FX 146
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"DR" of this project. The purpose of the "DR" circuitry is to filter out or at
least diminish other signals while boosting signals in the 146 MHz region.
2. The First IF converts the incoming 146.52 MHz signal down to 21.4 MHz.
In order to do this job, the IC's 1st IF Mixer needs ANOTHER frequency
source to mix with the signal presented by the antenna through the filters
and preamps of Stage DR. This other signal must be VERY precise since
the mixer output is fed into a very sharp crystal filter at 21.40 MHz.
Specifically, the 1st IF mixer seeks a second signal that is 146.52 - 21.40 =
125.12 MHz.
3. Supplying the needed 125.12 MHz "local-oscillator" input signal is the job
of the tuneable or programmable oscillator section of any receiver, whether
AM-FM-SSB, HF or VHF, etc. In this case, take it on faith that the VCO
(Stage F), controlled by the PLL Frequency Synthesizer (Stage H) will
deliver the precise 125.12 MHz local oscillator signal needed by U1's 1st IF
Mixer. The VCO signal is applied to pin 1.
4. No matter what we tuned in, the 1st IF Mixer section of U1 delivers the
incoming signal at 21.4 MHz to the next section of U1. This next section
wants to do yet ANOTHER frequency conversion! Which is where we get
the ideas of "dual-conversion" and "2nd IF." This time, though, no more
"variable" input is expected from beyond the FM IC's basic functions. Crystal
Y1, 21.855 MHz, completes another internal oscillator. Its output is mixed
with the steady 21.4 MHz signal from the 1st IF Mixer, and we can start to
see the whole
picture:
5. The 146.52 MHz signal at the antenna has gone through quite a sorting
and converting process. It now appears to additional sections of U1 as a 455
KHz signal that needs "demodulating," a way of saying: detecting, analyzing,
decoding, or just making something intelligent out of it all. The MC13135
employs a conventional quadrature detector. Inductor L1 is the quadrature
coil, requiring a simple one-time adjustment.
The exact process of "detecting" intelligible FM voice or data from the 455
KHz 2nd IF is the job of the remaining sections of the MC13135 IC. Because
there's much more of this transceiver circuit to discuss and understand,
please study other sources if you are not clear on concepts such as Limiter,
FM Discriminator, Quadrature, phase shift, and so forth. As long as you just
see the general flow of how a 146.52 MHz. VHF FM signal can become
intelligible audio input to the U2 speaker amplifier (Stage "B"), we're doing
fine for now.
146.52 - 125.12 = 21.400 MHz
21.855 - 21.400 = 455 KHz
FX-146
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65
CRYSTALS:
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1 10.240 MHz, has 2 leads (Y2, PLL reference frequency)
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1 21.855 MHz, has 2 leads (Y1, receiver IF oscillator)
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1 21.4 MHz crystal filter, has 3 leads (FL1, 1st IF filter)
INDUCTORS:
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1 455 KHz shielded inductor, marked LB53303HK (L1)
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1 Shielded variable coil, marked 8488 5-5 (L7)
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4 .015 uH, 1.5 turns, .125" diameter (L2,5,6,14)
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1 .04 uH, 4 turns, red color, (L10)
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1 500 uH noise suppression choke, large with black shrink tubing over
the body (L20)
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2 .33 uh., upright-style green molded mini-inductor: markings include 2
orange stripes (L11,L21)
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1 .33 uh., axial leads, wirewound with orange-orange-silver bands (L18)
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2 2.2 uh. axial leads, molded inductor, marked with two red, one gold
bands (L8,L17)
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1 tinned wire to make: 1.5 turn, .375" diameter, hand-wound (L13)
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4 tinned wire to make: 2.5 turn, .375" diameter, hand-wound
(L12,15,16,22)
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2 small size tinned wire and cores to make: RF chokes, hand-wound on
ferrite cores (L9,L19)
SPECIALIZED COMPONENTS:
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1 Ramsey FX-series main and low pass filter printed circuit boards
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2 ferrite cores (for making L9,L19)
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1 455 KHz ceramic filter, molded cube, 3 pins. (FL2)
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1 100 pf SMT chip capacitor (C75)
INTERNAL ALIGNMENT COMPONENTS:
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1 10K trimmer potentiometers, marked 103 (R46)
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3 35 pf trimmer capacitors (C74,C81,C84)
FIXED-VALUE CAPACITORS (RF-critical picofarad values):
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3 3.9 pf (C66,C79,C94)
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2 8.2 pf (C27,104)
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3 10 pf (C21,C56,C82)
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1 15 pf (C76)
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1 18 pf (C73)
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2 22 pf (C51,C78)
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1 27 pf (C14)
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4 39 pf (C71,C72,C80,C87)