[time-nuts] Close-in phase noise question...

[Bruce Griffiths]

Brian

Low frequency noise modulates the switching times of the mixer
components and hence produces close in phase noise.
Diode mixers especially those using Schottky diodes have lower flicker
noise than active mixers.

Passive FET mixers are not immune to flicker noise.
The level of such flicker noise increases with the RF current level.

Bruce


wa1zms at att.net wrote:
> Bruce-
>
> OK... So, linear operation does therefore seem to be the preferred
> way to operate these MMICs rather than operation into compression.
> That's what I seem to be observing if only because my final RF
> frequency is so high and RX bandwidth so low.
>
> Having said that, if my frequency synthesis scheme involves a mixer
> does the same effect of low frequency noise to phase noise conversion still
> take place? After all, the mixer element is typically into compression
> if it's a FET based mixer. I assume a diode mixer is more immune to
> similar effects?
>
> I'm trying to grow my intuitive understanding of the subtle sources of
> noise. But I don't recall Maas giving much info on this topic in
> his otherwise excellent text.
>
> As always, thanks for your sagely advice.
>
> -Brian
>
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of Bruce Griffiths
> Sent: Monday, December 15, 2008 5:00 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Close-in phase noise question...
>
>
> wa1zms at att.net wrote:
>   
>> Looking for comment here...
>>
>> The background:
>> I'm working on a sub mm-wave LO chain for
>> a ham radio application. While chasing issues
>> of close-in phase (ie: within 1KHz of RF
>> carrier) by peeling the "layers of the onion",
>> I'm starting to question the performance of
>> the MMICs that are used as buffers and amps
>> following my Wenzel reference OCXOs.
>>
>> Question(s):
>> Should any MMIC be allowed to be driven
>> close to compression or into compression
>> when striving for best close-in noise?
>>
>> I know and have seen the NF of a MMIC
>> degrade while in compression, but my
>> target right now is close-in noise rather
>> than broadband noise.
>>
>> My design, in summary, takes 5MHz up to 630GHz
>> via several multipliers and PLL stages.
>>
>> -Brian
>>
>>     
> Brian
>
> The increased nonlinearity when driven into compression will enhance the
> conversion of low frequency noise to phase noise.
>
> Bruce
>
>