[time-nuts] Noise Floor

[Bruce Griffiths]

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-measurements-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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