A lower limit to the phase noise floor of an opamp can be estimated from
its output noise and the output signal level.
Half of the output noise power contributes to PM, the other half to AM.
At low offsets modulation of internal capacitances by power supply noise
and opamp noise modulates the phase shift produced by the opamp.
Such modulation occurs even when the amplifier isnt driven into saturation.
The same PM mechanisms occur in discrete amplifiers.
Keeping the dc gain low is helpful in reducing phase noise.
This doesnt mean AC coupling is a useful solution it isnt as a high dc
gain from an input base of a BJT to its collector will produce
significant modulation of its collector base capacitance and the phase
shift of the amplifier. Shunting output load resistors with inductors
can be useful in reducing such phase modulation, merely using a
capacitor to couple the output to the next stage has no effect on close
in phase noise produced by the amplified input noise (or power supply
noise - e.g. via a bias divider).
Discrete JFETs tend to be somewhat noisier than BJTs for low impedance
sources so using a JFET input device isnt usually as effective as using
a BJT at least for 50 ohm sources.
Bruce
Bill Fuqua wrote:
Discrete amplifiers are always less noisy than integrated amplifiers.
If you want really low noise design a one with JFETs and Bipolar
transistors.
I am trying to understand the contribution to phase noise by the opamps.
Perhaps the "threshold" is shifting and amplifier is being driven to
saturation?
I am new to this group but have had lots of RF experience and weak signal
detection experience.
73
Bill wa4lav
Date: Tue, 28 Feb 2012 07:34:09 -0500
From: Bob Camp <li...@rtty.us>
Hi
Very cool. How much power can you run through the device? Put another
way, if you drive it with +13 dbm do all the numbers get 5 db better?
I doubt very many of us will be worrying about weather it's below
-153 at 10 Hz or not?
Bob
On Feb 28, 2012, at 5:42 AM, Bruce Griffiths wrote:
> The attached plot indicates the phase noise performance obtainable
with a wideband FET (OPA653) input opamp.
> With a 10MHz +9dBm input, the phase noise floor is around
-163dBc/Hz at 1kHz offset and around -154dBc/Hz at 10Hz offset.
> A quieter test source would be useful particularly for offsets
below 10Hz.
>
> Bruce
> <OPA653PN2.gif>_______________________________________________
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William L. Fuqua III P.E.
Sr. Electrical Engineer
CP 177 Chemistry Physics Building
Department of Physics and Astronomy
University of Kentucky
Lexington,KY 40506-0055
Phone: 1-859-257-4155
e-mail: wlfuq...@uky.edu
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