Q: How to amplify 15uV, 1.5MHz signal to read on Scopemeter

[JimG]

I need some ideas - hope this is the right group.

To complete the alignment of a thermocouple/millivolt calibrator, I
need to tweak a chopper amp to minimize output noise.

Doing this requires picking up a 1.5MHz square wave that should have
an amplitude of around 15uV. To see the waveform and adjust the pot I
probably need to amplify this to 5mV, a gain of around 300.

Is there a simple way to do this, and still maintain something that
resembles the original signal?

I am using a Fluke 96 II Scopemeter (50MHz) with a 10:1 probe.

TIA.

Jim

 

[Phil Allison]

"JimG"

To complete the alignment of a thermocouple/millivolt calibrator, I
need to tweak a chopper amp to minimize output noise.

Doing this requires picking up a 1.5MHz square wave that should have
an amplitude of around 15uV. To see the waveform and adjust the pot I
probably need to amplify this to 5mV, a gain of around 300.

** Who needs to actually see it?

1.5 MHz is at the top end of the AM radio band and such radios are sensitive
down to a few uVs.

Be simple to pick up and monitor the signal on a ( reasonable quality)
portable radio and tweak it to minimum signal strength. Maybe use a 10pF cap
to couple the signal into the antenna circuit.

If you have the SW band available too, then check on 4.5 and 7.5 and 10.5
MHz as well.



........ Phil

 

[JimG]

** Who needs to actually see it?

Manufacturer advises that adjusting the pot to produce waveform with
visually flat tops/bottoms and square corners is most effective. There
may be other ways, but for now I am trying to follow instructions.

Thanks for the reply.

Jim

 

[Phil Allison]

"JimG"
"Phil Allison"

Manufacturer advises that adjusting the pot to produce waveform with
visually flat tops/bottoms and square corners is most effective.

** But never at signal levels like 15 uV @ 1.5 MHz - you utter dimwit.

There
may be other ways, but for now I am trying to follow instructions.

** You asked us for a simple way to minimise the output noise level.

I gave you a very neat and easy one.

You snipped it, ignored it and abused me for offering it.

So - go **** yourself -

you vile little shithead.




....... Phil

 

[JimG]

"JimG"
"Phil Allison"


** But never at signal levels like 15 uV @ 1.5 MHz - you utter dimwit.





** You asked us for a simple way to minimise the output noise level.

I gave you a very neat and easy one.

You snipped it, ignored it and abused me for offering it.

So - go **** yourself -

you vile little shithead.

Didn't ignore your advice, and will probably try it. I have a
portable radio that will probably do the trick.

But advice from manufacturer is that the proper scope (which I do not
have) can read signals at that level. I'm just trying to figure out
if there's a way to do it "their way" with what I have.

Can't figure out how I abused you, but I appreciate your advice
anyway.

Jim

 

[Phil Allison]

"JimG"
"Phil Allison"

Didn't ignore your advice,

** You did, far as I can see - shithead.

and will probably try it.

** Don't strain yourself - wanker.

I have a portable radio that will probably do the trick.

** Don't strain yourself - wanker.

But advice from manufacturer is that the proper scope (which I do not
have) can read signals at that level.

** Yawn.

Utterly detail free - so uncheckable bollocks.

I'm just trying to figure out
if there's a way to do it "their way" with what I have.

** Yawn - who here knows what YOU have ??

Except a fucking bad attitude.

Can't figure out how I abused you,

** Yawn - there is **** all you have ever figured out about anything.

but I appreciate your advice anyway.

** Reeks of insincerity.

Your Q is way OT for this NG anyhow.

Drop dead, shithead.





........ Phil

 

[Rich Grise]

On Sat, 27 Oct 2007 00:28:24 +1000, "Phil Allison" [the usual]
Can't figure out how I abused you, but I appreciate your advice
anyway.

Don't worry about Jim - he flames everybody.

