Datasheet LTC6903, LTC6904 (Analog Devices) - 8

LTC6903/LTC6904
Theory oF operaTion
The LTC6903/LTC6904 contain an internal feedback loop lower frequency ranges is very low because of the high which controls a high frequency square wave VCO oper- output divisor. ating between 34MHz and 68MHz. The internal feedback The higher frequency settings will display some deter- loop frequency is set over an octave by a 10-bit resistor ministic jitter from coupling between the control loop DAC. The VCO tracks the internal feedback loop frequency and the output. This shows up in the frequency spectrum and the output frequency of the VCO is divided by one of as spurs separated from the fundamental frequency by sixteen possible powers of two. 1MHz to 2MHz. Higher VCO frequencies and lower output divider settings can result in higher output jitter. Random jitter at the
applicaTions inForMaTion Frequency Setting Information
4.25MHz and 8.5MHz yielding an OCT value of 12 or The frequency output of the LTC6903/LTC6904 is deter- 1100. Substituting the OCT value of 12 and the desired mined by the following equation: frequency of 6.5MHz into the previous equation results in: 2078 Hz ( ) f OCT = 2 • 2078(Hz)•2(10+12) DAC = 2048– = 707.113  DAC  2 – 6.5e6(Hz)  1024 Rounding 707.113 to the nearest integer yields a DAC where DAC is the integer value from 0-1023 represented value of 707 (or a 10-bit digital word of 1011000011.) by the serial port register bits DAC[9:0] and OCT is the
Table 1. Output Frequency Range vs OCT Settling
integer value from 0-15 represented by the serial port
(Frequency Resolution 0.001 • f)
register bits OCT [3:0].
f ≥ f < OCT
Use the following two steps to choose binary numbers 34.05MHz 68.03MHz 15 “OCT” and “DAC” in order to set frequency “f”: 17.02MHz 34.01MHz 14 8.511MHz 17.01MHz 13 1) Use Table 1 to Choose “OCT” or use the following formula, rounding down to the integer value less than or 4.256MHz 8.503MHz 12 equal to the result. 2.128MHz 4.252MHz 11 1.064MHz 2.126MHz 10 f 532kHz 1063kHz 9 OCT =   3 322 . log 266kHz 531.4kHz 8  1039 133kHz 265.7kHz 7 2) Choose “DAC” by the following formula, rounding DAC 66.5kHz 132.9kHz 6 to the nearest integer: 33.25kHz 66.43kHz 5 2078(Hz)•2(10+OCT) 16.62kHz 33.22kHz 4 DAC = 2048– 8.312kHz 16.61kHz 3 f 4.156kHz 8.304kHz 2 For example, to set a frequency of 6.5MHz, first look 2.078kHz 4.152kHz 1 at Table 1 to find an OCT value. 6.5MHz falls between 1.039kHz 2.076kHz 0 69034fe 8 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Power Requirements Serial Port Electrical Characteristics Timing Characteristics Typical Performance Characteristics Pin Functions Block Diagram Timing Diagrams Theory of Operation Applications Information Package Description Revision History Typical Application Related Parts