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I was watching a video on impedance matching experiments and, supposing you
had a 75 ohm signal generator connected to a 1 M-ohm oscilloscope input, one
option that was suggested was to add a 75 ohm terminator at the scope input. The
only diagram I can find of this shows a physical connection of the BNC terminator
(mounted on a BNC T connector) to ground.

is that physical connection from the back of the terminator necessary to successfully terminate and if so, to what ground
do you connect? Or, do the terminators by themselves take care of the connection to ground through the BNC
connection?

A through teminator will give better performance at high frequencies. You can get away with a Tee and terminator up to a few MHz, but things go downhill very quickly after that.

A thru terminator is a 50 ohm load with an inline connection to the output which is expected to be 1 Meg or more. Surprisingly, none of the Chinese eBay thru terminators landed in the junk bin when I was testing devices using one of Leo Bodnar's 100 ps pulse units.

If you're using a 75 ohm signal generator, make sure you're using 75 ohm cable and BNCs in addition to the 75 ohm thru.


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Usually the terminator contains the ground connection.? Going for your description rather than a drawing or photo.?Jim FSent from my Verizon, Samsung Galaxy smartphone

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.

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.

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

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

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

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
or . There are lots more out
there.

Craig

Now that I've read and processed more about impedance than I care to admit, I'm wondering if there's anyone in the group has some extra BNC pass-through terminators that they don't need. Namely 50, 75, and 600 ohm for now (I don't have the option to change input impedance on my scopes).

thanks if anyone has these and would be willing to part with them.

David,

These should be good quality:



I have 8 very good generic Chinese thru terminators which I bought on eBay. But I can't find any information about who I bought them from. TDR testing on my 11801 shows no reflections below 500 MHz which is all you can really expect out of BNC and usable to 3 GHz. My 200 MHz Instek MSO-2204EA shows no reflection, whereas my 1.5 GHz LeCroy DDA-125 shows some.

I strongly urge you to get one of Leo's pulsers:



and learn to do TDR testing of connectors and cable. That will allow you to buy connectors on eBay and return those which test bad. The major problem I have found with Chinese BNC adapters is the male connectors have intermittent connections. Unfortunately, I only figured how to test them long after the return period had ended.

I'd been playing around testing connectors on my 11801 using some 20 GHz SD-26 heads and the calibrator output and am quite blown away by the sensitivity. I can see the reflection from an SMA-F to N-F and an N-M to BNC-F stack and from an SMA-F to BNC-F. In both cases the reflections are above 700 MHz.

Reg

Thanks Reginald! The at answer was worth the price just for the TDR link.
Do you know if there's a Mac equivalent out there somewhere? Looks like I
need Windows for that one. On the terminators, I think I'd be willing to
pay extra if it meant no hassles on the terminators. I've done some
searching and see them out there. I thought I'd check here first given that
I've shipped stuff around the world to list members "in need" for nothing
more than the cost of postage and I know there are others on this list that
sometimes do the same with their surplus (something I'm very very thankful
to have benefitted from already).

On Thu, Apr 4, 2019 at 5:32 PM Reginald Beardsley via Groups.Io <pulaskite=
[email protected]> wrote:

David,

These should be good quality:




I have 8 very good generic Chinese thru terminators which I bought on
eBay. But I can't find any information about who I bought them from. TDR
testing on my 11801 shows no reflections below 500 MHz which is all you
can really expect out of BNC and usable to 3 GHz. My 200 MHz Instek
MSO-2204EA shows no reflection, whereas my 1.5 GHz LeCroy DDA-125 shows
some.

I strongly urge you to get one of Leo's pulsers:




and learn to do TDR testing of connectors and cable. That will allow you
to buy connectors on eBay and return those which test bad. The major
problem I have found with Chinese BNC adapters is the male connectors have
intermittent connections. Unfortunately, I only figured how to test them
long after the return period had ended.

I'd been playing around testing connectors on my 11801 using some 20 GHz
SD-26 heads and the calibrator output and am quite blown away by the
sensitivity. I can see the reflection from an SMA-F to N-F and an N-M to
BNC-F stack and from an SMA-F to BNC-F. In both cases the reflections are
above 700 MHz.

Reg

You *only* need to use a PC to set the output voltage differently from the default. Otherwise the USB is just for power. So you can use the USB port on a DSO.

I did run the Windows app once. I'm a hardcore Unix guy. I use Solaris like God intended. But for general use there is no reason to change the default settings. I've never changed the settings of the 100 ps pulse version. And I'm pretty sure I left the square wave unit at the setting Leo supplied it at.

The 100 ps pulse version (available by request at the same price) is better for TDR, but the square wave is more useful if you need to adjust an analog scope. Because the 11801 has a 200 KHz square wave I've been playing with square wave TDR instead of the impulse TDR I was doing when I first tested the Chinese connectors. That was prior to getting the 11801 so I only tested them to 1.5 GHz.

My major project (i.e. will take several years) is FOSS DSO FW that does what I consider a proper job. I've not found a single DSO that I consider proper, even at the price level of a Keysight MSOX3104T or R&S RTM3104. My hope is to hijack Chinese COTS DSOs and install my FW in place of the factory FW.

A significant motivation for acquiring the 11801 is to be able to verify signal skew in FPGA DSP stages rather than blindly relying on the simulator software. I have almost finished my first reading of "VLSI DSP Systems" by K.K. Parhi. After that I will resume reading "FPGA based implementations of Digital Signal Processing" by Roger Woods et al. Woods made so many references to Parhi I decided to get a copy and was delighted at the decision. My second reading of Parhi will coincide with writing Verilog code to implement the various topologies Parhi describes and comparing the simulation results to measurements. I hope to be able to measure timings to 3-5 ps which in turn places very severe demands upon fixtures and cables. So I have a lot to learn.

That problem, has in turn led to the need to refine my understanding of microwave construction. So I have begun reading "Planar Microwave Engineering" by Thomas H. Lee. It's 850 pages, but a pure delight to read as he is broadly literate and writes superb prose.

Aside from stackable arbitrary filter operations such as the LeCroy provides, I plan to include the ability to do vector network analysis to the limits of the DSO using TDR. I spent my career in reflection seismology, so I can directly interpret the TDR trace in terms of impedance contrasts by inspection. At least if I have enough BW. I did encounter a connector stack yesterday that would need 50 GHz to resolve. Not quite sure how to get that BW at a price I can afford.

BTW Leo's pulsers are AC coupled and will tolerate an SOLT calibration. So you can readily do VNA if you know how to use MATLAB or Octave.

Have Fun!
Reg