re low noise regulators
I checked both datasheets, the 723 and the LT1762. It was specsmanship that
rated the 723 as having lower noise. Here's how:
The internal comparison opamp in the 723 was OK for its day back in Fairchild's
history but by today's standards, it is noisy and has poor gain and BW. So in
order to make the regulator low noise, a 5uF capacitor had to be placed across
the ref comparison input.
The low noise, and even the standard linear regulators today are much quieter
and few people need 5uV of power supply noise, especially given that the
circuitry we place on the power rails will almost certainly spoil that.
So no one specs putting a 5uF cap on the ref line. The LTC LT1762 does suggest a
Cbyp of .01uF will provide a noise of 20uV broad bandwidth as you correctly
stated. I am certain that putting a higher value cap and perhaps even a RC
filter on the reference input will lower this substantially. But again, I
suggest that it will become meaningless to all but Super-Audio Snake Oil
people because any circuitry powered from the produced Vcc line will add more
noise than the regulator. Even with the now ancient history LM317 style
adjustables, I always put a tantalum cap across the reference to ground. It
does wonders for getting rid of traces of hum.
Just for your collection I will send John two App notes in PDF form to post.
They cover the subject of reducing spikes and switching regulator noise.
regards and 73,
Jeffrey Pawlan WA6KBL
Pawlan Communications
Jeffrey Pawlan wrote:
I checked both datasheets, the 723 and the LT1762. It was specsmanship that
rated the 723 as having lower noise. Here's how:
The internal comparison opamp in the 723 was OK for its day back in Fairchild's
history but by today's standards, it is noisy and has poor gain and BW. So in
order to make the regulator low noise, a 5uF capacitor had to be placed across
the ref comparison input.
Nonsense, if the amplifier was the largest contributor to the noise this
would have virtually no effect as it only filters the reference noise.
The low noise, and even the standard linear regulators today are much quieter
Rubbish look at the output noise from any 78xx regulator.
and few people need 5uV of power supply noise,
Just those who want low flicker phase noise RF amplifiers.
especially given that the
circuitry we place on the power rails will almost certainly spoil that.
So no one specs putting a 5uF cap on the ref line. The LTC LT1762 does suggest a
Cbyp of .01uF will provide a noise of 20uV broad bandwidth as you correctly
stated.
At 5V output, again specmanship at work.
I am certain that putting a higher value cap and perhaps even a RC
filter on the reference input will lower this substantially.
Without some idea of the internal circuit configuration how can you be
sure of this?
It may well make the noise worse than when using a single capacitor or
event cause the device to oscillate. the performance as there's nowhere
to put it.
It would appear from the 1761 datasheet that it merely filters the
1761's noisy internal reference or at least ensures that the midband
gain from the reference to the output is unity.
But again, I
suggest that it will become meaningless to all but Super-Audio Snake Oil
people because any circuitry powered from the produced Vcc line will add more
noise than the regulator. Even with the now ancient history LM317 style
adjustables, I always put a tantalum cap across the reference to ground . It
does wonders for getting rid of traces of hum.
Just for your collection I will send John two App notes in PDF form to post.
They cover the subject of reducing spikes and switching regulator noise.
regards and 73,
Jeffrey Pawlan WA6KBL
Pawlan Communications
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.
"Nonsense", rubbish"??
Not the words I want to hear from a fellow Time Nut.
Bruce, next time how about giving constructive criticism without belittling
other peoples efforts?
Remember that this group contains everyone from professionals to complete
newcomers. We want to encourage and grow the group by collective help and
encouragement, not by insulting them.
Season of goodwill to all men?
Cheers
Rob Kimberley
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Bruce Griffiths
Sent: 14 December 2007 04:56
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] re low noise regulators
Jeffrey Pawlan wrote:
I checked both datasheets, the 723 and the LT1762. It was specsmanship
that
rated the 723 as having lower noise. Here's how:
The internal comparison opamp in the 723 was OK for its day back in
Fairchild's
history but by today's standards, it is noisy and has poor gain and BW. So
in
order to make the regulator low noise, a 5uF capacitor had to be placed
across
the ref comparison input.
