Date   
Re: Impedance matching question

David Berlind
 

That was a decent video, thank you. It doesn't seem like he had problems
with the terminators themselves. In fact, he uses them to address
mismatches. I'd be curious to know (from anyone who knows) why maximum
power (matched impedance) is ideal in some situations, but a low-Z to
high-Z arrangement is the ideal in other situations (ie: guitar to
amplifier or microphone to PA). I realize the outcome in the latter
situation; you preserve the audio highs and lows. But need some schooling
on why to maximize voltage at the load (vs. maximizing transfer power) in
those situations and why the mismatch isn't destructive to the signal. And
correspondingly, why the same isn't true of the impedance match between
output tubes and the primary of an output transformer since its a similar
audio application. Again, I understand the requirements and outcomes... but
am confused about the underlying physics.

Thanks in advance for this help.

On Mon, Mar 25, 2019 at 8:09 PM Roy Thistle <roy.thistle@...>
wrote:

Hi:
If you haven't watched EEVblog #652, he demonstrates a couple of problems
that can arise when using coax, terminators, and Ts, to connect a sig gen
to a scope.
Cheers.



Re: 11801 is reassembled, up and running :-)

Albert Otten
 

Attachments are not allowed here Reg. Via the website you can create a new Album in the Photos section and put your picture(s) there. If you do so, it's handy for "us" if you insert a link to the album in your messages.
Albert

On Tue, Mar 26, 2019 at 02:06 PM, Reginald Beardsley wrote:


I've attached a photo showing the calibrator output trace. The measured rise
time is shown in the lower left at 32 ps. I've got smoothing on.

Re: Impedance matching question

Craig Sawyers
 

That was a decent video, thank you. It doesn't seem like he had problems with the terminators
themselves. In fact, he uses them to address mismatches. I'd be curious to know (from anyone who
knows) why maximum power (matched impedance) is ideal in some situations, but a low-Z to high-Z
arrangement is the ideal in other situations (ie: guitar to amplifier or microphone to PA). I
realize the
outcome in the latter situation; you preserve the audio highs and lows. But need some schooling on
why to maximize voltage at the load (vs. maximizing transfer power) in those situations and why the
mismatch isn't destructive to the signal. And correspondingly, why the same isn't true of the
impedance match between output tubes and the primary of an output transformer since its a similar
audio application. Again, I understand the requirements and outcomes... but am confused about the
underlying physics.
It is to do with the frequency range. As soon as the length of the cable becomes a significant
fraction of the electrical wavelength in the cable, you need to impedance match. That is because
energy is reflected at an impedance discontinuity, so you end up with standing waves along the length
of the cable. For a 1 metre long coax the wavelength becomes a significant fraction of the cable
length by about 10MHz, so you need to impedance match.

With audio, the wavelength is so long (at 20kHz it is about 10km in a typical coax cable) you
absolutely do not need to match. Hence you guitar example.

Going way back long distance telegraph and telephone lines were significant fractions of an audio
wavelength in the cables, and they needed to impedance match.

Craig

Re: 11801 is reassembled, up and running :-)

Bob Koller
 

They must have changed the cal output circuitry on the later models.

Re: Impedance matching question

David Berlind
 

Thanks Craig... so, if I were to summarize what you wrote, at such short
distances, there's really no opportunity for a reflected signal to go out
of phase with the incident signal?

In watching the EEVblog videos, he's clearly using pretty high frequencies
(well out of the audio spectrum). So, your explanation is consistent with
that.

So, two questions remain.

1. why is an impedance match between output tubes and the output
transformer primary so important given the short physical differences. Or,
maybe the tube specs are not showing the actual impedance, but rather the
recommend Hi-Z on the load end to offer the optimal Low-Z to Hi-Z ratio?

2. Why is a Low-Z to Hi-Z ratio desired in audio applications vs. an
impedance match? I understand your point that it doesn't matter at low
distances, but Low-Z to Hi-Z appears to be an objective (iow, the objective
is to avoid a match, by orders of magnitude). Does the higher resultant
voltage (amplitude) at the load spread the signal out in a way that give
the amp more to work with from a fidelity POV?

