[time-nuts] low power, but quiet, oscillators

Magnus Danielson magnus at rubidium.dyndns.org
Mon Feb 6 19:35:40 EST 2017


On 02/07/2017 12:36 AM, jimlux wrote:
> On 2/6/17 2:37 PM, Bob Camp wrote:
>> Hi
>> One of the most basic reasons for putting out > +20 dbm is that you
>> had a spec of -195 dbc / Hz for the noise floor :)
>> Some of these specs *are* a bit mutually exclusive.
> Sure.. And to be honest, I'm not sure that some of the folks coming up
> with paper requirements for these speculative low power transmitters are
> aware of that.  They take dBc values from 1 Watt transmitters and assume
> you can meet that with your 1 mW transmitter.
> Then again couldn't you cool your oscillator.. that gets the T part of
> the kT down lower <grin>
> Cool that puppy down to <1K and get 25dB noise improvement, eh?

Your 50 ohm termination resistor will be a great source of that noise.
For a narrow-band fixed signal you can terminate with whatever reactive 
network you feel confident with instead. If you match impedance well 
enough it will work fairly well. Some oscillators have far-out 
impedances far from 50 Ohm anyway so impedance matching is so-so and 
most of the noise comes from the termination resistor.

Besides, for the deep space stuff you have cheap access to 2.7 K or so 
anyway, right? :)


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