Empathy List Archives

DC

David C. Partridge

Tue, Jan 29, 2013 3:53 PM

I've returned to the 7081 that was giving me troubles in the AC buffer circuit.

Probing the output test point (TP 705) I'm seeing about 100mV pp (approx) at about 10MHz. This would likely explain the strange results I was getting last year. However I'm puzzled as to where it can be coming from!!! Any suggestions gratefully received.

Regards,
David Partridge

I've returned to the 7081 that was giving me troubles in the AC buffer circuit. Probing the output test point (TP 705) I'm seeing about 100mV pp (approx) at about 10MHz. This would likely explain the strange results I was getting last year. However I'm puzzled as to where it can be coming from!!! Any suggestions gratefully received. Regards, David Partridge

JL

J. L. Trantham

Wed, Jan 30, 2013 4:14 AM

Is it possible that it is, somehow, 'picking up interference' from your
'house standard'? How close to 10.000 000 000 MHz is it?

100 mV P-P is fairly substantial. Otherwise, is it oscillating?

Good luck.

Joe

-----Original Message-----
From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On
Behalf Of David C. Partridge
Sent: Tuesday, January 29, 2013 9:54 AM
To: 'Discussion of precise voltage measurement'
Subject: [volt-nuts] 7081 AC buffer again

I've returned to the 7081 that was giving me troubles in the AC buffer
circuit.

Probing the output test point (TP 705) I'm seeing about 100mV pp (approx) at
about 10MHz. This would likely explain the strange results I was getting
last year. However I'm puzzled as to where it can be coming from!!! Any
suggestions gratefully received.

Regards,
David Partridge

volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

Is it possible that it is, somehow, 'picking up interference' from your 'house standard'? How close to 10.000 000 000 MHz is it? 100 mV P-P is fairly substantial. Otherwise, is it oscillating? Good luck. Joe -----Original Message----- From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On Behalf Of David C. Partridge Sent: Tuesday, January 29, 2013 9:54 AM To: 'Discussion of precise voltage measurement' Subject: [volt-nuts] 7081 AC buffer *again* I've returned to the 7081 that was giving me troubles in the AC buffer circuit. Probing the output test point (TP 705) I'm seeing about 100mV pp (approx) at about 10MHz. This would likely explain the strange results I was getting last year. However I'm puzzled as to where it can be coming from!!! Any suggestions gratefully received. Regards, David Partridge _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.

EB

Ed Breya

Wed, Jan 30, 2013 4:57 AM

I agree with JL - it's either picking up an available frequency, or
making its own. Oscillation can happen with transistors, especially any
common-base amplifiers or emitter followers that don't have enough
degeneration - but that is usually in the VHF range. For MHz-type
oscillations, look for opamp loops that went unstable for some reason -
bad grounds or bypassing can do it.

An old trick for VHF is to poke around with the tip of a wood-sheathed
lead pencil - the partially-conductive graphite core and the lossy
wooden capacitance to an even lossier hand would sometimes damp
oscillations and point directly to the problem, so to speak. For lower
frequency you need brute force - try a small screwdriver, a meter probe
lead, or tweezers held in the hand, and just poke around on various
nodes, without shorting anything out. If that's not enough, a small RC
series damper - say 1000 pF and a few hundred ohms - with a clip lead to
ground should show some results. All you're looking for is some effect -
it should either reduce or aggravate the problem when you are around the
problem circuit. You will naturally be injecting all kinds of line
frequency and RF interference into any high-Z circuits, so ignore that
and just look for the effects at the target frequency.

Ed

I agree with JL - it's either picking up an available frequency, or making its own. Oscillation can happen with transistors, especially any common-base amplifiers or emitter followers that don't have enough degeneration - but that is usually in the VHF range. For MHz-type oscillations, look for opamp loops that went unstable for some reason - bad grounds or bypassing can do it. An old trick for VHF is to poke around with the tip of a wood-sheathed lead pencil - the partially-conductive graphite core and the lossy wooden capacitance to an even lossier hand would sometimes damp oscillations and point directly to the problem, so to speak. For lower frequency you need brute force - try a small screwdriver, a meter probe lead, or tweezers held in the hand, and just poke around on various nodes, without shorting anything out. If that's not enough, a small RC series damper - say 1000 pF and a few hundred ohms - with a clip lead to ground should show some results. All you're looking for is some effect - it should either reduce or aggravate the problem when you are around the problem circuit. You will naturally be injecting all kinds of line frequency and RF interference into any high-Z circuits, so ignore that and just look for the effects at the target frequency. Ed

DC

David C. Partridge

Wed, Jan 30, 2013 10:21 AM

House standard should be pretty close as it's a Trimble Thunderbolt GPS. However, I don't think that is the culprit as thjis is just "close to" 10MHz, rather than "spot on".

