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MC13110A Datasheet(PDF) 28 Page - Motorola, Inc |
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MC13110A Datasheet(HTML) 28 Page - Motorola, Inc |
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28 / 68 page  MC13110A/B MC13111A/B 28 MOTOROLA ANALOG IC DEVICE DATA IF Limiter and Demodulator The limiting IF amplifier typically has about 110 dB of gain; the frequency response starts rolling off at 1.0 MHz. Decoupling capacitors should be placed close to Pins 31 and 32 to ensure low noise and stable operation. The IF input impedance is 1.5 k Ω. This is a suitable match to 455 kHz ceramic filters. Figure 48. IF Limiter Schematic Lim Out Lim In Limiter Stage RPI CPI Figure 49. Limiter Input Impedance Unit Input Impedance (RPI) Input Impedance (CPI) Lim In 1538 Ω 15.7 pF Figure 50. Quadrature Detector Demodulator Schematic Q Coil Lim Out1 C28 10 p Rext 22.1 k Toko Q Coil 7MCS–8128Z The quadrature detector is coupled to the IF with an external capacitor between Pins 27 and 28. Thus, the recovered signal level output is increased for a given bandwidth by increasing the capacitor. The external quadrature component may be either a LCR resonant circuit, which may be adjustable, or a ceramic resonator which is usually fixed tuned. (More on ceramic resonators later.) The bandwidth performance of the detector is controlled by the loaded Q of the LC tank circuit (Figure 50). The following equation defines the components which set the detector circuit’s bandwidth: (1) RT = Q XL, where RT is the equivalent shunt resistance across the LC tank. XL is the reactance of the quadrature inductor at the IF frequency (XL= 2π f L). The 455 kHz IF center frequency is calculated by: (2) fc = [2π (L Cp)1/2] – 1 where L is the parallel tank inductor. Cp is the equivalent parallel capacitance of the parallel resonant tank circuit. The following is a design example for a detector at 455 kHz and a specific loaded Q: The loaded Q of the quadrature detector is chosen somewhat less than the Q of the IF bandpass for margin. For an IF frequency of 455 kHz and an IF bandpass of 20 kHz, the IF bandpass Q is approximately 23; the loaded Q of the quadrature tank is chosen slightly lower at 15. Example: Let the total external C = 180 pF. (Note: the capacitance is the typical capacitance for the quad coil.) Since the external capacitance is much greater than the internal device and PCB parasitic capacitance, the parasitic capacitance may be neglected. Rewrite equation (2) and solve for L: L = (0.159)2/(C fc2 ) L = 678 µH ; Thus, a standard value is chosen: L = 680 µH (surface mount inductor) The value of the total damping resistor to obtain the required loaded Q of 15 can be calculated from equation (1): RT = Q(2π f L) RT = 15(2π)(0.455)(680) = 29.5 kΩ The internal resistance, Rint at the quadrature tank Pin 27 is approximately 100 k Ω and is considered in determining the external resistance, Rext which is calculated from: Rext = ((RT)(Rint))/(Rint – RT) Rext = 41.8 kΩ;Thus, choose a standard value: Rext = 39 kΩ In Figure 50, the Rext is chosen to be 22.1 kΩ. An adjustable quadrature coil is selected. This tank circuit represents one popular network used to match to the 455 kHz carrier frequency. The output of the detector is represented as a “S–curve” as shown in Figure 52. The goal is to tune the inductor in the area that is most linear on the “S–curve” (minimum distortion) to optimize the performance in terms of dc output level. The slope of the curve can also be adjusted by choosing higher or lower values of Rext . This will have an affect on the audio output level and bandwidth. As Rext is increased the detector output slope will decrease. The maximum audio output swing and distortion will be reduced and the bandwidth increased. Of course, just the opposite is true for smaller Rext. A ceramic discriminator is recommended for the quadrature circuit in applications where fixed tuning is desired. The ceramic discriminator and a 5.6 k Ω resistor are placed from Pin 27 to VCC . A 22 pF capacitor is placed from Pin 28 to 27 to properly drive the discriminator. MuRata Erie has designed a resonator for this part (CDBM455C48 for USA & A/P regions and CDBM450C48 for Europe). This resonator has been designed specifically for the MC13110/111 family. Figure 51 shows the schematic used to generate the “S–curve” and waveform shown in Figure 54 and 55. |
Similar Part No. - MC13110A |
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Similar Description - MC13110A |
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