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SL1452 Arkusz danych(PDF) 4 Page - Zarlink Semiconductor Inc |
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SL1452 Arkusz danych(HTML) 4 Page - Zarlink Semiconductor Inc |
4 / 8 page 3 SL1452 1 2p=LC SL1452 QUADRATURE DEMODULATOR The SL1452 FM demodulator has a simple application with very low external component count. This is demonstrated by the applications circuit diagram Fig. 4, but as with most integrated circuits, particularly those working at high frequencies, some attention to good RF layout techniques and correct component selection will ensure optimum results. A good layout can usually be ensured by the simple precau- tion of keeping all components close to the SL1452, maintaining short lead lengths and ensuring a good low impedance ground plane. Double sided board layout enables these objectives to be easily met, but is not essential for satisfactory operation. All coupling and decoupling capacitors should be chosen for low impedance characteristics at high frequencies, multilayer ce- ramic types usually providing small size and adequate high frequency performance. For the quadrature coil tuning capacitor a fairly stable component should be selected to prevent excessive drift. The power supply decoupling capacitor from pin 6 to ground should be 0.1 µF minimum but the input coupling and decoupling values can be smaller, about 330pF being adequate. The only remaining components to be selected are those forming the quadrature circuit on pins 2 and 3 and some care in the determination of values for these is required if maximum performance is to be obtained. First determine the quadrature circuit operating frequency, which is a quarter of the input frequency on pin 8 due to the two internal 42 stages (see Fig.2). Choose suitable values for L and C to resonate at the correct frequency using: f = The value of C should by greater than 15pF to prevent stray capacitance effects introducing errors and distortion of the demodulation curve, but the use of very large capacitances with small inductance values will lower the impedance of the tuned circuit at the required Q value, reducing the drive level to the demodulator and thereby restricting the video output available. In general, for operation in the 400MHz to 600MHz range, an inductance value between 40nH and 60nH is recommended. Once suitable L and C values have been determined, the working Q for the quadrature circuit should be set, the Q value determining the video output level and bandwidth. Video output is proportional to Q whereas video bandwidth is inversely proportional. The effect of Q variations on video bandwidth and amplitude can be determined from Table 1 and the graphs in Fig. 5. 330 SL1452 4 3 2 1 5 6 7 8 0·04 µ 27p 1n 1 5V 0·1 µ 1n VIDEO OUTPUT 612MHz INPUT 0V A value for total damping resistor value to obtain the required Q can be calculated from: R = Q2 πfL The internal 800 Ω resistance between pins 2 and 3 must be allowed for when calculating R. Example Design a quadrature circuit to demodulate a carrier on pin 8 with centre frequency 480 MHz and video bandwidth of 10MHz. For L = 40nH, fQUAD = 120MHz, C = 43·98pF (nearest preferred value 47pF) From Table 1, Q required is approximately 6, therefore total R required is: R = Q2 πfL = 6323 π 3480310630·0431026 4 =181 ohms Allowing for the internal 800 Ω resistance between pins 2 and 3 (see Fig.3), the external resistance required is 234 Ω. ; choose 270 Ω. It should be remembered that the internal 800 Ωresistance is subject to production tolerances and if fairly close control of video bandwidth is required, the L and C ratio may require some adjustment to ensure that the external R is sufficiently low to swamp the effect of internal resistance changes. The value of 270 Ω obtained in the example is low enough to allow adequate control. In order to overcome the effects of component tolerances, it will usually be necessary to make either the L or C a variable component, the value being adjusted to obtain best linearity. Q 10 6 4 Bandwidth 7·5MHz 14MHz 23MHz Table 1 Fig. 4 Typical application |
Podobny numer części - SL1452 |
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Podobny opis - SL1452 |
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