So yes, oscillation is likely.

Regards,
David Partridge
-----Original Message-----
From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On Behalf Of J. L. Trantham
Sent: 30 January 2013 04:15
To: 'Discussion of precise voltage measurement'
Subject: Re: [volt-nuts] 7081 AC buffer again

Is it possible that it is, somehow, 'picking up interference' from your 'house standard'? How close to 10.000 000 000 MHz is it?

100 mV P-P is fairly substantial. Otherwise, is it oscillating?

Good luck.

Joe

House standard should be pretty close as it's a Trimble Thunderbolt GPS. However, I don't *think* that is the culprit as thjis is just "close to" 10MHz, rather than "spot on". So yes, oscillation is likely. Regards, David Partridge -----Original Message----- From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On Behalf Of J. L. Trantham Sent: 30 January 2013 04:15 To: 'Discussion of precise voltage measurement' Subject: Re: [volt-nuts] 7081 AC buffer *again* Is it possible that it is, somehow, 'picking up interference' from your 'house standard'? How close to 10.000 000 000 MHz is it? 100 mV P-P is fairly substantial. Otherwise, is it oscillating? Good luck. Joe

DC

David C. Partridge

Wed, Jan 30, 2013 10:24 AM

Ed, thank you, that pencil trick is one I don't know. One of the problems with being "self taught".

Mind you, if it is FB round the op-amp, then anywhere in the FB loop might well show as a culprit :(

Regards,
David Partridge
-----Original Message-----
From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On Behalf Of Ed Breya
Sent: 30 January 2013 04:57
To: volt-nuts@febo.com
Subject: Re: [volt-nuts] 7081 AC buffer again

I agree with JL - it's either picking up an available frequency, or making its own. Oscillation can happen with transistors, especially any common-base amplifiers or emitter followers that don't have enough degeneration - but that is usually in the VHF range. For MHz-type oscillations, look for opamp loops that went unstable for some reason - bad grounds or bypassing can do it.

An old trick for VHF is to poke around with the tip of a wood-sheathed lead pencil - the partially-conductive graphite core and the lossy wooden capacitance to an even lossier hand would sometimes damp oscillations and point directly to the problem, so to speak. For lower frequency you need brute force - try a small screwdriver, a meter probe lead, or tweezers held in the hand, and just poke around on various nodes, without shorting anything out. If that's not enough, a small RC series damper - say 1000 pF and a few hundred ohms - with a clip lead to ground should show some results. All you're looking for is some effect - it should either reduce or aggravate the problem when you are around the problem circuit. You will naturally be injecting all kinds of line frequency and RF interference into any high-Z circuits, so ignore that and just look for the effects at the target frequency.

Ed

volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

Ed, thank you, that pencil trick is one I don't know. One of the problems with being "self taught". Mind you, if it is FB round the op-amp, then anywhere in the FB loop might well show as a culprit :( Regards, David Partridge -----Original Message----- From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On Behalf Of Ed Breya Sent: 30 January 2013 04:57 To: volt-nuts@febo.com Subject: Re: [volt-nuts] 7081 AC buffer *again* I agree with JL - it's either picking up an available frequency, or making its own. Oscillation can happen with transistors, especially any common-base amplifiers or emitter followers that don't have enough degeneration - but that is usually in the VHF range. For MHz-type oscillations, look for opamp loops that went unstable for some reason - bad grounds or bypassing can do it. An old trick for VHF is to poke around with the tip of a wood-sheathed lead pencil - the partially-conductive graphite core and the lossy wooden capacitance to an even lossier hand would sometimes damp oscillations and point directly to the problem, so to speak. For lower frequency you need brute force - try a small screwdriver, a meter probe lead, or tweezers held in the hand, and just poke around on various nodes, without shorting anything out. If that's not enough, a small RC series damper - say 1000 pF and a few hundred ohms - with a clip lead to ground should show some results. All you're looking for is some effect - it should either reduce or aggravate the problem when you are around the problem circuit. You will naturally be injecting all kinds of line frequency and RF interference into any high-Z circuits, so ignore that and just look for the effects at the target frequency. Ed _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.

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7081 AC buffer again