[time-nuts] Noise Floor

[Charles Steinmetz]

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