Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old January 12th 20, 03:36 PM posted to sci.electronics.design,sci.electronics.repair
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Default Noise on VCO voltage ramp

Gentlemen,

I've got this weird problem with the network analyser I posted about
recently. The problem *seems* to be noise spikes on the DC voltage
control ramp which are causing spurious spikes in the frequency domain
on the RF output when viewed on an RF spectrum analyser. The peculiar
thing is these spikes on the RF output are not random; they come and
go according to the point the control voltage has reached as it ramps
up. Curiously, I cannot see anything amiss with the ramp signal when I
check it with a scope. I'd have expected to see at least *some* sign
of instability, but it appears rock steady. I'm just wondering if an
ordinary oscilloscope is really fast enough to pick out these
transients, though. Maybe they're there even though I can't see them.
Any suggestions as to what other instrument might be better suited to
this purpose? A DSO, maybe? Or if not, optimising the settings on the
analogue scope to have the best chance of spotting them?
thanks!
--

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Old January 12th 20, 09:21 PM posted to sci.electronics.design,sci.electronics.repair
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Default Noise on VCO voltage ramp

On 2020-01-12 09:36, Cursitor Doom wrote:
Gentlemen,

I've got this weird problem with the network analyser I posted about
recently. The problem *seems* to be noise spikes on the DC voltage
control ramp which are causing spurious spikes in the frequency domain
on the RF output when viewed on an RF spectrum analyser. The peculiar
thing is these spikes on the RF output are not random; they come and
go according to the point the control voltage has reached as it ramps
up. Curiously, I cannot see anything amiss with the ramp signal when I
check it with a scope. I'd have expected to see at least *some* sign
of instability, but it appears rock steady. I'm just wondering if an
ordinary oscilloscope is really fast enough to pick out these
transients, though. Maybe they're there even though I can't see them.
Any suggestions as to what other instrument might be better suited to
this purpose? A DSO, maybe? Or if not, optimising the settings on the
analogue scope to have the best chance of spotting them?
thanks!


Finding an asynchronous glitch is hard on an analog scope.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

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Old January 12th 20, 10:36 PM posted to sci.electronics.design,sci.electronics.repair
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Default Noise on VCO voltage ramp

On Sun, 12 Jan 2020 15:21:39 -0500, Phil Hobbs
wrote:

Finding an asynchronous glitch is hard on an analog scope.


It is to a major extent reproduceable, though, Phil. I can manually
override the ramp and set a voltage which will show up a glitch in the
frequency domain, which is a useful plus. You reckon it would be
better to use a DSO for this? I do have one.
--

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Old January 12th 20, 11:17 PM posted to sci.electronics.design,sci.electronics.repair
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Default Noise on VCO voltage ramp

On 2020-01-12 16:36, Cursitor Doom wrote:
On Sun, 12 Jan 2020 15:21:39 -0500, Phil Hobbs
wrote:

Finding an asynchronous glitch is hard on an analog scope.


It is to a major extent reproduceable, though, Phil. I can manually
override the ramp and set a voltage which will show up a glitch in the
frequency domain, which is a useful plus. You reckon it would be
better to use a DSO for this? I do have one.


Frequency domain glitch? You mean a discrete spur?

A glitch is a transient time-domain animal. A DSO can save a single
instance, or (with averaging) ignore the noise and asynchronous
background and just reproduce the glitch itself.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

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Old January 12th 20, 11:22 PM posted to sci.electronics.design,sci.electronics.repair
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Default Noise on VCO voltage ramp

On Sun, 12 Jan 2020 14:36:27 +0000, Cursitor Doom
wrote:

I've got this weird problem with the network analyser I posted about
recently. The problem *seems* to be noise spikes on the DC voltage
control ramp which are causing spurious spikes in the frequency domain
on the RF output when viewed on an RF spectrum analyser.


In other words, it's picking up garbage from something in your shop
(or cave). I suggest you start by moving or removing all the
switching power supplies, wall warts, light dimmers, desk lamps, or
gizmos within a few feet that might have a switching power supply
inside. Next, cover the network analyzer with some aluminum foil to
see if the glitches are being delivered via RF. My desktop wi-fi
access point does that sometimes. If that also fails, drag your
network analyzer and scope to another part of your shop (or cave),
that's away from all the gizmos, and see if the problem persists. In
other words, determine if the garbage is internally generated,
externally radiated, or conducted via the power lines, clip leads,
cables, etc.

If you really want to see the noise, glitch, trash, garbage, or
whatever signal, set you scope so that the horizontal trigger is off
and the horizontal sweep is free running. Then, slooooooowly vary the
horizontal sweep until you see fairly stable glitches. If the noise
is periodic, you should be able to see something.


--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558


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Old January 13th 20, 01:18 AM posted to sci.electronics.design,sci.electronics.repair
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Default Noise on VCO voltage ramp

On Sun, 12 Jan 2020 17:17:04 -0500, Phil Hobbs
wrote:

Frequency domain glitch? You mean a discrete spur?
A glitch is a transient time-domain animal.


I could have put it better I guess. What I mean is I'm seeing thin
spikes in the RF output spectrum of the VNA at other frequencies than
the desired one. I'm guessing these spikes are the result of unwanted
transient voltage spikes present on the DC VCO control voltage which
sweeps the RF output frequency from 4Mhz through to 1.3Ghz.

A DSO can save a single
instance, or (with averaging) ignore the noise and asynchronous
background and just reproduce the glitch itself.


