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

Luis Cupido cupido at mail.ua.pt
Mon Dec 15 21:44:46 EST 2008


Brian,

I also think the linear operation would be better, but not
so sure if bipolar transistors wouldn't be preferred
over MMIC's for this (on bottom part of the spectrum)
(ok... is not so handy).

What I can certainly add to the discussion is that
power amplification followed by higher rate varactor
multiplication is considerably better than a multiple
lower multiplication ratios chain.
I had that experience on the 411GHz where from a 70MHz(*)
xtal osc amplified to several Watt driving varactors and
cavities. I jump with few steps to about 45GHz as opposed
to a DB6NT like LO chain which was noticeably worst in
the close in noise.

Using mixers without driving hard the LO or the RF with
same marginal level on both ports is possible but you will
be in trouble with level settings :-(
Not sure how much you would gain there... theoretically
something... but then AM to PM conversion is against you so
not sure if better or worst.

Luis Cupido
ct1dmk.

(*) x12 x9 x3 x2
w/ 9th harm corner cube harm mixer
if my memory serves me well....


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
> 
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