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A fellow engineer raised a question re the 75 ohm
characteristic impedance of the HP8714ET I just purchased off eBay.
Although I have minimum-loss matching pads ordered and enroute, does
anyone here know just what would be involved if I were to open up the
unit and try to change the impedance with some part swaps? The firmware
already supports a choice of 75 ohm or 50 ohms as the impedance, for
Smith charts, etc. I suspect that before I do that, I had better get the
service manuals, but that's for the future - thus far I'm less than 1/4
of the way thru reading the user manual, and I have a bunch of other
documentation to read as well. I'm very pleased with my purchase, if only
for the education I'm getting!
Steve Hendrix
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Look in the service manual. It depends on the instrument. I remember looking at an HP 'scope that had 75¦¸ input long ago and it was the same as the 50¦¸ version but with an internal minimum loss pad that could easily be removed and have the N connector on the front panel replaced with a 50¦¸ version. Other things, like the 8753 test sets require a complete replacement of the samplers, which can easily cost more than the analyzer.
On Tue, Feb 5, 2019 at 6:55 PM Steve Hendrix < SteveHx@...> wrote:
A fellow engineer raised a question re the 75 ohm
characteristic impedance of the HP8714ET I just purchased off eBay.
Although I have minimum-loss matching pads ordered and enroute, does
anyone here know just what would be involved if I were to open up the
unit and try to change the impedance with some part swaps? The firmware
already supports a choice of 75 ohm or 50 ohms as the impedance, for
Smith charts, etc. I suspect that before I do that, I had better get the
service manuals, but that's for the future - thus far I'm less than 1/4
of the way thru reading the user manual, and I have a bunch of other
documentation to read as well. I'm very pleased with my purchase, if only
for the education I'm getting!
Steve Hendrix
|
From an cost standpoint, it would only be worth converting to 50 Ohms if you get the parts for free. Here are some pictures of the RF deck in the 75 Ohm design:
The main expense is in the gold plated microcircuit in the front (part number 5086-7937). You would need to find the equivalent part for a 50 Ohm analyzer and that will probably cost more than it's worth. There are other expensive parts too, e.g. the two Type N connectors and possibly the coax cables inside.
For what it's worth, and I suspect you don't want to hear this, I would suggest returning the 75 Ohm analyzer if you can and waiting until you find one that's right. Even if you get a partial refund, you will probably be happier in the end. If not, use the minimum loss pads. They will work and you will be able to calibrate the analyzer correctly with a 50 Ohm cal kit, but you will lose 2 times 5.7dB in sensitivity on transmission and reflection measurements.
Vladan
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At 2019-02-05 03:33 PM, pianovt via Groups.Io wrote:
From an cost standpoint, it
would only be worth converting to 50 Ohms if you get the parts for
free.
Thank you for the confirmation. That's exactly the sort of expert
knowledge I was looking for with my question.
For what it's
worth, and I suspect you don't want to hear this, I would suggest
returning the 75 Ohm analyzer if you can and waiting until you find one
that's right.
For now it makes sense to me to keep it, and count the cost as a cost of
my own education. I can learn a whole bunch using this unit, and for now
at least, it goes well beyond my needs in terms of precision and dynamic
range. If I learn enough, I might decide I need something fancier, but
for now it's a tremendous bargain for me.
Use the minimum
loss pads. They will work and you will be able to calibrate the analyzer
correctly with a 50 Ohm cal kit,
Thank you for that confirmation. I was hoping for such, but couldn't
absolutely prove it in my own head. I need to play with the calibration a
bit to fully understand what parameters they're actually saving
inside.
but you will lose 2
times 5.7dB in sensitivity on transmission and reflection
measurements.
Good, at least I'm on the right track. I not only lose 11.4 dB of dynamic
range, but I also have a ready-made attenuator so can put an extra 5.7 dB
into the transmission port before I blow anything up. Just a bit more
protection against my fumble-fingers!
Thank you for the detailed response to my question!