Nonsense, if the amplifier was the largest contributor to the noise this
would have virtually no effect as it only filters the reference noise.
The low noise, and even the standard linear regulators today are much
quieter
Rubbish look at the output noise from any 78xx regulator.
and few people need 5uV of power supply noise,
Just those who want low flicker phase noise RF amplifiers.
especially given that the
circuitry we place on the power rails will almost certainly spoil that.
So no one specs putting a 5uF cap on the ref line. The LTC LT1762 does
suggest a
Cbyp of .01uF will provide a noise of 20uV broad bandwidth as you
correctly
stated.
At 5V output, again specmanship at work.
I am certain that putting a higher value cap and perhaps even a RC
filter on the reference input will lower this substantially.
Without some idea of the internal circuit configuration how can you be
sure of this?
It may well make the noise worse than when using a single capacitor or
event cause the device to oscillate. the performance as there's nowhere
to put it.
It would appear from the 1761 datasheet that it merely filters the
1761's noisy internal reference or at least ensures that the midband
gain from the reference to the output is unity.
But again, I
suggest that it will become meaningless to all but Super-Audio Snake Oil
people because any circuitry powered from the produced Vcc line will add
more
noise than the regulator. Even with the now ancient history LM317 style
adjustables, I always put a tantalum cap across the reference to ground .
It
does wonders for getting rid of traces of hum.
Just for your collection I will send John two App notes in PDF form to
post.
They cover the subject of reducing spikes and switching regulator noise.
regards and 73,
Jeffrey Pawlan WA6KBL
Pawlan Communications
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
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and follow the instructions there.
Well, not to put too fine a point on it, but it WAS nonsense and rubbish.
The assertion in question had no basis in reality. I was pretty darned sure
about that, because I had just made the same bad assumption at my own
workbench.
In my case I had assumed that the Jim Williams-certified Low Noise(tm)
LT1762 was the best part for a VCO power supply, or at least as good as
anything else on the market. I knew very well what a 723 was -- every
80s-vintage boat anchor from HP and Tektronix around here is full of them.
But I didn't bother to check its data sheet, and my PLL fell short of its
expected PN level as a result. I wish somebody had said "Nonsense, rubbish"
about two months ago when I was laying out the board! Heh...
-- john, KE5FX
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com]On
Behalf Of Rob Kimberley
Sent: Friday, December 14, 2007 9:22 AM
To: 'Discussion of precise time and frequency measurement'
Subject: Re: [time-nuts] re low noise regulators
"Nonsense", rubbish"??
Not the words I want to hear from a fellow Time Nut.
Bruce, next time how about giving constructive criticism without
belittling
other peoples efforts?
Remember that this group contains everyone from professionals to complete
newcomers. We want to encourage and grow the group by collective help and
encouragement, not by insulting them.
Season of goodwill to all men?
Cheers
Rob Kimberley
Rob Kimberley wrote:
"Nonsense", rubbish"??
Not the words I want to hear from a fellow Time Nut.
Bruce, next time how about giving constructive criticism without belittling
other peoples efforts?
Remember that this group contains everyone from professionals to complete
newcomers. We want to encourage and grow the group by collective help and
encouragement, not by insulting them.
Season of goodwill to all men?
Cheers
Rob Kimberley
No insults or belittling intended, they were merely robust comments
about the statements made.
If you ever think that anything I say is nonsense (or rubbish) please
don't hesitate to say so, I will not be offended in any way.
Bruce
A more accurate summary of the noise characteristics of voltage
regulators is something like (neglecting the effects of excessive noise
on the regulator input voltage):
-
For all regulators when the reference isn't low pass filtered the
reference noise dominates the output noise. Even when the reference
noise is low pass filtered it dominates the regulator output noise at
low frequencies. -
When the reference is low pass filtered the error amplifier noise
dominates the midband regulator output noise. -
The regulator output noise scales with output voltage. However a
capacitor bypassing one of the output voltage setting resistors in a
conventional voltage regulator circuit can reduce the midband output noise.
