Circuit Description - Ramsey Electronics AA7B User Manual

CIRCUIT DESCRIPTION

The AA7B is the next generation of our popular AA7. While the AA7 is a
great product it has been around for a long time in relative terms of electronic
equipment. Because of ever changing technology there comes a time when
certain components are no longer produced and those still available are very
expensive, which makes them unacceptable for our use. This was the case
with the AA7, hence the AA7B was designed.
The basic idea of the AA7B is to make a small antenna, like the whip
antenna supplied, act like a larger antenna, or an antenna with a gain factor.
Put in simple terms, it is an amplifier which will amplify the very small radio
frequency signals picked up by an antenna!
Let's take a look at the schematic diagram. We'll follow the signal from input
to output to get a general idea how this kit works, and why.
The power supply
The power for your AA7B is supplied from either an external 9VDC to 20VDC
power supply or an internal 9VDC battery. If an external supply is being
used it will connect to the AA7B by its plug being inserted into Jack J3. The
plug being installed causes the 'SW' terminal inside of J3 to disconnect from
the 'RING' terminal. When this connection is broken the negative terminal
of the 9 Volt battery is removed from the circuit, and power from the external
supply will be connected to VR1. This is done to protect the battery from
being damaged (i.e. blowing up, catching fire, etc) by applying power from an
external source. NEVER attempt to charge a non-rechargeable battery.
NEVER attempt to charge a rechargeable battery without the proper
charging system. If the plug is removed from J3 the contact between 'SW'
and 'RING is reconnected and the battery power is reconnected to VR1.
VR1 is a 5 volt regulator that provides clean regulated 5 volts to all the active
devices in the AA7B. Capacitors C5 and C13 are filter capacitors and along
with VR1 assure a clean DC supply. R11 limits the current used by power
indicator LED D1 to less than 1mA.
Notice that the output of VR1 is connected to U2 Pin 14 even with the power
switch (S1:B) in the off position. This is the power connection to U2 and is
necessary because U1, U2, and U5 need power to operate properly even
when the unit is off. BUT WAIT! Isn't the battery going to run down because
there is still power being consumed? The amount of power required to
operate VR1, the relays, and U2 is less than or equal to the amount of power
to operate your watch. Since the capacity of a 9 volt battery is many times
greater than a watch battery, if you turn off your AA7B and come back in a
year you'll probably find it still working fine!
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AA7B 4