Ramsey Electronics FX Series Manual page 71

diminish other signals while boosting signals in the 223 MHz region.
2. The First IF converts the incoming 223.50 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 223.50 + 21.40 = 244.90 MHz.
3. Supplying the needed 244.90 MHz "local-oscillator" input signal is the job of
the tunable 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
244.90 MHz local oscillator signal needed by U1's 1st IF Mixer. The VCO
signal is applied to pin 2.
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! This is where we get the ideas of
"dual-conversion" and "2nd IF." This time, though, no more "variable" input is
expected 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
and we can start
picture:
5. The 223.50 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 223.50 MHz VHF FM signal can become intelligible
audio input to the U2 speaker amplifier (Stage "B"), we're doing fine for now.
244.90 - 223.50 = 21.400 MHz
21.855 - 21.400 = 455 KHz
the 1st IF Mixer,
to see the whole