Ramsey Electronics FZ-146 Specifications download pdf (Page 101)

FX-146 • 101

Stage G: The FX- Transceiver PLL Synthesizer

A reassuring feature of the FX Transceiver is that if you build it correctly and

make just a few adjustments, it will work fine whether you understand how it

works or not. For most amateurs and beginners, the functions of the PLL

Frequency Synthesizer and VCO will present the greatest challenge to

understanding. In fact, these stages, which are at the heart of the

transceiver's design, are not easy to explain in everyday terms, because

some level of engineering training or serious amateur curiosity is definitely

required to catch on to the many concepts and principles involved. In short,

don't feel bad if the functions of U5 through U10 seem somewhat mysterious

to you at first.

The author's suggestion is that you study the FX transceiver's more complex

sections simply for the fun and satisfaction of it, catching on to a little bit here

or a new concept there as you go along. You'll get some insights from our

published circuit descriptions, and you'll pick up more from studying other

publications such as the ARRL HANDBOOK, discussing the circuit with

friends with engineering backgrounds, or doing whatever helps you best

catch on to new ideas and concepts.

The VCO, integral to the PLL, was discussed and tested in the preceding

assembly stage.

What we have so far is a master oscillator for the transceiver that can be

tuned by varying the voltage applied to the varactor diodes. All we really

need now is a super reliable way of controlling the voltage applied to those

varactor diodes so that we can put that oscillator on any VHF channel we

want with 5 KHz precision. And this control obviously must be extremely

stable, since a change of only a few picofarads can tune through many MHz

of VHF spectrum. How can we get such precision performance from small

tuning diodes, a very ordinary shielded coil, and a transistor with its

supporting capacitors and resistors?

We assume already that the answer must lie in the functions of the

Frequency Synthesizer IC. However, the key to grasping what this IC really

does lies in grasping TWO concepts: frequency synthesis and the "Phase

Locked Loop" or PLL. The more of a handle that you can get on these two

interconnected ideas, the less mysterious will be your FX transceiver as well

most other modern radio gear from ham transceivers to car stereos to

cellular phones. To begin, let's over simplify as much as we can.

UNDERSTANDING THE PLL:

In the following block diagram, the notion of "Loop" is seen in the simple fact

that the VCO output is routed right back into the VCO itself via the other

circuit sections. Try to imagine the unending and rapid circle of activity in

which the VCO output is repeatedly affecting its own oscillating frequency,

self-adjusting many times per second. Hence, an un-ending circle or loop.

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Ramsey Electronics FZ-146 Specifications Page 101