Re: #synth ref frequency #synth

F1BFU - Fr - 79

This is explain in assembly manual of Si5351A of QRPLabs.

"The SiLabs Si5351A chip is a cousin of the famous and popular Si570 chip, but is much less expensive. Unlike the Si570 however, the Si5351A has no quartz crystal inside. An external reference oscillator or crystal must be provided. The reference frequency may be 25MHz or 27MHz. In this module, a 27MHz crystal is used. This frequency is chosen because it is possible to configure the chip to produce the exact 1.46Hz tone spacing for WSPR, on any amateur radio band from 2200m (136kHz) to 2m (145MHz). A 25MHz crystal cannot provide WSPR tone spacing on the 2m band. In summary, the Si5351A chip synthesizes output frequencies in three stages. The block diagram of the chip is shown to the above (from the Si5351A datasheet). First, a crystal reference oscillator is multiplied up to internal frequency in the range 600-900MHz. There are two PLL’s in the chip, each may be selected to choose a different internal frequency. At the second stage one of the PLL frequencies is divided down to each of up to three required output frequencies. Both the upward multiplication to the internal PLL Output frequency, and the division down to the output frequency, use fractional ratios – an integer plus a fractional part consisting of 20- bit numerator and denominator. Optionally a third division stage may be configured to divide each output by a power of 2, for a maximum division ratio of 128. It is used for generating low frequency outputs between 8kHz and 500kHz."

73 QRO
Gilles F1BFU /Fr

Le mer. 21 nov. 2018 à 21:38, DC1MAK <dc1mak@...> a écrit :
Hi Hans, hi all,

somwhere i read about, that the reference clock of the Silab synth in a U3S MUST be at 27 MHz.
Of course this depends on the firmware of the U3S.
But i think, i heard that there is another cause.
Something with the 1.4648 Hz spacing of the wspr tones ?
Does anybody know about it and can explain that
to me ?

Gilles - F1BFU/FR

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