On Tue, Mar 26, 2019 at 9:59 AM Craig Sawyers <c.sawyers@...>
wrote:

That was a decent video, thank you. It doesn't seem like he had problems
with the terminators
themselves. In fact, he uses them to address mismatches. I'd be curious
to know (from anyone who
knows) why maximum power (matched impedance) is ideal in some
situations, but a low-Z to high-Z
arrangement is the ideal in other situations (ie: guitar to amplifier or
microphone to PA). I
realize the
outcome in the latter situation; you preserve the audio highs and lows.
But need some schooling on
why to maximize voltage at the load (vs. maximizing transfer power) in
those situations and why the
mismatch isn't destructive to the signal. And correspondingly, why the
same isn't true of the
impedance match between output tubes and the primary of an output
transformer since its a similar
audio application. Again, I understand the requirements and outcomes...
but am confused about the
underlying physics.
It is to do with the frequency range. As soon as the length of the cable
becomes a significant
fraction of the electrical wavelength in the cable, you need to impedance
match. That is because
energy is reflected at an impedance discontinuity, so you end up with
standing waves along the length
of the cable. For a 1 metre long coax the wavelength becomes a significant
fraction of the cable
length by about 10MHz, so you need to impedance match.

With audio, the wavelength is so long (at 20kHz it is about 10km in a
typical coax cable) you
absolutely do not need to match. Hence you guitar example.

Going way back long distance telegraph and telephone lines were
significant fractions of an audio
wavelength in the cables, and they needed to impedance match.

Craig





Re: 11801 is reassembled, up and running :-)

Reginald Beardsley
 

The user manual I quoted has a revision date of May 1989. The August 1988 service manual states the calibrator is < 35 ps. The February 1989 revision page states the same rise time.

As I noted previously. I also got a 300 ps rise time from the calibrator due to something not being set properly. This persisted when I used autoset, but eventually went away after I power cycled it a couple of times and used the autoset again. The behavior suggested an off by 10x error in the timebase generator strobe line. But I have far too little experience with the instrument to be sure of anything.

Re: Impedance matching question

Dale H. Cook
 

At 09:58 AM 3/26/2019, Craig Sawyers wrote:

Going way back long distance telegraph and telephone lines were significant fractions of an audio wavelength in the cables, and they needed to impedance match.
Through the tube era and the discrete solid state era professional balanced audio lines in radio and recording studios were also impedance matched at 600 ohms (earlier 500 ohms) because they were transformer coupled and power sourced. The constant impedance was implemented for maximum power transfer. It was not until the advent of high quality op amps that voltage sourced balanced audio lines became the norm in studios.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html

Re: 11801 is reassembled, up and running :-)

Albert Otten
 

On Tue, Mar 26, 2019 at 03:28 PM, Reginald Beardsley wrote:


The user manual I quoted has a revision date of May 1989. The August 1988
service manual states the calibrator is < 35 ps. The February 1989 revision
page states the same rise time.

As I noted previously. I also got a 300 ps rise time from the calibrator due
to something not being set properly. This persisted when I used autoset, but
eventually went away after I power cycled it a couple of times and used the
autoset again. The behavior suggested an off by 10x error in the timebase
generator strobe line. But I have far too little experience with the
instrument to be sure of anything.


Re: Impedance matching question

Craig Sawyers
 

1. why is an impedance match between output tubes and the output transformer primary so
important given the short physical differences. Or, maybe the tube specs are not showing the actual
impedance, but rather the recommend Hi-Z on the load end to offer the optimal Low-Z to Hi-Z ratio?
Because there is a mismatch between the high plate resistance of the output pentodes or tetrodes
(about 4.5k-ohms in push pull) and the loudspeaker. The impedance transformation goes as the square
root of the turns ratio. So to match 4.5kk to 8 ohms needs a turns ratio of root(4500/8) = 24:1 turns
ratio. To deliver 30W into 8 ohms (typical for 6550's in push pull) needs 21V peak, times 24 = 500V.
The anode/plate voltage will be 560V - which is consistent with a 500V signal peak.

The details are of course more complicated than that - so you'll just have to read around.

