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MC13030DW bảng dữ liệu(PDF) 10 Page - Motorola, Inc |
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MC13030DW bảng dữ liệu(HTML) 10 Page - Motorola, Inc |
10 / 16 page MC13030 10 MOTOROLA ANALOG IC DEVICE DATA secondary, but, like with the one for the first mixer, if the secondary is tuned, the tap can be adjusted for the impedance of the 455 kHz filter. Wideband filters usually have a higher terminating resistance than the narrowband ones. The recommended coil is made this way. The IF amplifier is basically a transconductance amplifier because the output is a current source. The output is also internally connected to a high impedance AM detector. gm for the IF amplifier is ≈ 0.028 mho. The voltage gain will be the detector coil impedance x 0.028. This can be designed to give the desired audio output level for a given RF input level. If it is set too high, the receiver may oscillate with no input signal. The application circuit was designed for a relatively narrow bandwidth, so a tapped detector coil is used to get the desired gain. If a wide bandwidth receiver is desired, the detector coil can be untapped, and a resistor can be added across the coil to get the desired Q. The detector output on Pin 13 is a low impedance. It supplies the IF AGC signal to Pin 12, so the audio must be filtered out. The time constant of this filter is up to the designer. The main requirement is usually the allowable audio distortion at 100 Hz, 80% modulation. If the time constant is made too long, the audio level will be slow to correct when changing stations. The Signal Strength (S) output is dependent only on the IF amplifier input level. Its maximum voltage is about 5.0 V with a 75 k load resistor. The range can be reduced by using a lower value for the resistor on Pin 11. The S signal will stop increasing when the RF AGC circuits become active, so if the RF AGC threshold is set too low, or there is too much loss from the Mixer2 output to the IF input, the maximum S signal will be reduced. The desired load resistor on Pin 11 (R11) can be determined using the curve of Pin 11 current versus IF input. Setting the RF AGC threshold is probably the most difficult because a trade–off between allowable interference and suppression of desired signals must be made. First select the values for both mixers: a. Using the formula Pin = IP3 – DR/2 Select the desired dynamic range and calculate the maximum input levels for both mixers. Remember that all levels must be in dB, dB µV or dBm. Let DR = 50 dB. IP3 for Mixer2 = 112 dB µV. Therefore, Pinmax = 87 dBµV. IP3 for Mixer1 = 127 dB µV. Therefore, Pinmax = 102 dBµV. b. First, adjust the resistor from Pin 6 to ground to give the desired maximum input level to Mixer2. From the curve of Pin 6 current versus Mixer2 input level, R6 = 1.2/110 µA = 11 k. Rint = 39 k, so R6ext = 15 k. c. From the curve of Pin 6 current versus Mixer1 input level, determine how much more gain would be required in the Mixer1 AGC circuit to achieve the desired dynamic range for Mixer1. From the curve of Relative Sensitivity versus R7 determine the value of R7. Alternatively, R7 can be adjusted to give the desired maximum input level to Mixer1. The resulting R7 may be too small to set the AGC threshold of Mixer1 as low as desired. Also, if R7 is less than 680 Ω, the AGC sensitivity for the Mixer1 input falls off at higher frequencies, so in these cases, the resistor from Pin 6 to ground must be reduced to achieve the desired level because the overload of Mixer1 provides the most important spurious response rejection. However, if the AGC level is set too high, the IF in signal may become too large and the IF amplifier can overload with strong signals. The values used in the application are more conservative. The gain from the antenna input to the point being measured are shown on the AM radio application. These are helpful when calculating audio sensitivity and troubleshooting a new radio. |
Số phần tương tự - MC13030DW |
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Mô tả tương tự - MC13030DW |
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