Well, Jeff has posted some advice for me on that aspect which I intend
to try out tomorrow using my Tek DSO which is probably much better
suited to the task than the analogue one I've hithertofore been
using..
--

No deal? No problem! :-D
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Old January 13th 20, 01:24 AM posted to sci.electronics.design,sci.electronics.repair
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Default Noise on VCO voltage ramp

On Sun, 12 Jan 2020 14:22:35 -0800, Jeff Liebermann
wrote:

In other words, it's picking up garbage from something in your shop
(or cave). I suggest you start by moving or removing all the
switching power supplies, wall warts, light dimmers, desk lamps, or
gizmos within a few feet that might have a switching power supply
inside. Next, cover the network analyzer with some aluminum foil to
see if the glitches are being delivered via RF.


Yeah, I did try all that and it didn't help, Jeff. Also there's no
glitches or noise when I pull the RF signal away, so at least it's not
something coming from the spectrum analyser.

My desktop wi-fi
access point does that sometimes. If that also fails, drag your
network analyzer and scope to another part of your shop (or cave),
that's away from all the gizmos, and see if the problem persists. In
other words, determine if the garbage is internally generated,
externally radiated, or conducted via the power lines, clip leads,
cables, etc.


I *suspect* noise is somehow getting onto the DC VCO ramp voltage and
manifesting as spikes in the RF output of the VNA accordingly.
Fortunately, there is a jack in the back of the VNA for the
application of an external VCO control voltage, so I plan to avail
myself of that with a big old linear bench supply and see if the
problem goes away.

If you really want to see the noise, glitch, trash, garbage, or
whatever signal, set you scope so that the horizontal trigger is off
and the horizontal sweep is free running. Then, slooooooowly vary the
horizontal sweep until you see fairly stable glitches. If the noise
is periodic, you should be able to see something.


Thanks, Jeff. Duly noted!

--

No deal? No problem! :-D
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Old January 13th 20, 01:40 AM posted to sci.electronics.design,sci.electronics.repair
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Default Noise on VCO voltage ramp

On 2020-01-12 19:18, Cursitor Doom wrote:
On Sun, 12 Jan 2020 17:17:04 -0500, Phil Hobbs
wrote:

Frequency domain glitch? You mean a discrete spur? A glitch is a
transient time-domain animal.


I could have put it better I guess. What I mean is I'm seeing thin
spikes in the RF output spectrum of the VNA at other frequencies
than the desired one.


Which is a discrete spur. Gotcha.

I'm guessing these spikes are the result of unwanted transient
voltage spikes present on the DC VCO control voltage which sweeps the
RF output frequency from 4Mhz through to 1.3Ghz.


If there's a forest of them, spaced at equal intervals, they could be
coming from a periodic time-domain source.


A DSO can save a single instance, or (with averaging) ignore the
noise and asynchronous background and just reproduce the glitch
itself.


Well, Jeff has posted some advice for me on that aspect which I
intend to try out tomorrow using my Tek DSO which is probably much
better suited to the task than the analogue one I've hithertofore
been using..


Yeah, I'm fonder of analog scopes than JL is, but my beautiful Tek 2467
hasn't been used in quite awhile, whereas I use the 1180x, TDS 694C and
TDS 784As most days that I'm in the lab. For manual driving, the 2467
has the best triggering of the lot, but it's very hard to beat a DSO for
general use.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

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Old January 13th 20, 02:46 AM posted to sci.electronics.design,sci.electronics.repair
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Default Noise on VCO voltage ramp

On Mon, 13 Jan 2020 00:24:38 +0000, Cursitor Doom
wrote:

If you really want to see the noise, glitch, trash, garbage, or
whatever signal, set you scope so that the horizontal trigger is off
and the horizontal sweep is free running. Then, slooooooowly vary the
horizontal sweep until you see fairly stable glitches. If the noise
is periodic, you should be able to see something.


Thanks, Jeff. Duly noted!


Oh, and btw, I know you like details such as this, I'll be using my
Tek 2232 DSO for this measurement. I don't really get on with digital
scopes, but seems like there's little alternative for this job.

--

No deal? No problem! :-D
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Old January 13th 20, 02:59 AM posted to sci.electronics.design,sci.electronics.repair
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Default Noise on VCO voltage ramp

On Sun, 12 Jan 2020 19:40:23 -0500, Phil Hobbs
wrote:

I'm guessing these spikes are the result of unwanted transient
voltage spikes present on the DC VCO control voltage which sweeps the
RF output frequency from 4Mhz through to 1.3Ghz.


If there's a forest of them, spaced at equal intervals, they could be
coming from a periodic time-domain source.


There *are* equally-spaced spikes at the lower end of the spectrum, up
to about 100Mhz, but thereafter they assume a far more random
appearance - although they are NOT random as they reappear at the same
place on the spectrum and at the same amplitude with each sweep. Plus
there *is*lower-level truly random noise around 500-600Mhz - so looks
like 3 different and unrelated faults to fix!! I do love my vintage
test gear, but I often spend more time fixing it than using it!

Yeah, I'm fonder of analog scopes than JL is, but my beautiful Tek 2467
hasn't been used in quite awhile, whereas I use the 1180x, TDS 694C and
TDS 784As most days that I'm in the lab. For manual driving, the 2467
has the best triggering of the lot, but it's very hard to beat a DSO for
general use.


I only have one DSO out of 13 scopes in all: the Tek 2232. It's
nothing special; it was okay in its day I guess, but I'm sure John's
cheapo Chinese Rigol could **** all over it in a head-to-head
"test-off" lol.
--

No deal? No problem! :-D


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