NS公司的LMH1982是带Genlock的多速率视频时钟发生器,时钟速率高达3Gbps,可用在高清(HD),标清(SD)视频如视频同步,串行数字接口(SDI),串化器和解串器(SERDES),视频转换,视频编辑,以及其它广播和专业级视频系统.本文介绍了LMH1982的主要特性,方框图,以及各种视频时钟应用电路和评估板的多种电路图与材料清单(BOM).

LMH1982 Multi-Rate Video Clock Generator with Genlock
The LMH1982 is a multi-rate video clock generator ideal for use in a wide range of 3-Gbps (3G), high-definition (HD), and standard-definition (SD) video applications, such as video
synchronization, serial digital interface (SDI) serializer and deserializer (SerDes), video conversion, video editing, and other broadcast and professional video systems.
The LMH1982 can generate two simultaneous SD and HD clocks and a Top of Frame (TOF) pulse. In genlock mode, the devices phase locked loops (PLLs) can synchronize the output signals to H sync and V sync input signals applied to either of the reference ports. The input reference can have analog timing from Nationals LMH1981 multi-format video sync separator or digital timing from an SDI deserializer and should conform to the major SD and HD standards. When a loss of reference occurs, the device can default to free run operation where the output timing accuracy will be determined by the external bias on the free run control voltage input.
The LMH1982 can replace discrete PLLs and field-programmable gate array (FPGA) PLLs with multiple voltage controlled crystal oscillators (VCXOs). Only one 27.0000 MHz
VCXO and loop filter are externally required for genlock mode. The external loop filter as well as programmable PLL parameters can provide narrow loop bandwidths to minimize
jitter transfer. HD clock output jitter as low as 40 ps peak-topeak can help designers using FPGA SerDes meet stringent SDI output jitter specifications.
The LMH1982 is offered in a space-saving 5 mm x 5 mm 32-pin LLP package and provides low total power consumption of about 250 mW (typical).
主要特性:
■Two simultaneous LVDS output clocks with selectable frequencies and Hi-Z capability:
SD clock: 27 MHz or 67.5 MHz
HD clock: 74.25 MHz, 74.25/1.001 MHz, 148.5 MHz or 148.5/1.001 MHz
■Low-jitter output clocks may be directly connected to an FPGA serializer to meet SMPTE SDI jitter specifications
■Top of Frame (TOF) pulse with programmable output format timing and Hi-Z capability
■Two reference ports (A and B) with H and V sync inputs
■Supports cross-locking of input and output timing
■External loop filter allows control of loop bandwidth, jitter transfer, and lock time characteristics
■Free run or Holdover operation on loss of reference
■User-defined free run control voltage input
■I2C interface and control registers
■3.3V and 2.5V supplies
主要应用:
■Video genlock and synchronization
■FPGA SDI SerDes recovered clock generation
■Triple rate 3G/HD/SD-SDI SerDes
■Video capture, conversion, editing and distribution
■Video displays and projectors
■Broadcast and professional video equipment



图1. LMH1982功能方框图



图2.用作三速SDI视频的模拟基准时钟



图3.用作三速SDI视频的SDI基准时钟



图4.三速SDI环通(loop-through)方框图
3.用作三速SDI视频的SDI基准时钟



图5.用于三速SDI视频的组合Genlock或环通

LMH1982评估板
The LMH1982 evaluation board was designed by National Semiconductor (NSC) to evaluate the performance and operation of the LMH1982 multi-rate video clock and timing generator with the LMH1981 SD/HD video sync separator.
The evaluation board provides input ports to receive analog or digital reference signals, SMA connector ports to transmit the differential output clocks, and headers to access various input/output signals. On-board toggle switches allow control over the sync inputs and control inputs, such as device reset. A USB interface board is also provided to allow programming of the LMH1982 through a PCs USB port using NSCs LMH1982 evaluation software.



图6. LMH1982评估板建立的简化功能图



图7. LMH1982评估板电路图(1):带外接电源输入插头的LP38693 LDO稳压器

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