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 <lists@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>_______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. 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: wlfuqu00@uky.edu

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 <lists@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>_______________________________________________ >> > time-nuts mailing list -- time-nuts@febo.com >> > To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> > and follow the instructions there. > > 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: wlfuqu00@uky.edu > >

You need to look at the driving impedance before you declare one technology "quieter" than another. That is, you have voltage noise and current noise. For low driving impedance, bipolar will be quieter since current noise will not be significant, plus a bipolar will have lower thermal noise. For high impedance, JFET may be a better solution. Opamps are around 1nv/rootHz these days. That isn't all that easy to achieve discretely. Noise is proportional to the square root of the bandwidth, so less is more. But that is broadband noise. I'm not sure about phase noise. On 2/28/2012 10:41 AM, 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 <lists@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>_______________________________________________ >> > time-nuts mailing list -- time-nuts@febo.com >> > To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> > and follow the instructions there. > > 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: wlfuqu00@uky.edu > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >