Ramsey Electronics FX-440 Manual page 83

theoretical test signal of 446.00 MHz as well as EVERY other signal in
the radio spectrum. Since that's a bit much to expect the receiver to
handle, we will build up that network of filters, tuned circuits and RF
preamplifiers that constitutes Stage ·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 440 MHz region.
2. The First IF converts the incoming 446.00 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
446.00 - 21.40
=
424.60 MHz.
3. Supplying the needed 424.60 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 424.60 MHz local oscillator signal needed by
U1 's 1st IF Mixer. The VCO signal is applied to pins 18 and 19 in a
·push-pull" effect through T1.
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
~et
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:
446.00 - 424.60
=
21.400 MHz
21.855 - 21.400
=
455 KHz
5. The 446.00 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 MC3335 employs a conventional quadrature detector. Inductor
L 1 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 MC3335 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 446.00 MHz. UHF FM
signal can become intelligible audio input to the U2 speaker amplifier
(Stage "B"), we're doing fine for now.
FX-440
Assm. 23