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MC13028AP Folha de dados(PDF) 9 Page - Motorola, Inc |
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MC13028AP Folha de dados(HTML) 9 Page - Motorola, Inc |
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9 / 20 page  MC13028A 9 MOTOROLA ANALOG IC DEVICE DATA The phase locked loop (PLL) in the MC13028A is locked to the L–R signal. This insures good stereo distortion performance at the higher levels of left only or right only modulations. Under normal operating conditions, the PLL remains locked because of the current flow capability of the loop driver circuit. This high gain, high impedance circuit performs optimally when the current flow is balanced. The balanced condition is enhanced by the loop driver filter circuit connected between Pin 14 and ground. The filter circuit consists of a 47 Ω resistor in series with a 47 µF capacitor. The 47 Ω resistor is to set the Fast Lock rate. It is recommended that the capacitor be a very low leakage type electrolytic, or a tantalum composition part because any significant amount of leakage current flowing through the capacitor will unbalance the loop driver circuit and result in less than optimum stereo performance, see Figures 10 and 11. The pilot tone detector circuit is fed internally from the Q detector output signal. The circuit input employs a low pass filter at Pin 11 that is designed to prevent the pilot tone detector input from being overloaded by higher levels of L–R modulation. The filter is formed by a 0.22 µF capacitor and the input impedance of the first amplifier. A pilot I detector circuit employs a capacitor to ground at Pin 9 to operate in conjunction with an internal resistor to create an RC integration time. The value of the capacitor determines the amount of time required to produce a stereo indication. This amount must include the time it takes to check for the presence of detector falsing due to noise or interference, station retuning by the customer, and pilot dropout in the presence of heavy interference. The pilot Q detector utilizes a filter on its pilot tone PLL error line at Pin 10. This capacitor to ground (usually 0.47 µF) is present to filter any low frequency L–R information that may be present on the error line. If the value of this capacitor is allowed to be too small, L–R modulation ripple on the error line may get large enough to cause stereo dropout. If the capacitor value is made too large, the pilot tone may be prevented from being reacquired if it is somehow lost due to fluctuating field conditions. A 1.0 V reference level is created internally from the VCC source to the IC. This regulated line is used extensively by circuits throughout the MC13028A design. An electrolytic capacitor from Pin 7 to ground is used as a filter for the reference voltage. DISCUSSION OF GRAPHS AND FIGURES If the general recommendations put forth in this application guide are followed, excellent stereo performance should result. The curves in Figures 2 through 7 depict the separation and the distortion performance in stereo for 30%, 50%, and 65% single channel modulations respectively. The data for these figures were collected under the conditions of VCC = 8.0 V and RO = 10 k in both the left and the right channels as applied to the application circuit of Figure 1. A very precise laboratory generator was used to produce the AM Stereo test signal of 450 kHz at 70 dB µV fed to Pin 4. An NRSC post detection filter was not present at the time of these measurements. The audio separation shows an average performance at 30% and 50% modulations of – 45 dB in the frequency range of 2.0 kHz to 5.0 kHz. The corresponding audio distortions under these conditions are about 0.28% at 30% modulation, and about 0.41% at 50% modulation. Figure 6 shows that the typical separation at 65% modulation in the 2.0 kHz to 5.0 kHz region is about – 37 dB, and the corresponding audio distortion shown in Figure 7 is about 1.0%. The performance level of these sinusoidal signals is somewhat less than those discussed in the previous paragraph due to the internal operation of the clamping circuits. In the field, the transmitters at AM Stereo radio stations are not usually permitted to modulate single channel levels past 70%. Therefore these conditions do not occur very often during normal broadcast material. The roll–off at both the low and high frequencies of the 30% single channel driven responses is due to the fact that a post detection bandpass filter of 60 Hz to 10 kHz was used in the measurement of the data, while a post detection filter of 2.0 Hz to 20 kHz was used for the collection of data in the 50% and 65% modulation examples. The tighter bandwidth was used while collecting the performance data at 30% modulation levels in order to assure that the distortion measurement was indicative of the true distortion products measured near the noise floor and thus not encumbered by residual noise and hum levels which would erroneously add to the magnitude of the harmonic distortion data. Note in Figure 8 the traces of noise response for the four different bandwidths of post detection filtering. It can be seen that the noise floors improve steadily with increasing levels of incoming 450 kHz as the value of the lower corner frequency of the filter is increased. Data for the stereo noise floors was collected with the decoder in the forced stereo mode. |
Nº de peça semelhante - MC13028AP |
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Descrição semelhante - MC13028AP |
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