MC13030
secondary, but, like with the one for the first mixer, if the
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.
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
First select the values for both mixers:
d. Using the formula P = IP3 – DR/2
in
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
internally connected to a high impedance AM detector. g for
m
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.
for Mixer2 = 112 dBµV. Therefore, P
for Mixer1 = 127 dBµV. Therefore, P
e. 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,
= 87 dBµV. IP3
= 102 dBµV.
inmax
inmax
R6 = 1.2/110 µA = 11 k. R = 39 k, so R
= 15 k.
int
6ext
f. 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.
9–165
MOTOROLA ANALOG IC DEVICE DATA
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