Cheers!
Rich

 

[Ninja]

Are you considering building a scope preamp to make this adjustment? This
could be a bit more challenging than you think. At 15 uV signal levels, you
have to think about the thermal noise floor. What the source impedance of
the device you're looking at? And what bandwidth (rise time) do you need on
the square wave you're looking at? Calculate the noise floor [sqrt(4KTBR)]
and compare the noise floor to your signal level.

If it's a 50 ohm output, a cheap and easy low-noise preamp is a 50 ohm gain
block like the Minicircuits GALI-39 ($1.19). It's 20 dB of gain (voltage
gain of 10) with 2.4 dB noise figure will get your signal (+ noise) above
the noise floor so you can finish the job with an op-amp. These monolithis
gain blocks are a low cost, low parts count way to go, but with gain well
into the GHz region, you have to know what you're doing layout-wise to make
an amplifier rather than an oscillator.

 

[JimG]

On Sat, 27 Oct 2007 00:28:24 +1000, "Phil Allison" [the usual]
Can't figure out how I abused you, but I appreciate your advice
anyway.

Don't worry about Jim - he flames everybody.

Cheers!
Rich

Thanks, Rich.

I've been flamed before, and I'll be flamed again. The price paid for
needing help, I suppose.

Jim

 

[JimG]

Are you considering building a scope preamp to make this adjustment?

Yes, I am.

In fact, I've made a little progress with a crude circuit using a
LF411CN op amp and a negative feedback loop (yielding a gain of around
320).

This
could be a bit more challenging than you think. At 15 uV signal levels, you
have to think about the thermal noise floor. What the source impedance of
the device you're looking at? And what bandwidth (rise time) do you need on
the square wave you're looking at? Calculate the noise floor [sqrt(4KTBR)]
and compare the noise floor to your signal level.

Your warning about the noise is right on the money. There is
something vaguely resembling a square wave on my scope, but the noise
overwhelms it much of the time.

But even with the extra noise, I think I am actually getting usable
results. The signal from the calibrator's chopper amp gets pretty
strong a half turn either side of the "sweet spot." And with the
source signal stronger, it becomes easily visible above the noise.

So by splitting the difference between the pot dial positions at which
the square wave just becomes visible on the scope, I don't think I am
more than 1/2 turn off - which for this application is probably going
to be close enough.

Although I was advised by the manufacturer to adjust to "square up"
the waveform, it now seems obvious that the amplitude of the signal is
what varies the most with pot rotation.

If it's a 50 ohm output, a cheap and easy low-noise preamp is a 50 ohm gain
block like the Minicircuits GALI-39 ($1.19). It's 20 dB of gain (voltage
gain of 10) with 2.4 dB noise figure will get your signal (+ noise) above
the noise floor so you can finish the job with an op-amp. These monolithis
gain blocks are a low cost, low parts count way to go, but with gain well
into the GHz region, you have to know what you're doing layout-wise to make
an amplifier rather than an oscillator.

The output impedance of the calibrator is very low - less than 0.05
ohm. Does the low impedance make the preamp you suggested more, or
less, feasible?

Thanks.

Jim

 

[Phil Allison]

"JimG"


** **** off - WANKER !!




...... Phil

 

[John Popelish]

JimG wrote:
(snip)

In fact, I've made a little progress with a crude circuit using a
LF411CN op amp and a negative feedback loop (yielding a gain of around
320).

(snip)

Not at 1.5 MHz, you didn't. That would take an opamp with a
gain bandwidth of at least 320*1.5 MHz = 480 MHz, at least.

The LF411 has a gain bandwidth of about 3 MHz, capable of a
gain of 2, at most, with 1.5 MHz signals.
http://cache.national.com/ds/LF/LF411.pdf

Now, if you have actually been talking about a 1.5 kHz
signal, then that is a horse of a different color.

 

[Phil Allison]

"John Popelish"

JimG wrote:
(snip)
(snip)

Not at 1.5 MHz, you didn't. That would take an opamp with a gain
bandwidth of at least 320*1.5 MHz = 480 MHz, at least.

The LF411 has a gain bandwidth of about 3 MHz, capable of a gain of 2, at
most, with 1.5 MHz signals.
http://cache.national.com/ds/LF/LF411.pdf

Now, if you have actually been talking about a 1.5 kHz signal, then that
is a horse of a different color.

