[time-nuts] Noise Floor

kb8tq at n1k.org kb8tq at n1k.org
Fri Mar 27 00:05:42 UTC 2020 

Hi

So here's an OPA 693 running on the TI demo board. One trace has a 50 ohm
termination on the 
input. The other trace has no termination. Yes that's not the best way to
run a splitter. Input signal
level to the splitter is same / same for both runs. 

No big surprise, phase noise is almost 6 db better. ADEV is still doing
pretty well. It and the phase
are impacted by something at the longer tau's. Is it the mixer and
temperature? Is it the sun going 
down and things cooling off? Not at all clear. The main point is that the
termination isn't helping
the phase noise any.  Throw away a resistor and nose goes down by 5 db ....

Bob

-----Original Message-----
From: time-nuts <time-nuts-bounces at lists.febo.com> On Behalf Of Bruce
Griffiths
Sent: Thursday, March 26, 2020 5:01 PM
To: Discussion of precise time and frequency measurement
<time-nuts at lists.febo.com>
Subject: Re: [time-nuts] Noise Floor

The PN floor is somewhat lower with a 15dBm rather than a 4dBm input

http://www.ko4bb.com/getsimple/index.php?id=phase-noise-and-other-measuremen
ts-with-a-timepod

Bruce

> On 27 March 2020 at 06:11 kb8tq at n1k.org mailto:kb8tq at n1k.org wrote:
> 
> 
>     Hi
> 
>     Ok so here's a "new" part as opposed to one that is many decades old.
Again
>     this is a part that has been talked about
>     more than once on the list. The demo board is an early one, but I
assume it
>     is representative of how the CMOS output
>     part you get today works.
> 
>     There are two runs. The first pretty clearly shows the part warming
up. The
>     second still shows temperature effects,
>     they just are not as obvious. If you *really* need to hit < 1x10^-15
at 100
>     seconds with this part, drafts are not a good thing.
> 
>     The part is being driven with 4 dbm at 5 MHz. Max input is rated as 10
dbm
>     so that's not as nutty as it sounds. 5 MHz ( or
>     even 10 MHz) is well below the design target for this part. There's
not a
>     lot on the datasheet to compare to. As with the previous
>     part, ADEV is pretty good (out to 100 sec). Phase noise is not in the
10811
>     class wideband. If you need low wideband phase noise
>     as a logic signal at 5 MHz, you need to do something different. For
ADEV,
>     this will do just fine.
> 
>     Bob
> 
>     -----Original Message-----
>     From: time-nuts <time-nuts-bounces at lists.febo.com
mailto:time-nuts-bounces at lists.febo.com > On Behalf Of Charles
>     Steinmetz
>     Sent: Wednesday, March 25, 2020 5:23 PM
>     To: time-nuts at lists.febo.com mailto:time-nuts at lists.febo.com
>     Subject: Re: [time-nuts] Noise Floor
> 
>     Taka Kamiya wrote:
> 
>         > > I've been playing around with Clifton amplifier as well. Mine,
input is
> > 
> >     > terminated with 50 ohm register, and rest is unmodified, so it has
6dB gain.
>     I have a 10dB pad on input side. I, too, noticed there will be a
severe
>     clipping with driving it too hard. * * * I with there was a little
>     more room there....
> 
>     Bob wrote:
>     >> Maybe there's some noise in those resistors
>     >> Well..... maybe not so much.
>     >>
>     >> If you drive this board so it has a couple db more output, it goes
into
>     clipping. When that happens .... yuck.
>     >> Noise and ADEV both are massively impacted.
>     >> You very much do *not* want to overdrive this board. * * *
>     >> I would stick with 12 dbm or less
> 
>     You can buy some headroom by raising the supply voltage. There is an
>     on-card LM78L09 9v voltage regulator (U902) that can safely be raised
to
>     12v (LM78L12). This will get you cleanly to and a bit past the
>     traditional +13 dBm (1v rms) standard reference level. Of course, you
>     will need to make sure the raw supply voltage is >15v.
> 
>     As to noise, the 200 ohm resistor on the opamp's noninverting input
>     (R901) accounts for nearly 6dB of the amplifier noise (assuming an
>     effective source impedance of 50 ohms). Reducing this to, say, 33 ohms
>     will lower the noise floor a few dB.
> 
>     Finally, the state of CFB video amplifier development has advanced
>     dramatically in the 20 years since the AD8007 was introduced. New
>     amplifiers with supply voltage ratings up to 36v are available
(allowing
>     about 10dB greater headroom than the 8007), and many of the newer
opamps
>     clip much more gracefully than the 8007 when you do hit the limit (but
>     you really want not to do that in any measurement application).
> 
>     Many of these new CFB amplifiers have been discussed here on the list,
>     and each has its own fans. One I like that doesn't get mentioned much
>     is the LME49713. It is discontinued, but still available from
Rochester
>     Electronics and others. But there are lots of good choices.
> 
>     Best regards,
> 
>     Charles
> 
> 
> 
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