Low Phase Noise Reference Buffer/Logic Converter Converts Sine Waves to Logic Levels with Lowest Additive Jitter The LTC®6957 is a dual output buffer/driver/logic translator, ideal for converting sine waves into low phase noise logic level signals. The device converts any DC to 300MHz reference frequency into dual LVPECL, LVDS or CMOS outputs with exceptionally low additive jitter (less than 200fsRMS total jitter). The LTC6957 also features a proprietary, selectable, input stage bandwidth-limiting feature, which substantially improves the additive phase noise for slow slewing signals by up to 3dB to 4dB. Additive Phase Noise at 100MHz Features Low Phase Noise Buffer/Driver Three Logic Output Types Available: LVPECL (LTC6957-1) LVDS (LTC6957-2) In-Phase CMOS (LTC6957-3) Complementary CMOS (LTC6957-4) Additive Jitter: <45fsRMS (LVPECL) 3.3V Supply Operation Low Output Skew: 3ps Typical Fully Specified from –40°C to 125°C Available in 12-Pin MSOP & 3mm × 3mm DFN Packages n –140 SINGLE-ENDED SINE WAVE INPUT AT +7dBm (500mVRMS) FILTA = FILTB = GND n n n n n n n n –145 PHASE NOISE (dBc/Hz) n LTC6957-2 (LVDS) –150 LTC6957-4 (CMOS) –155 –160 n LTC6957-3 (CMOS) –165 100 L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. LTC6957-1 (LVPECL) 1k 10k 100k OFFSET FREQUENCY (Hz) 1M The LTC6957 includes input filtering with three narrowband settings in addition to the full bandwidth limitation of the circuit design (Table 1). For slow slewing signals (ie <100MHz sine wave signals), substantially lower additive phase noise can be achieved using this feature. Table 1 The optimal filter setting depends on the clock frequency, amplitude, and waveform shape, with the major determinant being the slew rate at the input of the LTC6957. Table 2 shows the slew rate ranges most suitable for the four specific filter settings. FILTA FILTB BANDWIDTH Low Low 1200MHz (Full Bandwidth) High Low 500MHz (–3dB) Low High 160MHz (–3dB) High High 50MHz (–3dB) Table 2 Another way to look at this is to consider the case of sine waves, for which the frequency ranges will depend on the input amplitudes, as shown in Table 3. The LTC6957-1 100MHz phase noise plots in the figures below illustrate the trade-offs between filter settings at various input slew rates. For a 100MHz sine wave, a 10dBm input level results in a slew rate of 628V/µs. Table 2 indicates the best filter setting to use is FILTA = FILTB = L which is supported by the blue trace in Figure a. FILTA FILTB INPUT SLEW RATE (V/µs) Low Low >400 High Low 125 to 400 Low High 40 to 125 High High <40 Table 3 FREQUENCY RANGE With 0dBm at 100MHz, the input slew rate is 198V/µs. Table 2 indicates that the best filter setting is FILTA = H, FILTB = L which is supported by the green trace in Figure b. As the input decreased 10dB from Figure a to Figure b, the blue trace rose 5dB while the green trace only rose 3dB. INPUT AMPLITUDE (dBm) With –10dBm at 100MHz, the input slew rate is 63V/µs. Table 2 recom mends the best filter setting is FILTA = L, FILTB = H which is supported by the red trace in Figure c. As the input decreases 10dB from Figure a to Figure b, then 10dB from Figure b to Figure c, the red trace rose 3dB, then 4dB, while the blue and green traces rose much faster. –5 –10 FILTA = L, FILTB = L (MHz) FILTA = H, FILTB = L (MHz) FILTA = L, FILTB = H (MHz) FILTA = H, FILTB = H (MHz) 10 >63 20 to 63 6.3 to 20 <6.3 5 >112 35 to 112 11 to 35 <11 0 >200 63 to 200 20 to 63 <20 >112 35 to 112 <35 >200 63 to 200 <63 LTC6957-1 (LVPECL) Additive Phase Noise at 100MHz with Varying Input Amplitudes FILTA = FILTB = H –150 –155 FILTA = L, FILTB = H FILTA = H, FILTB = L –160 –145 –150 FILTA = FILTB = H FILTA = L, FILTB = H –155 –160 FILTA = FILTB = L 1k 10k 100k OFFSET FREQUENCY (Hz) (a) 1M –165 100 1k 10k 100k OFFSET FREQUENCY (Hz) (b) 1M FILTA = FILTB = H FILTA = FILTB = L –145 –150 FILTA = H, FILTB = L FILTA = L, FILTB = H –155 –160 FILTA = H, FILTB = L FILTA = FILTB = L –165 100 –140 SINGLE-ENDED SINE WAVE INPUT, 100MHz AT 0dBm (632.5mVP-P) LTC6957-1 PHASE NOISE (dBc/Hz) PHASE NOISE (dBc/Hz) –145 –140 SINGLE-ENDED SINE WAVE INPUT, 100MHz AT 10dBm (2VP-P) LTC6957-1 PHASE NOISE (dBc/Hz) –140 SINGLE-ENDED SINE WAVE INPUT, 100MHz AT –10dBm (200mVP-P) LTC6957-1 –165 100 1k 10k 100k OFFSET FREQUENCY (Hz) 1M (c) www.linear.com/6957 n 1-800-4-LINEAR 0313