[time-nuts] 10MHz standard for comms receivers

Bob kb8tq kb8tq at n1k.org
Mon Oct 1 17:24:31 EDT 2018


Hi

At least on the parts I’ve used, spread spectrum is a software controlled feature.
You use it or not depending on what you are trying to do. There are a lot of systems
out there that have fairly tight timing needs (though not time nut level stuff). 

Yes, this all *assumes* you are writing code from scratch for the micro. That may or
may not be the case……

The main reason for spread spectrum is to make EMI requirements easier to pass. 
Once you get to a small enough part, it really doesn’t generate all that much EMI internally.
Again, a lot of assumptions get into that. 

Bob

> On Oct 1, 2018, at 3:54 PM, jimlux <jimlux at earthlink.net> wrote:
> 
> On 10/1/18 10:44 AM, Dana Whitlow wrote:
>> Isn't it the case that computer clocks these days are subject to two
>> influences that make them worthless for timing?
>> 1. Deliberate random FM to spectrally spread RFI leakage.
>> and
>> 2, Wild variations of clock speed according to usage needs of the moment,
>>     in order to reduce average power consumption and thermal loading.
> 
> 
> yes and no..
> 
> microcontrollers and things with microcontrollers do use spectrum dithering, it's less common in a PC.
> 
> While the "instruction rate" might vary with the needs of the moment and die temperature, there's usually some clock and corresponding counter that runs at a relatively constant rate so the CPU knows what time it is.
> 
> That said, a 10 ppm (or even 50 ppm) oscillator in a PC would be "high performance"
> 
> My macbook air shows 56.652 in ntp.drift
> 
> 
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