Creating high frequency waves with low frequency crystals

[loopy]

G'Day all,

Is there any known method whereby you can generate a high frequency signal (say 10GHz) using a lower frequency crystal (say 10MHz)? Or is this limited by Nyquist's theorm of sampling?

Cheers,


Brad

 

[Nigel Goodwin]

Re: Creating high frequency waves with low frequency crystal

G'Day all,

Is there any known method whereby you can generate a high frequency signal (say 10GHz) using a lower frequency crystal (say 10MHz)? Or is this limited by Nyquist's theorm of sampling?

It's commonly done, you simply use frequency multipliers to increase the crystal frequency - although such a large change would require a good few stages.

A more modern technique would be to use a PLL, and lock a free running oscillator to a lower frequency crystal.

 

[loopy]

Thanks Nigel - can you explain (or point me in the direction of a good tutorial) how frequency multipliers work, especially in conjunction with a PLL? The way I understood it was a PLL was just a system for creating an accurate signal?

Cheers,


Brad

 

[Nigel Goodwin]

Multipliers are simply amplifier stages, with a tuned output - so you input (for example) 12MHz, and tune the output to 24MHz (the second harmonic), this gives an output at twice the input frequency. The next stage would be tuned to 48MHz, doubling the frequency again, the final multiplier could be tuned to 144MHz (third harmonic), giving a total of 12 times multiplication.

PLL's work differently, by division, rather than multiplication.

To use the same 12MHz to 144MHz example again, you would have a free running VCO (voltage controlled oscillator) at around 144MHz, and a fixed crystal controlled oscilator at 12MHz. The output of the VCO is applied to a chain of frequency divider chips (digital logic), which divide by 12 times - thus giving an output roughly around 12MHz.

This output is then compared with the crystal output, using a phase comparator - this produces an error voltage dependent on the error between them. This error voltage is used to feed the VCO - correcting it's frequency until the two signals (12MHz crystal and divider output) are identical. This is an ongoing process, and keeps the 144MHz output accurately locked to the 12MHz reference.

 

[loopy]

Thanks again Nigel - that's pretty much how I understood PLL's to work, but the frequency multiplier stuff is interesting....

Thanks again



Brad

 

[Nigel Goodwin]

Thanks again Nigel - that's pretty much how I understood PLL's to work, but the frequency multiplier stuff is interesting....

It's a standard transmitter technique, going back to valve transmitters. Crystals only have a limited frequency range, so for higher frequencies you have to multiply it.