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R48 and C85 form yet another low pass filter to ensure that any 5 KHz
"whine" will not get into the VCO. Because the DC charge developed in C85
(.1 uf) would slow down the PLL during major frequency swings, such as just
going from transmit to receive, D8 and D10 are set up back-to-back across
voltage dropping R48.
Whenever there is a major frequency shift (which means a significant VCO
control voltage change), one way or the other, one diode or the other is
switched on to short out R48 and discharge C85. This lets the PLL re-lock
instantly; C85 recharges and the diodes become no factor in the circuit. The
"lock detect" output (pin 28) of this Motorola PLL IC is a fine feature that
could be used many different ways in this circuit. We could have set it up to
tell an LED just to alert you that you are "UL" (unlucky, unlocked??) Instead,
we decided to protect your investment in the transmitter RF section of your
transceiver and keep our FCC smiling. The lock detector gives a strong
series of pulses when the PLL is unlocked. When the PLL is locked, only a
tiny sawtooth wave appears at pin 28. The "lock detect" voltage is watched
by U5:B. If "unlock" pulses appear, they are integrated through R90 and C96
as a fairly clean DC voltage charge built up in C96. If this charge causes
U5B to swing low, bias is removed from Transmit Buffer Q10. No damage is
done, and no offending signals can be emitted.
We've toured "The Loop." Now, let's build it and enjoy what it can do!
STAGE G: PLL SYNTHESIZER CIRCUIT ASSEMBLY
Since our most immediate goal is a functioning, programmable receiver, it is
useful to know that the receiver portion could work fine WITHOUT the four
binary adder IC's (U7-U10) or the secondary diode programming matrix. In
practice, this proposition need NOT be pursued, because it would require 16
wire jumpers to connect the A inputs of the adders to the summing outputs.
Also, the programming formula would involve addition of the receiver IF
frequency. For example, to receive 146.52 MHz., we would have to
determine N as (146520 - 21400) ÷ 5. Consequently, we can see that it will
actually EASIER to install the 4 IC's, install diodes in the RECEIVE line of
the second matrix and proceed to find "N" simply by dividing our desired
frequency by 5 KHz.
CONSTRUCTION PROCEDURE NOTE:
The suggested order of assembly for the PLL Frequency Synthesizer portion
of your transceiver is exactly that: a suggestion. This stage involves some
repetitive work that may seem extra easy, but it also can become easier to
make mistakes: skipping connections, causing solder bridges, etc. Some
builders may prefer to push ahead and do all of the repetitive soldering as a
single operation, while others do better work with frequent breaks in the
phases that involve repetition of steps. The suggested order of assembly is
for the benefit of the latter group. If you wish to proceed differently, feel free
to do so. Just be sure to check off the respective assembly steps as you
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CCW
= see CW below.
COR
= "Carrier Operated Relay," a switching circuit activated by the
detection of a carrier signal in a receiving circuit.
"CW"
= TWO common meanings in electronics: "Continuous Wave" for the
communicating of Morse code signals, or "Clockwise" to designate a point
on a variable control. "CCW" therefore means "counterclockwise."
DECIMAL
= in computer or programming context, "decimal" refers to
counting by tens, our traditional way, in contrast to your computer's binary
(counting by 1's and 0's), or the hexadecimal system (counting by 16's.) See
"N" and BINARY.
DIP
= "Dual Inline Package," referring to IC's and their sockets or any other
component ("DIP Switch," etc.). The "line" refers to rows of PC-board or
perfboard holes that are 0.1" apart. "Dual" means two rows. Therefore "SIP"
would mean a SINGLE row device.
DISCRIMINATOR
= FM terminology for "detector." See: FM.
DUAL-MODULUS
= as in dual-modulus prescaler or divider: a circuit which
divides a frequency by two different ratios depending on pin selection, for
example: 64 or 65. (See: PRE - SCALE)
FILTER-ACTIVE
FILTER-CERAMIC
FILTER-CRYSTAL
Filters are used in DC, Audio and RF
FILTER-HI-PASS
circuits. Understanding them is essential
FILTER-LOW -PASS
electronics know-how. The Radio Amateurs’
FILTER-BANDPASS
Handbook covers the subject of filters very
FILTER-PASSIVE
well.
FM
("Frequency Modulation") = Changing the transmitter frequency in exact
pace with speech or sound variations.
FREQUENCY SYNTHESIS
= To "synthesize" anything is to create an
imitation or simulation from something else. The "basic" ways for generating
useful RF frequencies are a crystal oscillator or the L-C oscillator where the
frequency is determined by coil (L) and capacitor (C). However we can
process the output of an L-C oscillator through digital circuitry to simulate
many different, precise crystal-like frequencies, using only one crystal as a
reference standard. See also PLL, PHASE DETECTOR, VCO.
FSK
= "Frequency Shift Keying." TRUE FSK actually moves the RF
frequency (transmitted and received) from one pre-determined point to
another, in contrast to "AFSK" (Audio Frequency Shift Keying") where the
transmitted signal remains steady and all shifting is done by audio tones.
GROUND-PLANE
= ALL sections of a PC-board which are mechanically or
electrically connected to DC and RF ground. In the Ramsey FX design,
almost ALL of the top or upper side (component side) of the board is a
groundplane. The term is also used in antenna design: if your 2M or 1.25M
antenna has a vertical radiating element, plus four (or more) horizontal