2. Why is a Low-Z to Hi-Z ratio desired in audio applications vs. an impedance match?
Because if you do an impedance match at audio you lose half the signal. So your signal to noise ratio
goes down by 6dB. Which is why no-one does it - there is absolutely no upside.

We're kind of off-topic here. If you want to discuss audio, try
https://www.diyaudio.com/forums/index.php or https://www.stevehoffman.tv/ . There are lots more out
there.

Craig

Re: Repairability of SD-24, SD-30 & SD-32 sampling heads

Egge Siert
 

Hi Reginald,

It is in the archives:

https://groups.io/g/TekScopes/topic/7659008#141241

Enjoy.

Egge Siert

Switching power supplies

Brendan
 

I have a few questions about power supplies and would like some opinions. Are switching powers supplies inherently harder on components than linear power supplies? If you picked up a 70's-80's vintage scope with a SMPS would you replace power supply components before using it as a daily driver? Or do you treat all power supplies the same and visually inspect, check for ripple and call it good? From my reading it seems that when a SMPS melts down the chain reaction damage has the possibility of being bad.


Brendan

Re: Switching power supplies

Chuck Harris
 

Tough question.

Yes, switching supplies are inherently harder on
components than linear, but they also use much better
parts. And they are more likely to be designed using
math, rather than rules of thumb. And they are more
likely to have extensive protection devices to prevent
catastrophic failures from occurring.

Linear supplies are heavy, are more likely to break your
toes, and make copious amounts of heat. That heat soaks
into everything around them, and causes component failure.

Tektronix used some really uber parts in their 70's vintage
supplies. In some cases, I cannot find modern parts that
were as good as what they used.

I would take it through calibration, testing the ripple
and voltages, and just use it. If it fails, fix the failure.

Odds are very, very, good that in prophylactically replacing
parts you are going to introduce failures, now and in the
future. Replacing a 10,000 hour rated part with a 1000 hour
part, isn't going to improve reliability.

-Chuck Harris

Brendan via Groups.Io wrote:

I have a few questions about power supplies and would like some opinions. Are switching powers supplies inherently harder on components than linear power supplies? If you picked up a 70's-80's vintage scope with a SMPS would you replace power supply components before using it as a daily driver? Or do you treat all power supplies the same and visually inspect, check for ripple and call it good? From my reading it seems that when a SMPS melts down the chain reaction damage has the possibility of being bad.


Brendan

Re: Switching power supplies

Tom Gardner
 

On 26/03/19 20:37, Brendan via Groups.Io wrote:
I have a few questions about power supplies and would like some opinions. Are switching powers supplies inherently harder on components than linear power supplies? If you picked up a 70's-80's vintage scope with a SMPS would you replace power supply components before using it as a daily driver? Or do you treat all power supplies the same and visually inspect, check for ripple and call it good? From my reading it seems that when a SMPS melts down the chain reaction damage has the possibility of being bad.
Replace Rifa mains interference suppression delayed action smoke generators on sight; if there is any sign of cracking in the transparent case, don't even turn it on.

Where there are tantalum beads that are operating near their rated voltage (e.g. a 15V tant on a 13V rail), replace those with a higher voltage.

Visually inspect, measure, replace if there's a problem.

Otherwise, if it ain't broke, don't fix it.

11801 calibrator rise time

Reginald Beardsley
 

I think I figured out why the calibrator rise time measurement doesn't read to spec. I warmed up the 11810 for about 40 minutes to do the Enhanced Accuracy adjustments. That seemed to go fine except afterwards when I measured the rise time I was getting >400 ps. The 11801 will remember the incorrect settings through a power cycle.

If you do the Loop Gain adjustment in the Enhanced Accuracy menu it resets something which results in incorrect values. If you go to the Utility menu and press Initialize you should now be able to set the Trigger to Internal, press Autoset, adjust the time base, turn on Hardware measurement and get <35 ps rise time.

Unfortunately, it does not appear to be possible to measure the rise time on more than one one channel.

I think a bunch of the Chinese DSO designers must have spent a lot of time using an 11801 and thought that was what a scope UI should look like. Even after reading all the way through the User manual the UI is still confusing because of the strange locations of various settings.

