spurious performance in frac-N mode.
28 CPT[1] Charge pump test mode: 00 for normal operation; 10 for CP in source mode; 11 for CP in sink mode — Reserved On the ADF4350, users set bit 27 to program the prescaler value. On the MAX2870, the prescaler is automatically set. Users must set only CPT[1:0] to 00 for normal operation.
27 CPT[0] PR1
 Prescaler
26–15 P[11:0] Phase value P12– P1 Phase value N/A
14–3 M[11:0] Modulus value M12– M1 Interpolator modulus N/A
2–0 ADDR[2:0] Register address C3– C1 Register address N/A

Register 2


Bit Position MAX2870 ADF4350 Difference
Bit ID Name/Description Bit ID Name/Description
31 LDS Lock detect speed — Reserved The MAX2870 provides 4ns lock detect precision when PFD frequency is greater than 32MHz (Note 2).
30–29 SDN[1:0] Frac-N noise mode
00 = low-noise mode
01 = reserved
10 = low-spur
mode 1
11 = low-spur
mode 2		 L2– L1 Noise mode
00 = low-noise mode
01 = reserved
10 = reserved
11 = low-spur mode		 The MAX2870 provides two modes of dithering for spur mitigation. Users should choose the one that works best in their system.
28–26 MUX[2:0] MUX_OUT configuration
 M3– M1 MUXOUT The MAX2870 has the same mux output, plus an extra serial output to be read out of Register 6 (Note 3).
25 DBR Reference doubler mode RD2
 Reference doubler N/A
24 RDIV2 Reference Div2 mode RD1 Reference divideby2 N/A
23–14 R[9:0] Reference divider mode R10– R1 10-bit R counter N/A
13 REG4DB Double buffer D1 Double buffer N/A
12–9 CP[3:0] Charge-pump current CP4– CP1 Charge pump current setting N/A
8 LDF Lock-detect function U6 LDF N/A
7 LDP Lock-detect precision U5 LDP N/A
6 PDP Phase detector polarity U4 PD POLARITY N/A
5 SHDN Power-down mode U3 Power down N/A
4 TRI Charge-pump three-state mode U2 Cp three state N/A
3 RST Counter reset U1 Counter reset N/A
2–0 ADDR[2:0] Register address C3– C1 Register address N/A

Register 3


Bit Position MAX2870 ADF4350 Difference
Bit ID Name/Description Bit ID Name/Description
31–26 VCO[5:0] Manual VCO selection and VCO subband when VAS is disabled — Reserved The MAX2870 allows the user to disable VCO autoselection (VAS) machine and manually select VCO subband. Use this function with a VCO lookup table method to achieve faster PLL lock time.
25 VAS_SHDN VAS SHDN:
0 = VAS enabled;
1 = VAS disabled		 — Reserved
24 RETUNE Sets VAS response to temperature drift — Reserved
23–19 >— Reserved — Reserved N/A
18 — Reserved F1 CSR Yes
17 — Reserved — Reserved N/A
16–15 CDM[1:0] Clock divider mode:
10 = phase mode		 C2– C1 Clock div mode:
10 = resync mode		 The MAX2870 only provides a phase change from the current phase, where the ADF4350 allows a phase relative to a sync pulse.
14–3 CDIV[11:0] Clock divider value D12– D1 12-bit clock divider value These bits define the low-frequency clock ratio on both parts; depending on operation mode, they have different meanings (Note 4).
2–0 ADDR[2:0] Register address C3– C1 Register address N/A

Register 4


Bit Position MAX2870 ADF4350 Difference
Bit ID Name/Description Bit ID Name/Description
31–26 — Reserved — Reserved Set to 011000 on the MAX2870.
25–24 BS_MSBs[1:0] Band-select MSBs — Reserved Because the MAX2870 allows higher PDF frequency, these additional bits increase the band-select divider range from 8 to 10 bits.
23 FB VCO feedback mode D13 Feedback select N/A
22–20 DIVA[2:0] RFOUT_ output divider mode D12– D10 RF divider select The MAX2870 provides more RFOUT divider ratio options (Note 5).
19–12 BS[7:0] Sets band-select clock-divider value. MSBs are located in bits <25:24> BS8– BS1 8-bit BAND select clock divider value Same for the 8-bit band-select divider. The MAX2870 offers two more MSBs (see BS_MSBs[1:0]).
11 — Reserved D9 VCO power down Yes
10 — Reserved D8 Mute till lock detect Yes
9 BDIV RFOUTB output path select D7 AUX out select N/A
8 RFB_EN RFOUTB output mode D6 AUX out enable N/A
7–6 BPWR[1:0] RFOUTB output power D5– D4 AUX out power N/A
5 RFA_EN RFOUTA output mode D3 RF out enable N/A
4 APWR[1:0] RFOUTA output power D2– D1 Output power N/A
2–0 ADDR[2:0] Register address C1 Register address N/A

