Ramsey Electronics FZ-146 Specifications download pdf (Page 82)

FX 146 • 82

"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

1 2 ... 77 78 79 80 81 82 83 84 85 86 87 ... 144 145

No comments

Ramsey Electronics FZ-146 Specifications Page 82