** LOL

I kept wondering if that 15 uV figure ought to be 15 mV.

But of course, why the heck would a thermocouple amp need to chop at 1.5 MHz
??




........ Phil

 

[JimG]

JimG wrote:
(snip)
(snip)

Not at 1.5 MHz, you didn't. That would take an opamp with a
gain bandwidth of at least 320*1.5 MHz = 480 MHz, at least.

The LF411 has a gain bandwidth of about 3 MHz, capable of a
gain of 2, at most, with 1.5 MHz signals.
http://cache.national.com/ds/LF/LF411.pdf

Now, if you have actually been talking about a 1.5 kHz
signal, then that is a horse of a different color.

Yeah, you're right.

The circuit schematic shows 1.2MHz (I misread it to say 1.5MHz
initially). But I found some text in another part of the service
manual says 1.2kHz.

The 1.2kHz must be correct - scope trace shows a period of around
800uS.

The 15uV, OTOH, seems about right. When the pot is well-adjusted, I
am seeing an amplified signal that is a little less than 1/3 of a 10mV
division on the scope. When poorly adjusted, the amplified signal is
10-20mV, corresponding roughly to 30-60uV from the calibrator.

Jim

 

[Phil Allison]

"JimG = UTTER FUCKING MORON "

Yeah, you're right.

The circuit schematic shows 1.2MHz (I misread it to say 1.5MHz
initially). But I found some text in another part of the service
manual says 1.2kHz.

** YOU FUCKING MORON !!!

The 1.2kHz must be correct - scope trace shows a period of around
800uS.

** YOU FUCKING FUCKWIT MORON !!!


FUUUUUUUUUUK OFFFFFFFFFFFFFF



........... Phil

 

[Daniel Mandic]

.......... Phil

Who is that to your left ))


you Beginner........... ;-))))))))))



Best regards,

Daniel Mandic

 

[Daniel Mandic]

** YOU FUCKING MORON !!!




** YOU FUCKING FUCKWIT MORON !!!


FUUUUUUUUUUK OFFFFFFFFFFFFFF



.......... Phil

why do you explode all the time when someone won't do it as you say,
even you are right ?


Write for example: Well, if you know it better then do as you wish.



Best Regards,

Daniel Mandic

 

[JimG]

Looks like my crude preamp circuit is going to work. I've been able
to increase the gain to 1500, which at 1.2kHz is holding up OK on the
LF411CN op amp.

The source signal from the chopper amp is indeed close to 15uV p-p,
which gets amplified to almost 25mV. This is a couple of divisions on
my Scopemeter and is easily visible.

But there is still a fair amount of noise on top of the basic 1.2kHz
signal. Any further suggestions for cheap ways to reduce this noise
are welcome.

It's not clear to me if something like the Minicircuits GALI-39 that
ninja suggested is still appropriate for a 1.2kHz signal with 0.05 ohm
output impedance.

Jim

 

[John Popelish]

"John Popelish"



** LOL

I kept wondering if that 15 uV figure ought to be 15 mV.

But of course, why the heck would a thermocouple amp need to chop at 1.5 MHz
??

Perhaps it is a micron sized thermocouple measuring the
temperature of a speck of radium as it radioactively decays.

Yeah, that is probably it. ;-)

 

[John Popelish]

Yeah, you're right.

The circuit schematic shows 1.2MHz (I misread it to say 1.5MHz
initially). But I found some text in another part of the service
manual says 1.2kHz.

The 1.2kHz must be correct - scope trace shows a period of around
800uS.

That one letter change means that this signal is a good
candidate for a simple opamp gain block. Though, I would
still reserve 90% of the opamp gain bandwidth for the
harmonics of the signal, so I could see the wave shape,
clearly. But it also means that you don't need to use a
10:1 probe on the scope, so you need only 1/10th of the gain
you thought you did. Just add a 100 ohm resistor between the
opamp output and the scope probe to keep the probe from
capacitively destabilizing the opamp and overshooting the
edges of the wave.