Register 5


Bit Position MAX2870 ADF4350 Difference
Bit ID Name/Description Bit ID Name/Description
31–25 — Reserved — Reserved N/A
24 F01 F01 — — The MAX2870 provides an additional method to control int/frac mode (Note 1).
23–22 LD[1:0] Lock-detect pin function D15– D14 LD PIN mode N/A
21–19 — Reserved — Reserved N/A
18 MUX[3] MUX MSB — Reserved The MAX2870 provides additional serial data output on MUX pin (Note 3).
17 — Reserved — Reserved N/A
16–3 — Reserved — Reserved N/A
16–3 — Reserved — Reserved N/A
2–0 ADDR[2:0] Register address C3– C1 Register address N/A

Loop Filter

The MAX2870's KVCO (typical 100MHz/V) is different than the ADI4350's (typical 33MHz/V). To keep PLL parameters (e.g., CP current, N divider, Fcomp, loop BW, and phase margin) unchanged, the loop filter components must be recalculated (Figure 1). Using ADI4350's loop filter on the MAX2870 without careful evaluation could result in performance degradation or even unstable PLL.

Figure 1. Loop filter component labels.

Figure 1. Loop filter component labels.

The following example shows that to maintain the PLL's parameters, a different loop filter needs to be installed, depending on whether the MAX2870 or ADI4350 is being used.

PLL Condition and Loop Filter MAX2870 ADF4350 Notes
KVCO (MHz/V) 100 33 Different KVCO
ICP (mA) 2.5 2.5 Same
Fcomp (MHz) 10 10 Same
N 400 400 Same
Loop BW (kHz) 20.2 17.6 Similar loop BW
Phase margin (deg) 39 37.5 Similar phase margin
C1 (nF) 10 1.5 Different loop filter
C2 (nF) 68 22 Different loop filter
R2 (Ω) 220 430 Different loop filter
C3 (nF) 1.2 0.68 Different loop filter
R3 (Ω) 1000 820 Different loop filter

Notes

Note 1: The MAX2870 has two methods for switching between int/frac modes. In Method 2, this is independent of the FRAC[11:0] bit setting.

Int/Frac Mode Switching MAX2870 ADF4350 Notes
F01 INT FRAC11– FRAC00 FRAC11– FRAC00
Method 1 Integer mode 1 X All 0s All 0s N/A
Frac mode 1 X Not all 0s Not all 0s N/A
Method 2 Integer mode 0 1 X N/A The MAX2870 provides an additional method.
Frac mode 0 0 X N/A The MAX2870 provides an additional method.

Note 2: The MAX2870 allows users to set a PDF frequency of up to 105MHz in int mode, and up to 50MHz in frac mode. The table below summarizes the LD pin settings under different PDF frequencies.

LD Pin Function MAX2870 ADF4350 Notes
LD[1:0] LDS LDP LDF D15–D14 LDP LDF
Low 00 X X X 00 X X Same as ADF4350
Digital LD 01 0 0 0 01 0 0 Same as ADF4350, for PDF frequency < 32MHz, frac-N mode, 10ns precision
0 0 1 0 1 Same as ADF4350, for PDF frequency < 32MHz, integer mode, 10ns precision
0 1 0 1 0 Same as ADF4350, for PDF frequency < 32MHz, frac-N mode, 6ns precision
0 1 1 1 1 Same as ADF4350, for PDF frequency < 32MHz, integer mode, 6ns precision
1 X 0 — — The MAX2870 digital LD function for PDF frequency > 32MHz, frac-N mode, 4ns precision
1 X 1 — — The MAX2870 digital LD function for PDF frequency > 32MHz, int mode, 4ns precision
Analog LD 10 X X X 10 X X Same as ADF4350
High 11 X X X 11 X X Same as ADF4350

Note 3: The table below summarizes the MUX pin setting of the MAX2870 and ADF4350.

MUX Pin Function MAX2870 ADF4350 Notes
MUX[3] REG5[18] MUX[2:0] REG2[28:26] REG5[18] M3–M1 REG2[28:26]
Regular MUX pin function 0 000–111 Reserved 000–111 N/A
Serial data output 1 100 — — The MAX2870 configures the MUX_OUT pin as serial data output to read REG 06 (IC status bits). The ADF4350 does not have this function.

Note 4: Below is a comparison of the CDIV bits function under different modes.

Operation Mode MAX2870 ADF4350
CDIV[11:0], Reg3[14:3] D12-D1, Reg3[14:3]
Fast lock mode CDIV defines the fast lock timeout Defines the fast lock timeout
Phase adjust mode Adjusts the VCO phase relative to the current phase, so no resync period is necessary D12-D1 define the phase resync period.

Note 5: The MAX2870 provides more output divider ratio selection, allowing more design flexibility.

Output-Divider Ratio MAX2870 ADF4350
DIVA[2:0], Reg4[22:20] D12–D10, Reg4[22:20]
1 000 000
2 001 001
4 010 010
8 011 011
16 100 100
32 101 Not available
64 110 Not available
128 111 Not available