Ulrich Bangert wrote: > Hi foks, > > I want to put forward a similar but slightly different question: > > Suppose I need an clock running at around 50 Mhz for an DDS. Because of > the DDS it need not be exactly 50 MHz, can be 52 or 54 MHz too. > Basically this clock shall be derived from a 10 MHz source (OCXO, > Rubidium...) The OUTPUT of the DDS is to be used as an frequency > standard, with the DDS being an complete digital steering circuit. > If I have the choice to use > > a) an harmonic X5 multiplier for the 10 MHz signal > > or > > b) a 54 MHz VCXO with the following specs: 0.8 ps RMS jitter, noise > floor -145 db @ 100 kHz offset phase locked to the 10 MHz > > what is the prefered solution? Or is the answer dependent on what I plan > to use the frequency standard for? > > TIA > Ulrich Bangert, DF6JB > > Ulrich
Whilst in general the answer does depend on the application the following observations concerning the phase noise floor of the ~50MHz signal may be useful. With a state of the art OCXO with a phase noise floor of less than -170dBc/Hz multiplying by 5 with a low phase noise multiplier will raise the phase noise floor to around -156dBc/Hz somewhat less than that of your proposed VCXO. However if your 10MHz standard has a phase noise floor higher than -160dBc/Hz the 54MHz VCXO will have a lower phase noise floor. The phase noise at offsets closer to the carrier will usually be less when multiplying a low noise 10MHz reference than for a higher VCXO. If you only need to adjust the frequency by a few ppm then one can cleanup the spurs and phase noise of a DDS reducing them to very low levels by using a cascaded mix and divide technique like that in: http://www.karlquist.com/FCS95.pdf With such a circuit you can achieve a phase noise floor (if you use appropriate dividers especially in the last stage) approaching that of a good OCXO. With this technique there is no need to use a ~ 50MHz reference for the DDS if all you want is a corrected 10MHz signal. Bruce _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