The tip off came from this line on p 82 of the User manual:

"Whenever you begin a new task using the 11801. you should initialize the system so that all the settings are at "factory default" . That way you do not get unexpected results because of settings remaining from the last use of the 11801."

BTW I have a 2 port divider feeding the upper channel of my SD-22s and the waveforms look to be exactly the same. The pieces of hardline from the divider to the heads are different lengths and I have not been able to shift one relative to the other to see how closely they overlay as there is a 340 ps delay between the two. The divider increases the rise time to 42 ps. It's an MBC Technology unit. No frequency rating specified.

I just discovered that it reset the time base readings when I removed the splitter and turned off one channel to take a look at how much of the 7% overshoot was the splitter which it turns out is about 5% of the total.

There's a little bit of ringing on the calibrator step at 12.2 GHz which is probably the result of reflections where the semi-rigid SMA cable connects to the 3.5 mm calibrator and sampling head connectors.

I am quite agog at how precise this thing is. Getting the full performance out of it will take some serious skill.

Re: Switching power supplies

Glenn Little
 

My take on this after replacing thousands of surface mount electrolytic capacitors is replace them all when you get a new to you device that is more than ten or so years old.

Switchers are harder on electrolytics because they run at a higher frequency than a linear supply and the bean counters usually required the least expensive part to be installed to get the instrument out of warranty.

My take on surface mounted electrolytics is that the fail for two reasons. The seal where the positive lead exits the case was violated during the reflow process. Or the capacitors was not properly selected for the task at hand.

When I replace power supply capacitors in switchers, I select the highest temperature capacitors available with the highest ripple current rating that will fit into the available space. I do not use 85 degree C capacitors unless they are all that is made in that value and voltage.

When doing repair work on cameras and tape decks in TV stations, I never saw a failure of a capacitor that I replaced with the above guidelines. I may have not waited long enough or may have been lucky.

Large computer grade electrolytics are the exception. Do not replace unless needed. These do not usually cause board damage that cannot be repaired.

See: https://www.eetimes.com/author.asp?section_id=183&doc_id=1279791#

Hope this helps.

Glenn

On 3/26/2019 4:37 PM, Brendan via Groups.Io wrote:
I have a few questions about power supplies and would like some opinions. Are switching powers supplies inherently harder on components than linear power supplies? If you picked up a 70's-80's vintage scope with a SMPS would you replace power supply components before using it as a daily driver? Or do you treat all power supplies the same and visually inspect, check for ripple and call it good? From my reading it seems that when a SMPS melts down the chain reaction damage has the possibility of being bad.


Brendan


--
-----------------------------------------------------------------------
Glenn Little ARRL Technical Specialist QCWA LM 28417
Amateur Callsign: WB4UIV wb4uiv@... AMSAT LM 2178
QTH: Goose Creek, SC USA (EM92xx) USSVI LM NRA LM SBE ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"

Re: Repairability of SD-24, SD-30 & SD-32 sampling heads

Jim Ford
 

Hmmm...  Did he die-bond, wire-bond, and package the FPGA or just solder it onto the board?  Not so impressive if it's the latter.  Still, the link to the website shows microwave devices being bonded in hybrid packages to repair mixers in spectrum analyzers - now that's impressive!Jim FordSent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: Craig Sawyers <c.sawyers@...> Date: 3/25/19 5:07 PM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Repairability of SD-24, SD-30 & SD-32 sampling heads He clearly knows what he is doing. You don't knock up something with an Altera FPGA unless you have a major development lab at your disposal https://www.qsl.net/ct1dmk/gw_04.jpgCraig> -----Original Message-----> From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jose Luu> Sent: 25 March 2019 23:29> To: TekScopes@groups.io> Subject: Re: [TekScopes] Repairability of SD-24, SD-30 & SD-32 sampling heads>> https://www.qsl.net/ct1dmk/wbond_ex.html>> This guy seem to have made a home hybrid lab and repairs. Could former professionals comment ?>> Best> Jose>>> On Mon, Mar 25, 2019 at 9:30 PM Bob Koller via Groups.Io <testtech= yahoo.com@groups.io> wrote:>> > How well I know. My last job in the hybrid business was at Teledyne> > Microelectronics. Space Qual parts had reams of paperwork, x-rays,> > PIND test,photographs, etc...> > Tek, Keysight, and many other still , out of necessity, use hybrid> > technology with custom parts, in the front ends, and other places, of> > very high frequency instruments. But, commercial hybrid production> > has, I think, largely been supplanted by ASIC, FPGA, and more modern,> > more reliable, less expensive technology.> >> >> >> >>>

