Ramsey Electronics FZ-146 Specifications download pdf (Page 102)

FX 146 • 102

Inserted into this circle is the frequency synthesizer (U6 with U3), which

compares the output of the VCO with the output of its own 10.24 MHz.

Reference Oscillator (see Y2) and also with the frequency programming

which you have set up. After making these comparisons, it gives an

instruction to the VCO in the form of a precision voltage applied to the

varactor diodes, and the cycle repeats itself at lightning speed.

Try dividing 10240 KHz by 2048 on your calculator. If you get an answer of 5

(KHz) you are correct and perhaps on your way to seeing how the FX

transceiver can be programmed in 5 KHz steps! If you are curious about U3,

its job is to divide the VCO's VHF output down so that U6 need only analyze

("count") 1/64 of it in order to send voltage adjustments back to the VCO.

This prescaling keeps the input frequency to U6within specifications.

"Will the circle stay unbroken?" asks an old folksong. If the PLL's circle of

activity is broken by incorrect phasing, the loop is said to be "unlocked." If all

is going well, the loop is locked. The loop is UN-locked only when the phase

detector gets so unhappy that what is being fed into its two inputs is so far

"out of whack" that it cannot make corrections. If the loop is unlocked, no

useful control voltage reaches the VCO. If the VCO operates at all, it will be

at a wrong or unknown frequency.

The phase "detector" can also be called a phase "corrector." It does more

than just detect or look at the two incoming 5 KHz signals. It sends

correcting pulses to the VCO to keep it in phase with the 5 KHz output of the

crystal reference oscillator.

A good word for understanding "phase" is synchronization or "sync." The

process of tuning a musical instrument is a good illustration of sync or

phase. When two guitar strings are very close but not exactly on the same

note, there is a discernible third sound, a sort of rumble that disappears as

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Ramsey Electronics FZ-146 Specifications Page 102