Ramsey Electronics FZ-146 Specifications download pdf (Page 93)

FX-146 • 93

Stage E-F: The FX Transceiver VCO

The VCO (Voltage Controlled Oscillator) provides basic frequency control for

both transmit and receive modes. It is essential to understand its function in

the transceiver circuit. Q7 is the oscillator transistor and the key VCO

components are L7, D3 and D23.

Think of the VCO for now as just a simple 'VFO' type RF oscillator whose

output frequency can be changed by adjusting the slug in L7. Notice that

there are no capacitors, either fixed or variable, to form a 'tuned-circuit' with

L7. Instead, varactor diodes D3 and D23 perform this function. (A varactor

diode changes capacitance in step with the amount of voltage applied

across it.) If the output of this simple oscillator reached U1 through C35, you

would certainly be able to receive any signals on a frequency determined by

that oscillator frequency plus or minus 21.4 MHz, the Receiver 1st IF.

Similarly, if the oscillator output could reach Q9 and Q8 in the transmit

section, or even the transmit buffer, Q10, some sort signal could be put on

the air.

After the VCO is assembled on the PC board, the interested builder has the

option of experimenting with it in receive mode before proceeding. The VCO

must be capable of considerable frequency range. To receive a signal at 140

MHz, the VCO must tune to 140 MHz minus 21.4 MHz or 118.60 MHz. To

transmit at 160 MHz, it must tune to 160 MHz. This indicates a tuning range

of over 40 MHz!

For now, consider Q5 and Q16 to be buffer stages for oscillator Q7 and that

transistor Q4 does some nifty voltage filtering that we'll explain later on.

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 some precision 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. We know we can get stability by using a crystal oscillator,

but that would not allow us to operate on a multitude of channels without

changing crystals for each channel. 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 guess 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

understanding 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. The VCO is an integral part of the phase locked loop. You

can read ahead about PLL and synthesis in the next section, or you can

proceed with building and testing the VCO alone.

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Ramsey Electronics FZ-146 Specifications Page 93

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