Re: Switching power supplies

Kevin Oconnor
 

I have to agree with Chuck. Most test equipment with imbedded/integrated switchers are going to be very difficult to qualify parts if working. Their functionality can be extremely subtle. Blanket replacement of components will more likely cause additional reliability issues.
My 1978 Tek 485 has had spot repairs, but it’s still working fine. I’d never replace caps in it without good reason.
I have an HP 500MHz digital scope with one of this 3rd party modular switchers. (I forget the HP model but 5digits). Switcher went bad. No docs, no schematics. Really hopeless to diagnose. Best option was an eBay replacement switcher. $50 and never looked back.
Now if you are talkin about bad caps in an LCD monitor or TV, that’s where “replace em all” will serve u well! Lots of crap used in those devices.

K

Sent from kjo iPhone

SD-26 disassembly, how to separate sampling gate block from pcb

Albert Otten
 

Does anyone know how to remove the complete sampler block from the printed circuit board? I ask since I'd like to replace the channel select switches. The solder lips are hidden between sampler block and pcb. I removed the 6 block attachment screws but still I can't remove the block with reasonable force. Maybe more force is needed because there are several contact pins between this block and the pcb. The nice close-up repair photos by Leo Bodnar show the internals of the sampler block but not how to remove the block (the block perhaps was still in situ during repair).
It seems that the tiny plastic push pins of the switches have been cut away deliberately. I guess this has been done to prevent any accidental touches by an operator. I can still activate the switches using a small screw driver, so I don't want to take the risk of damaging the otherwise functioning head.

Albert

Re: Switching power supplies

Chuck Harris
 

SMD electrolytic capacitors are a tricky problem in several ways:

First, they are easily damaged by the heat of the reflow oven,
and solvents that may be used in cleaning flux,... though cleaning
flux isn't done much if at all, on consumer grade equipment.

Second, it is almost impossible to find out what was originally
installed. There are no standardized markings to identify the
manufacturer, or what grade capacitor was installed.

Third, it is hard for a re-worker to find a complete spectrum of
ESR values from which to select replacement capacitors.

When you grab an assortment of SMD electrolytic capacitors off
of ebay, you are unlikely to be able to match more than just the
capacitance and voltage of the original. Never the ESR or lifetime
rating.

When you buy from Mouser, or DigiKey, you will find that cute
little 20uf, 16V cap you need to replace, (because its ESR is 3 ohms
while its cohorts are all 0.5 ohm), can only be had with ESR's of
0.7 ohm, 3 ohms, and 12 ohms! Replace the 0.5 ohm ESR caps with a
0.7 ohm cap, and you have taken the expected life of the cap and cut
it in half (or more). Put in a 3 or 12 ohm version, and you will have
improved nothing.

Tricky indeed!

-Chuck Harris

Kevin Oconnor wrote:

I have to agree with Chuck. Most test equipment with imbedded/integrated switchers are going to be very difficult to qualify parts if working. Their functionality can be extremely subtle. Blanket replacement of components will more likely cause additional reliability issues.
My 1978 Tek 485 has had spot repairs, but it’s still working fine. I’d never replace caps in it without good reason.
I have an HP 500MHz digital scope with one of this 3rd party modular switchers. (I forget the HP model but 5digits). Switcher went bad. No docs, no schematics. Really hopeless to diagnose. Best option was an eBay replacement switcher. $50 and never looked back.
Now if you are talkin about bad caps in an LCD monitor or TV, that’s where “replace em all” will serve u well! Lots of crap used in those devices.

