MC13030 Просмотр технического описания (PDF) - Motorola => Freescale

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MC13030

Dual Conversion AM Receiver

Motorola => Freescale 

MC13030

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, dBµV or dBm. Let DR = 50 dB. IP3

for Mixer2 = 112 dBµV. Therefore, Pinmax = 87 dBµV. IP3

for Mixer1 = 127 dBµV. 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.

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MOTOROLA ANALOG IC DEVICE DATA

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