FX 146
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Stage M: Microphone Amplifier and PTT Circuit
If you have studied all preceding circuit explanations, you have a good idea
of what the Microphone and PTT circuitry is supposed to accomplish.
Understanding our design clearly and assembling it correctly will save many
headaches and will ensure reliable FX transceiver operation.
U4 is a "quad op amp" which means 4 operational amplifiers in one DIP
package. Two are used as a conventional microphone gain amplifier, and
the other two are used in the PTT (push to talk) circuit.
Capacitor C83 couples microphone audio to U4A and isolates the audio
(AC) from the PTT circuitry (DC). Op amps are designed to run from both a
positive and a negative voltage source. U4 is powered by a 8V
supply through the use of a voltage divider network (R59, R40). The gain of
the amplifier is established by the ratio of R56 to R58. A passive low pass
filter is formed by R51 and C89. The B section of U4 and its associated
components form an active low pass audio filter. The output of U4B is fed
through C62 to modulate the VCO control voltage as explained in Stage E-F.
Trimmer R46 permits adjustment of modulation level.
The purpose of Q11 is to shunt the microphone circuit straight to ground
during receive, so that it cannot possibly disturb the VCO. An accessory
modulation input is provided at PC-board point "PL" for direct injection of
DTMF or CTCSS tones, etc.
The PTT circuit is designed to accommodate the popular ICOM- compatible
speaker-mikes. Notice that a single line at J4 serves both audio and PTT
functions. The one shielded wire into the microphone takes care of not two
but three functions which could involve three conductors and a more
complex jack.
Three functions? First, we need to supply audio output from the microphone
element to the amplifer. Next, we need some kind of PTT switching
connection. Third, the electret microphone itself needs a small amount of
voltage to operate its internal FET source follower transistor.
Here's how we do it with one mike line. Pushing the button simply connects
the microphone element to the line. About 2 volts through R60 and R57
operate the microphone element which sends audio through C83 to U4A.
PNP transistor Q12 senses the tiny current draw of the microphone element
and switches the 8 volts at the emitter through to the collector. To state it
very simply, the output of U4 turns off PNP Q13 which had been supplying 8
volts to all "+8R" points of the circuit. And the output of U4C switches on
PNP Q14 to supply all "+8T" points.
Diodes D11 and D12 assure positive action, that Q13 and Q14 are fully on
or fully off when the op amp outputs swing. Releasing the mike button
instantly reverses the status of Q12, Q13 and Q14 to return to receive mode.
R70 limits the current drawn by D17 to a safe level. This LED usefully
FX-146
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There are several methods for quickly finding the required binary code for a
particular frequency and its "N" number:
1.
Descending Subtraction (see Programming Worksheet)
2.
Printed reference lists (see Popular 2 Meter Frequency Pairs)
3.
Computer programs (see our sample BASIC program)
We recommend strongly that you fully understand how to make the
calculation yourself, because that is your ONLY means for checking the
accuracy of printed information, computer programs or the operation of
experimental programming circuits. Even though there are 16 matrix
positions to program, there are some shortcuts to make the job easier for
normal ham band operation. Consider the upper and lower band edges.
Notice the values of the highest 6 positions are the same throughout the
band. We still must program in those six positions but we only need to
calculate for the remaining 10 (512 through 1) to program any 2 Meter band
frequency desired. The simplex calling frequency of 146.52MHz is the
demonstration and alignment standard for the FX-146 model.
"N" is quite easy to determine:
"N" for 146.520 MHz = = 29,304
The placement of diodes in the Programmable Offset Matrix follows the
same binary number principles as used for frequency programming. This
matrix is connected to the 16 programming inputs of U6 through the four 4
bit binary adders (U7-U10). Fewer programming positions are provided on
the board simply because there is no practical use for extremely large or
very tiny offsets. The 1 bit to 8K range provides plenty of flexibility for non-
standard channel spacing.
U7 through U10 are called "4 bit" binary adders because they each can
handle four binary addition operations. For each bit, there are A and B inputs
and one S (sum) output. Examine the schematic diagram closely, and you
will see that all the frequency programming lines are connected to "A" inputs
and all offset lines go to "B" inputs. Notice further that the binary positions of
both matrixes correspond to each other exactly: the 8K offset position goes
to B1 of U7 and the 8K frequency programming position goes to A1. Their
sum appears at S1 (pin 1) and goes to U6. And so forth for all the other
binary programming positions.
The programming for receive mode and standard repeater offsets is
silkscreened on the PC board itself. Assembly Stage H explains the theory
behind these positions. The +RPT "N" numbers are calculated in the same
way as for the Frequency Programming matrix. -RPT, RECV and other
"minus" offsets are calculated by straightforward "2's Complement" binary
addition. See Stage H for examples.