Using the 723 regulator as an example:
From the datasheet:
Vout = 5V
output noise (100Hz - 10kHz) 86uV rms (unfiltered reference)
Output noise (100Hz - 10kHz) 5uV rms (filtered reference)
If we assume that in both cases the noise spectrum is flat (i.e white)
in the 100Hz - 10KHz band
If we also assume that the noise bandwidth is 10KHz (difficult to know
more precisely as the details of the band limiting filters used arent given)
Then crude approximations of the noise of the reference and error
amplifier are:
Reference noise (when attenuated to 5V) 860nV/rtHz (in the 100Hz
to 10KHz band)
Error amplifier input noise 50nV/rtHz
(in the 100Hz to 10KHz band)
With an error of less than 30% (if high frequency band limiting filter
has a single pole noise bandwidth is actually ~16kHz) due to possible
error in noise bandwidth.
The reference noise (~1.2uV/rtHz at the 7V reference output) is somewhat
higher than that of a modern buried zener reference (50-100nV/rtHz).
The error amplifier input noise (50nV/rtHz) is also significantly higher
than that of most modern opamps.
Thus it is possible to design and build a voltage regulator using
discrete components, a low noise (buried zener) reference and a low
noise opamp that has lower output noise than a 723 based regulator.
To facilitate comparisons between regulators the estimates for the noise
of the 723 reference is perhaps best expressed in ppm/rtHz.
i.e. 723 reference noise is around 0.12ppm/rtHz in the (100Hz-10kHz) region.
A similar analysis is possible for other conventional regulators.
However if one requires more accurate characterisation of regulator
output noise there is no substitute for measuring the output noise of a
real regulator.
Comparing regulator output noise characteristics is perhaps easiest if
the relevant data (and assumptions made) are tabulated in the form of a
spreadsheet.
I am happy to do this (in both Excel and OO formats), just let me know
what regulators you would like to be included.
If anyone has actual measurements of regulator output noise noise please
post them together with actual circuit schematic and measurement method
details.
Bruce
Gentlemen,
over the last years I have been confronted with numerous nonsense and
rubbish technical ideas as part of my job. Over the years I came to the
conclusion that just for the sake of clarity it MUST be allowed to call
them "nonsense" and "rubbish" without applying any political or other
correctness. I stand for that claim even if it caused some "Kiss my
back" reactions from my boss form time to time.
Best regards
Ulrich Bangert
-----Ursprungliche Nachricht-----
Von: time-nuts-bounces@febo.com
[mailto:time-nuts-bounces@febo.com] Im Auftrag von Bruce Griffiths
Gesendet: Freitag, 14. Dezember 2007 22:11
An: Discussion of precise time and frequency measurement
Betreff: Re: [time-nuts] re low noise regulators
Rob Kimberley wrote:
"Nonsense", rubbish"??
Not the words I want to hear from a fellow Time Nut.
Bruce, next time how about giving constructive criticism without
belittling other peoples efforts?
Remember that this group contains everyone from professionals to
complete newcomers. We want to encourage and grow the group by
collective help and encouragement, not by insulting them.
Season of goodwill to all men?
Cheers
Rob Kimberley
No insults or belittling intended, they were merely robust
comments about the statements made. If you ever think that
anything I say is nonsense (or rubbish) please don't hesitate
to say so, I will not be offended in any way.
Bruce
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.
On Dec 16, 2007 6:09 AM, Ulrich Bangert df6jb@ulrich-bangert.de wrote:
Gentlemen,
over the last years I have been confronted with numerous nonsense and
rubbish technical ideas as part of my job. Over the years I came to the
conclusion that just for the sake of clarity it MUST be allowed to call
them "nonsense" and "rubbish" without applying any political or other
I certainly agree, as long as such criticisms are clearly directed to
the mistaken or false claims, and not an attack on the person who made
the claim.
-Michael