replacing crystal with synthesizer+vco

[HMZ]

hi
I want to replace a set of crystals in an oscillator with a circuit that can sweep frequency range from 34-62Mhz with same frequency stability .
I thought to use a synthesizer (Lmx2306)+VCO(MC1648) in pll loop but the vco range for this purpose is not proper .
Is this good way ?if not what shall i do?
thanks

 

[biff44]

You can easily do that. But you should realize that any spurious tones or phase noise on the "clock" input will be degraded by 20 Log N. In other words, lets say you had your 34 - 62 MHz "clock" synthesizer set at 34 MHz with 50 dBc down spurs at 1 MHz offset and phase noise of -80 dBc/Hz at 10 KHz. From this you wanted to generate a 3400 MHz output from the main PLL. Your 3400 MHz signal will have spurs 1 MHz away that are 10 dBc down, and the phase noise will be at least as bad as -40 dBc/Hz at 10 KHz.

20 Log of 3400/34 = 40 dB. It is hard to beat the goodness of a single frequency crystal oscillator!