Steve Hendrix
|
Hi Steve, an HP8714B just sold for $565 US. I suspect you paid much less for the 75 ohm version and as much for shipping..? My suggestion is to keep it and use it with 75/50 pads to learn about it while watching for a cheap 50 ohm version, not working if you have repair abilities, and use your 75 ohm version to troubleshoot and repair. Do you have the pads? I have a couple of new Greenpar 502-4718-701, 90-48 that you can have for the price of shipping. Contact me offline. Be aware that the 75 ohm Type N are different from the 50 ohm Type N and you could damage them by attaching the wrong one. Someone else may have a better explanation but 75 ohms seems to explain the different dimensions. PeterB
On Wed, Feb 6, 2019 at 6:54 AM Steve Hendrix < SteveHx@...> wrote:
At 2019-02-05 03:33 PM, pianovt via Groups.Io wrote:
From an cost standpoint, it
would only be worth converting to 50 Ohms if you get the parts for
free.
Thank you for the confirmation. That's exactly the sort of expert
knowledge I was looking for with my question.
For what it's
worth, and I suspect you don't want to hear this, I would suggest
returning the 75 Ohm analyzer if you can and waiting until you find one
that's right.
For now it makes sense to me to keep it, and count the cost as a cost of
my own education. I can learn a whole bunch using this unit, and for now
at least, it goes well beyond my needs in terms of precision and dynamic
range. If I learn enough, I might decide I need something fancier, but
for now it's a tremendous bargain for me.
Use the minimum
loss pads. They will work and you will be able to calibrate the analyzer
correctly with a 50 Ohm cal kit,
Thank you for that confirmation. I was hoping for such, but couldn't
absolutely prove it in my own head. I need to play with the calibration a
bit to fully understand what parameters they're actually saving
inside.
but you will lose 2
times 5.7dB in sensitivity on transmission and reflection
measurements.
Good, at least I'm on the right track. I not only lose 11.4 dB of dynamic
range, but I also have a ready-made attenuator so can put an extra 5.7 dB
into the transmission port before I blow anything up. Just a bit more
protection against my fumble-fingers!
Thank you for the detailed response to my question!
Steve Hendrix
|
Just to note¡
?
¡®90-48¡¯ is most probably the yeardate/week of manufacture code.
?
Nigel
?
toggle quoted message
Show quoted text
From: [email protected] [mailto:[email protected]]
On Behalf Of peter bunge
Sent: 06 February 2019 14:20
To: [email protected]
Subject: Re: [HP-Agilent-Keysight-equipment] HP8714ET Impedance
?
Hi Steve, an HP8714B just sold for $565 US.
I suspect you paid much less for the 75 ohm version and as much for shipping..?
My suggestion is to keep it and use it with 75/50 pads to learn about it while watching for a cheap 50 ohm version, not working if you have repair abilities, and use your 75 ohm version to troubleshoot and repair.
Do you have the pads? I have a couple of new Greenpar 502-4718-701, 90-48 that you can have for the price of shipping. Contact me offline.
Be aware that the 75 ohm Type N are different from the 50 ohm Type N and you could damage them by attaching the wrong one. Someone else may have a better explanation but 75 ohms seems to explain the different dimensions.
?
On Wed, Feb 6, 2019 at 6:54 AM Steve Hendrix <SteveHx@...> wrote:
At 2019-02-05 03:33 PM, pianovt via Groups.Io wrote:
From an cost standpoint, it would only be worth converting to 50 Ohms if you get the parts for free.
Thank you for the confirmation. That's exactly the sort of expert knowledge I was looking for with my question.
For what it's worth, and I suspect you don't want to hear this, I would suggest returning the 75 Ohm analyzer if you can and waiting until you find one that's right.
For now it makes sense to me to keep it, and count the cost as a cost of my own education. I can learn a whole bunch using this unit, and for now at least, it goes well beyond my needs in terms of precision and dynamic range. If I learn enough, I might decide
I need something fancier, but for now it's a tremendous bargain for me.
Use the minimum loss pads. They will work and you will be able to calibrate the analyzer correctly with a 50 Ohm cal kit,
Thank you for that confirmation. I was hoping for such, but couldn't absolutely prove it in my own head. I need to play with the calibration a bit to fully understand what parameters they're actually saving inside.
but you will lose 2 times 5.7dB in sensitivity on transmission and reflection measurements.
