AA7B
•
6
to Q3 pin 4 from the antenna.
The last thing in the signal path is U5, another electronic relay like U1. The
signal from Q2 is coupled to pin 1 of U5. For this discussion U5, like U1, is
configured to connect pins 1 and 5 together and the signal connected
through C12 to the output jack J2. This is the connection that goes to your
receiver’s antenna connection.
You may be wondering what the other capacitors, like C2, C4, C6, C8, C9,
C14 and C15 are for. Remember that a capacitor allows an AC signal to
pass through it but blocks DC. These capacitors essentially short the RF
signals that may appear on the power supply lines to ground because they
have very low resistance to the RF signals. The schematic only indicates the
electrical location in the circuit of the capacitors. They may even appear to
be all tied to the same place. However… if you look at their physical
placement on the circuit board you will notice that they are usually placed
very near a component, look at L1 and C6 for example. The physical
placement of the capacitor is very important in relation to other factors on the
board. At RF frequencies a piece of wire or copper trace on a circuit board
can act like an inductor if it is too long. This can have undesirable effects on
the circuit operation and will cause it not to work under the right conditions.
The amplifier bypass circuit
If you look below Q3 on the diagram you will see U2:A and U2:B. These are
CMOS 74HC00 logic NAND gates. Without going into the NAND gate
operation, just know that if a low signal is applied to the input, U2:A pins 1
and 2, the output pin 3 will be high. In other words the output is the opposite
of the input. The output of U2:A, pin 3, is connected to the input of U2:B,
pins 4 and 5. This means that the output of U2:B, pin 6, will be the same
level as the input of U2:A and the connection between U2:A and U2:B will be
opposite. There are actually 4 of these gates in U2, but we only need two.
The extra two gates have their inputs grounded to keep them from causing
noise.
Here’s how we control our electronic relays: Pins 4 and 6 of U1 and U5 are
connected to U2:A and U2:B’s outputs respectively. When power is turned
on a high level is applied to U2:A’s input. This makes U2:A’s output low and
U2:B’s output high, therefore the relays have pins 5 and 1 connected so the
signal is amplified. When power is turned off U2:A’s input goes low, the
relays switch, and pins 5 and 3 connect. If you follow the signal from J1
through U1 you will see that the signal is connected through C11 to U5 pin 3
and then out U5 pin 5 to J2. This has connected the antenna through the
AA7B directly to your receiver!
