SNAU305 February 2025 LMH1229 , LMH1239
The LMH12x9 can be configured to a target CDR loop bandwidth through register control in the CDR page. To optimize jitter between the phase-locked loop (PLL) and voltage-controlled oscillator (VCO) the loop bandwidth can be adjusted over all data rates. TI recommends to leave the CDR loop bandwidth at default for the majority of cases.
An external 470nF loop filter capacitor is needed when LOOP_BW_SEL is R or L.
| DATA RATE (Gbps) | LOOP_BW_SEL | EXTERNAL CAPACITOR | OVERRIDE & REG 0x23[6:7] | TARGET CDR LOOP BW (0.2 UI SINUSOIDAL JITTER) |
|---|---|---|---|---|
11.88 | H | No | 11 | 7MHz |
F | No | 01 | 7MHz | |
R | Yes | 10 | 700kHz | |
L | Yes | 00 | 350kHz | |
5.94 | H | No | 11 | 7MHz |
F | No | 01 | 7MHz | |
R | Yes | 10 | 600kHz | |
L | Yes | 00 | 300kHz | |
2.97 | H | No | 11 | 5MHz |
F | No | 01 | 5MHz | |
R | Yes | 10 | 460kHz | |
L | Yes | 00 | 230kHz | |
1.485 | H | No | 11 | 3MHz |
F | No | 01 | 3MHz | |
R | Yes | 10 | 240kHz | |
L | Yes | 00 | 120kHz | |
0.27 | H | No | 11 | 800kHz |
F | No | 01 | 800kHz | |
R | Yes | 10 | 50kHz | |
L | Yes | 00 | 30kHz |
| COMMAND | REGISTER | VALUE | MASK | //COMMENTS |
|---|---|---|---|---|
| RAW | FF | 04 | 07 | //Select CDR Register Page |
| RAW | 24 | 80 | 80 | //Override Loop_BW_SEL pin. |
| RAW | 23 | 00 | C0 | //Set the Loop_BW_SEL in to L mode. Requires an external 470nF filter capacitor. |
RAW | 23 | 60 | C0 | //Set the Loop_BW_SEL in to R mode. Requires an external 470nF filter capacitor. |
RAW | 23 | 80 | C0 | //Set the Loop_BW_SEL in to F mode |
RAW | 23 | C0 | C0 | //Set the Loop_BW_SEL in to H mode |