K

Sent from kjo iPhone



Re: Switching power supplies

David Kuhn
 

Switching supplies can be a problem when they fail. I recently had a
Agilent VXI E4808A chassis that the main power supply failed, at least
its 12 volts out did. That power supply is huge with a logic board mounted
about it ( I think it's a power supply monitor board). It's a lot of
physical work just to get it apart to check caps. Anyway, I doubt
HP/Agilent made that power supply themselves. I really don't know, as to
get information out of them is like pulling hen's teeth. They no longer
sell, or support, those 4 slot VXI chassis's, so it would be nice of them
to release the schematics, or service information. I think you can only
hope to buy a used chassis somewhere. I would love to have the schematics
for them.

In other switches that fail to start, I often find what I call the
"start-up" capacitor in the primary is open or leaking or has a high ESR.
I call it the "start-up cap" as I do not fully understand switching
supplies, and often if there are schematics, there is not a theory of
operation with them, but there is often a capacitor in the primary circuit
that looks to be a short to ground and then charge-up to create the initial
switch swing to get it going and then afterwards, the power supply self
sustains.

So a lot of times with a dead switcher, I have been able to fix them by
replacing the small electrolytic in the primary side of the circuit. It is
usually a 4.7uf or 10uf. Other than that, if the rest are not physically
leaking or swelled and the supply is working I don't touch it.

One instrument that I work on has an on-board +5volt switching regulator
circuit. It is very reliable. Two times (since 1999) I have seen the
switch regulator fail where the output drive to the FET shorts to ground
allowing the supply voltage (~+12volts) go through to the output with no
over-voltage protection, not even a +5.6volt zener to short. It blows
every TTL chip on the +5volt rail, and some some regulators that follow it.
Stupid, stupid German design. You have to be very urber careful probing
that +5volt switching regulator circuit. One slip of the scope probe and
you can simulate the switching regulator IC output shorting to ground
turning on the pass FET full time. So, I've learned "If it aint broke,
don't fix it"!

Sorry it that was slightly off topic.

Dave

On Wed, Mar 27, 2019 at 8:29 AM Chuck Harris <cfharris@...> wrote:

SMD electrolytic capacitors are a tricky problem in several ways:

First, they are easily damaged by the heat of the reflow oven,
and solvents that may be used in cleaning flux,... though cleaning
flux isn't done much if at all, on consumer grade equipment.

Second, it is almost impossible to find out what was originally
installed. There are no standardized markings to identify the
manufacturer, or what grade capacitor was installed.

Third, it is hard for a re-worker to find a complete spectrum of
ESR values from which to select replacement capacitors.

When you grab an assortment of SMD electrolytic capacitors off
of ebay, you are unlikely to be able to match more than just the
capacitance and voltage of the original. Never the ESR or lifetime
rating.

When you buy from Mouser, or DigiKey, you will find that cute
little 20uf, 16V cap you need to replace, (because its ESR is 3 ohms
while its cohorts are all 0.5 ohm), can only be had with ESR's of
0.7 ohm, 3 ohms, and 12 ohms! Replace the 0.5 ohm ESR caps with a
0.7 ohm cap, and you have taken the expected life of the cap and cut
it in half (or more). Put in a 3 or 12 ohm version, and you will have
improved nothing.

Tricky indeed!

-Chuck Harris

Kevin Oconnor wrote:
I have to agree with Chuck. Most test equipment with imbedded/integrated
switchers are going to be very difficult to qualify parts if working. Their
functionality can be extremely subtle. Blanket replacement of components
will more likely cause additional reliability issues.
My 1978 Tek 485 has had spot repairs, but it’s still working fine. I’d
never replace caps in it without good reason.
I have an HP 500MHz digital scope with one of this 3rd party modular
switchers. (I forget the HP model but 5digits). Switcher went bad. No docs,
no schematics. Really hopeless to diagnose. Best option was an eBay
replacement switcher. $50 and never looked back.
Now if you are talkin about bad caps in an LCD monitor or TV, that’s
where “replace em all” will serve u well! Lots of crap used in those
devices.

K

Sent from kjo iPhone