Wednesday, September 22, 2010

Frequency multipliers and their use

Sometimes, one may want to multiply a frequency by factors other than two (doubling, see earlier post).

Here is an example why you actually may want to do this. What about a crystal controlled 30m band (10.100...10.150MHz) signal generator to generate a frequency of 10.118MHz using cheaply available xtals?
  • 3 * 7.3729 - 12.000  =  22.1187 - 12.000 = 10.1187
Hmmm, now we have to triple one of the frequencies... Yep, no problem. The trick is to use generate a picket fence of harmonics and filter out the one we want.
The spectrum of a square wave signal consists of odd harmonics. Hence odd multiplications are available by simply "clipping" the original signal, in other words, convert it into a square wave, and filter out the harmonic of interest.
In the digital age, one may think of using a digital gate, e.g. XOR or NOT (inverter), to generate a nice frequency fence in the first place.

Back to our example, in order to create our signal, we would build a square-wave generator using a 7.3729MHz crystal, filter out the 22.1187Mhz contribution and mix it with 12.000MHz.

Closing with an academic one: One may want to multiply a signal by 6. This could be done by filtering the 3rd harmonic and double it by means of a diode doubler...