Ramsey Electronics WCT3 Manual page 6

amplifier stage formed by U1B and the resistors R5, R6, R8 and capacitor C7.
U1B is a high gain, high frequency, high input resistance, low noise amplifier.
There are 4 of these amplifiers in the U1 package and one is used for each of
the 4 sections of the WCT3. The reason an amplifier with the characteristics
of U1 is needed is because the detected signal level is extremely small, actu-
ally in the micro volt, (that's 0.000001 volts), range. The amplifier in the case
of the 125KHz signal amplifies the signal about 330 times so, for example, a
signal level of 100 micro volts, (0.0001 volts), will result in a level of 33 mil-
livolts, (0.033 volts), at the output of the amplifier. This gain is determined by
R6 and R8 and is calculated by dividing the value of R6 by R8. So R6 / R8 =
330,000 / 1000 which equals 330. Believe it or not we now have a signal level
that is easy to work with.
Now that we have a workable signal level it is time to indicate that a signal has
been detected. This is the job of the comparator circuit formed by U2B and
resistors R14 and R15. U2 , like U1, has 4 identical parts with one being used
for each of the WCT3 sections. What U2B does is look at the signal applied to
U2B pin 6 from amplifier U1B pin 7 and compares it to a predetermined volt-
age level set by R14 and R15 on U2B pin 5. This is a simple voltage divider
and for the 125KHz signal provides a level of about 685 millivolts. You will
notice that this level is shown on the schematic near the junction of R14, R15
and U2B pin 5. If the voltage level applied to U2B pin 6 from the amplifier
goes above 685mV the output, U2B pin 7, goes low and LED D7 lights indicat-
ing a signal has been detected.
Operation Of The Other Sections
If you look at the other sections of the schematic you will notice that they all
look about the same as the 125KHz section. The component values are dif-
ferent for each section but the basic operation is the same. The major differ-
ence is in the Input/Filter sections because it is necessary to allow signals of
different frequencies to be detected. Notice that the 20KHz section uses the
same transponder coil but has a different value for capacitor C17 compared to
C4 in the 125KHz section so only signals in the 20KHz frequency range will be
detected. Changing the capacitor value changes the resonance of the circuit.
The transponder coils, L1 and L2, are identical and have an inductance value
of about 7.5 millihenries. To learn more about resonance try searching on the
Web for "resonance frequency". There is lots of information available.
For the 315/433MHz section the filter has a different type of coil. It actually
works the same way as L1 and L2 but because of the much higher frequency
the coil is constructed differently. In this case the coil, M2, is actually formed
by a copper trace which is physically part of the circuit board. This is called a
strip-line inductor and makes it much easier to accurately construct the coil.
It's inductance value is about 0.18microhenries (0.00000018 henries), much
smaller than L1 and L2. If you look at the circuit board you will see M2 which
WCT3  6