[time-nuts] fast switching quiet synthesizer

Bob Camp kb8tq at n1k.org
Wed Oct 8 10:21:05 EDT 2014


Hi

On Oct 8, 2014, at 3:52 AM, Hal Murray <hmurray at megapathdsl.net> wrote:

> 
> bruce at ko4bb.com said:
>> Kratos   (www.kratosepd.com) do fast switching synthesiser subsystems that
>> can be locked to a reference.. 
> 
> What does "fast switching" mean in the context of a DDS?

Not much at all. Your DDS clock will be something like 1 GHz and it will “switch frequency (actually it switches phase)” on a clock edge.  You probably can’t load a billion updates a second through the update port. People do indeed load millions of updates a second.The DDS is quite happy to “follow” the updates. It’s one of the ways to get a clean(er) 10 MHz reference signal out of a DDS on a GPSDO. 

> 
> What does the spectrum of a DDS look like if I switch back and forth between 
> 2 frequencies at 1 KHz?  

Since it is phase coherent FSK with instantaneous phase change, it’s pretty much a textbook spectrum. Lots of energy at the switch points. It’s likely closer to the textbook than anything you can get out of a conventional (non DDS) signal generator. 


> Or what question should I be asking?

Is that waveform (and it’s spurs) OK in the application or not. Only Jim knows for sure …

Bob

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