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

Bob Camp kb8tq at n1k.org
Mon Feb 6 21:14:21 EST 2017


> On Feb 6, 2017, at 7:35 PM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> Hi,
> 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? :)

Ok so, I have a device that puts out 1 mw (0 dbm) of power. I want the phase noise to be -195 dbc. What 
source and load resistance do I use?  :)

I *can* use an infinite load, that will get me a whopping 3 db of noise improvement. That only 
leaves another 17 or so db still to be found. Of course I’m not going to deliver 1mw into
an infinite load so we now loop back through what does putting out 1 mw really mean? 

If my source is purely reactive I have a phase angle between voltage and current. I now have energy
coming out and not power. Back to the same question about 1 mw. 

Yes you can run in and out of rabbit holes for weeks on this one :)


> Cheers,
> Magnus
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