ABM1 • 5
2
nd
Band-pass Filter
The 2
nd
Band-pass filter is a copy of the first. Its job is to further filter the RF
signal. It consists of coupling capacitor C8, and filter components M3, C10,
L4, C11, C12, and M4.
AM Detector
The AM detector consists of C13; you guessed it, it’s another coupling ca-
pacitor that passes the signal to U2’s non-inverting input while isolating DC.
R6 provides the bias for the detector while C17 provides power supply filter-
ing. C15 is used to prevent high frequency signals from entering U2 directly
from the air. R2 sets the input impedance of the stage to 51 ohms. This is
important for proper frequency response from the 2
nd
Band-pass filter, it
needs to see a 50 ohm load. C14 essentially places the inverting input at AC
ground. The Logarithmic Amplifier U2 provides a high amount of gain, de-
tects or turns the RF signal in to audio, and provides a DC output voltage pro-
portional to its input signal strength.
Squelch
The squelch circuit allows the user to set the RF signal level that the Audio
Amplifier will amplify and pass. C19 is a coupling capacitor that passes the
detected audio from U2 to the audio amplifier section while preventing U2’s
DC output from being affected by the setting of R11. R9 prevents the AC
output of U2 from being shorted to ground if the squelch is closed. R3, R4,
and R5 make up a voltage divider that allows the user to set the voltage
(squelch level) at U3 pin 3 through R7 (filtered by C16) between 0.1 VDC and
2.0 VDC. U3 is an operational amplifier being used as a comparator. This
circuit compares the voltage from the voltage divider and DC level from the
Detector’s output through R8 (filtered by C18). When the Detector’s DC
level, which is proportional to U2’s RF input, at U3 pin 2 falls below the level
set by the operator (U3 pin 3). U3’s output at pin 1 goes high (9 Volts) turn-
ing on Q1. Q1 is an N-channel Enhancement Mode Field Effect Transistor.
The positive voltage from the comparator U3, pin 1, is felt on the gate of Q1,
turning it on, which results in a very low Drain to Source resistance. This low
Drain to Source resistance shorts the audio signal coming through R9 to
ground. This prevents the audio from passing to the audio amplifier so noth-
ing is heard in the headphones. When the input signal to U2 increases the
DC output voltage at U2 pin 4 will increase. When this voltage being felt on
U3 pin 2 increases above the voltage on U3 pin 3 (set by the user) U3 pin 1
will go low (0 Volts). With 0 Volts on the gate of Q1 it turns off which causes
the Drain to Source Resistance to become very high. With Q1’s Drain to
Source resistance high the detected audio from U2 is allowed to pass
through C19 and R9 into the Audio Amplifier. Now received audio is heard in
the headphones.
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