[time-nuts] AM vs PM noise of signal sources

Bob kb8tq kb8tq at n1k.org
Sun Jan 7 09:57:23 EST 2018


Hi

> On Jan 7, 2018, at 8:02 AM, Ulrich Rohde via time-nuts <time-nuts at febo.com> wrote:
> 
> This and related topics are  presented in :
>  
>  
> [BOOK] The design of modern microwave oscillators for wireless applications: theory and optimizationUL Rohde, AK Poddar, G Böck - 2005 - books.google.com
> Delivering the best possible solution for phase noise and output power efficiency in 
> oscillators This complete and thorough analysis of microwave oscillators investigates all 
> aspects of design, with particular emphasis on operating conditions, choice of resonators 
> and transistors, phase noise, and output power. It covers both bipolar transistors and FETs. 
> Following the authors' guidance, readers learn how to design microwave oscillators and …
>   Cited by 198 Related articles All 6 versions 
>  
>  
>  
>  
> 73 de Ulrich , N1 UL 


…. There’s also a few papers out there on the topic. I seem to remember sitting
through a few you wrote .

Bob

> In a message dated 1/7/2018 7:07:56 AM Eastern Standard Time, donaldbcollie at gmail.com writes:
> 
>  
> Does any limiter, soft or hard, [and perhaps any nonlinearity of power
> term 3 or greater in the amplifier of an oscillator] cause the "baseband
> 1/f noise to translate up to the resonator frequency [a form of
> crossmodulation]?. I wonder this because
> phase noise vs freq plots look a bit like the 1/f plots of a resistor, or
> active device, or power supply. Ceramic caps, and resonators [I`m thinking
> of quartz crystals] don`t pass much DC, and as I understand it, 1/f noise
> is associated with dc passing through resistors, or semiconductors. So the
> best way to go might be to have a very linear amplifier, which exhibits
> very low noise [perhaps 150dB below the operating level], with an AGC loop,
> that sets the operating levela little below the level at which the amp
> starts to clip - this could be done with a thermistor to avoid the AGC loop
> altering the [optimised] operating conditions of the amp. Alternatively you
> might be able to use a tetrode device like a dual gate MOSFET, and apply
> the AGC to the second gate. Thus you could keep the extremely linear amp
> extremely linear. [150dB below 1Volt RMS is 0.032uV RMS].
> Cheers!........................................................................................................................................................................Don
> ZL4GX
> 
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> On Sun, Jan 7, 2018 at 3:12 PM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> 
>> One point about oscillator design I've not yet seen mentioned is this: the
>> limiter
>> must not degrade the resonator Q when in action. Hence, a pair of diodes
>> connected in parallel back to back, across a shunt resonator, would be a
>> bad
>> thing to do from the perspective of low phase noise. A differential
>> amplifier
>> that limits by running out of current on peaks, driving a shunt resonator,
>> is
>> a much better way even though one pays a price in having more transistor
>> noise in the circuit.
>> 
>> I've long wondered if a very slow AGC might avoid the nonlinear mechanisms
>> issue except, of course, for things happening within the AGC loop's
>> bandwidth.
>> Is anybody reading this aware of what the truth really is?
>> 
>> Dana
>> 
>> 
>> On Sat, Jan 6, 2018 at 4:29 PM, Magnus Danielson <
>> magnus at rubidium.dyndns.org
>>> wrote:
>> 
>>> 
>>> 
>>> On 01/06/2018 10:31 PM, Joseph Gwinn wrote:
>>>>> ------------------------------
>>>>> 
>>>>> Message: 2
>>>>> Date: Sat, 6 Jan 2018 09:19:31 -0500
>>>>> From: Bob kb8tq <kb8tq at n1k.org>
>>>>> To: Discussion of precise time and frequency measurement
>>>>> <time-nuts at febo.com>
>>>>> Subject: Re: [time-nuts] AM vs PM noise of signal sources
>>>>> Message-ID: <DDEF34DD-AD21-44C6-9612-D877881078E5 at n1k.org>
>>>>> Content-Type: text/plain; charset=utf-8
>>>>> 
>>>>> Hi
>>>>> 
>>>>> The key point missing is the fact that any real oscillator must have
>>>>> a limiter
>>>>> in the loop. Otherwise it will “create one” by going over the max
>>>>> output of this or
>>>>> that amplifier. To the degree that the limiter has issues (limits
>>>>> poorly) you will get
>>>>> AM noise.
>>>> 
>>>> Hmm. Not strictly true. One can also use an AGC loop, like a wein
>>>> bridge oscillator. That said, some kind of softish limiter is commonly
>>>> used.
>>> 
>>> Regardless what non-linear mechanism in play, this remains a non-linear
>>> mechanism that achieves the goal. Choose wisely.
>>> 
>>> Cheers,
>>> Magnus
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