[time-nuts] Noise Floor

[kb8tq at n1k.org]

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> On Behalf Of Charles
Steinmetz
Sent: Wednesday, March 25, 2020 5:23 PM
To: 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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