Datasheet

Introduction

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1.6 27 MHZ VCXO and Loop Filter

The LMH1982 requires an external 27 MHz VCXO (X1) and loop filter circuitry for operation of the VCXO

PLL. The board is populated with a CTS 357-series 27.0000 MHz VCXO with ±50 ppm absolute pull range

(APR), which yields 1000 Hz/V nominal tuning sensitivity (K

VCO

). The VCXO input control voltage can be

measured at test point TP21.

The second-order loop filter consists of R

= 20 kΩ (R8), C

= 44 μF (C10 = C27 = 22 μF), and C

= 1 μF

(C28). The parallel combination of C10 and C27 form the series capacitor, C

. Based on the loop

response equations provided in the LMH1982 datasheet, this loop filter yields a nominal -3 dB loop

bandwidth (BW) of about 3 Hz and nominal damping factor of 0.8 assuming K

VCO

= 1000 Hz/V, I

CP1

= 250

μA (charge pump current for PLL 1), and FB_DIV = 1716 (feedback divider for NTSC input). This loop

filter was chosen to give good output jitter performance when the LMH1982 is genlocked to a clean black

burst or tri-level sync reference, such as from a Tektronix TG700 video generator.

It is possible to use different loop filter component values (or topologies) to meet output clock jitter and

lock time requirements for other input reference signals and applications. For example, to generate low-

jitter output clock from a high-jitter input reference (e.g. recovered H signal from an FPGA SDI receiver), a

narrowband loop filter (e.g. BW < 1 Hz ) is recommended for maximum jitter attenuation. In addition to

changing the loop filter components, I

CP1

can also be programmed to adjust the loop bandwidth. Refer to

the LMH1982 datasheet for more complete descriptions about designing the loop filter and optimizing the

VCXO PLL loop response.

The PCB layout of the external VCXO PLL circuitry is shown in Figure 2.

Figure 2. PCB Layout showing Loop Filter and VCXO

1.6.1 VCXO Power Supply Considerations

The VCXO and LMP7701 devices operate from a separate supply plane (VDD_VCXO) derived from the

board’s 3.3V supply. Resistor R5 is used to form a low-pass filter with the associated decoupling and

bypass capacitors to attenuate supply noise to these devices. Refer to the VCXO power supply and

ground routing in the PCB layout section.

AN-1841 LMH1982 Evaluation Board SNOA527A–May 2008–Revised April 2013

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