Good, at least I'm on the right track. I not only lose 11.4 dB of dynamic range, but I also have a ready-made attenuator so can put an extra 5.7 dB into the transmission port before I blow anything up. Just a bit more protection against my fumble-fingers!
Thank you for the detailed response to my question!
Steve Hendrix
|
N connectors are air dielectric coaxial transmission lines where they
mate. Impedance for an air dielectric coaxial line is:
Z0 = 60 ln(r0/ri)
So, a 75 ohm center conductor (ri) is necessarily smaller in diameter
than a 50 ohm center conductor for a given size shield diameter (r0).
If you mate a 50 ohm male N connector with a 75 ohm female N connector,
the usual result is to break one of the leaves of the center conductor
on the female connector.
The same does not apply to typical BNC connectors, as the 50 ohm BNC
has a teflon dielectric, but the 75 ohm BNC has an air dielectric, so
the center conductors are the same diameter.
More than you wanted to know.
-Chuck Harris
peter bunge via Groups.Io wrote:
toggle quoted message
Show quoted text
Hi Steve, an HP8714B just sold for $565 US.
I suspect you paid much less for the 75 ohm version and as much for
shipping..
My suggestion is to keep it and use it with 75/50 pads to learn about it
while watching for a cheap 50 ohm version, not working if you have repair
abilities, and use your 75 ohm version to troubleshoot and repair.
Do you have the pads? I have a couple of new Greenpar 502-4718-701, 90-48
that you can have for the price of shipping. Contact me offline.
Be aware that the 75 ohm Type N are different from the 50 ohm Type N and
you could damage them by attaching the wrong one. Someone else may have a
better explanation but 75 ohms seems to explain the different dimensions.
PeterB
[image: 75_50 ohm pads.JPG]
|
At 2019-02-06 09:19 AM, peter bunge wrote:
Hi Steve, an HP8714B just sold
for $565 US.
I suspect you paid much less for the 75 ohm version and as much for
shipping..?
Somewhat less, but not all that much. And shipping wasn't bad, as it came
from the same side of the continent as me.
My suggestion is to
keep it and use it with 75/50 pads to learn about it while watching for a
cheap 50 ohm version, not working if you have repair abilities, and use
your 75 ohm version to troubleshoot and repair.
That's my intent exactly, and it'll likely be a while before I'll have
need of that much additional capability.
Do you have the
pads? I have a couple of new Greenpar 502-4718-701, 90-48 that you can
have for the price of shipping. Contact me
offline.
Rats! I already order some and they're enroute. Thanks for the offer,
though.
Be aware that the
75 ohm Type N are different from the 50 ohm Type N and you could damage
them by attaching the wrong one. Someone else may have a better
explanation but 75 ohms seems to explain the different
dimensions.
Yes, someone else already pointed that out to me, and an article on N
connectors on Wikipedia that explains the differences quite well. It
appears HP used some good-quality connectors that are built for 75 ohms,
but have enough compliance in the fingers that grab the inner pin, that
they won't be damaged. Now that I know the difference, I think I'd better
make some time to apply my micrometer to my collection of various N
connectors and adapters, to be sure what's what.
Thanks for all the help!
Steve Hendrix
|
Yes 75 ohm N connectors are dimensioned differently than 50 ohm connectors. The difference is the center pin diameter. NEVER try and mate a 50 ohm to a 75 ohm as the 50 ohm pin is larger than the 75 ohm connector
/RANT
NEVER SHOULD HAVE MADE 2 VERSIONS WITH THE SAME OUTER THREAD
/RANTOFF
Content by Scott
Typos by Siri
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On Feb 6, 2019, at 4:21 PM, Steve Hendrix <SteveHx@...> wrote:
...
Be aware that the 75 ohm Type N are different from the 50 ohm Type N and you could damage them by attaching the wrong one. Someone else may have a better explanation but 75 ohms seems to explain the different dimensions. Yes, someone else already pointed that out to me, and an article on N connectors on Wikipedia that explains the differences quite well¡. I think I'd better make some time to apply my micrometer to my collection of various N connectors and adapters, to be sure what's what. Steve, Perhaps you would report any connector numbers you find and which is which. I have a dial calibers here that would let me figure our what connectors I have here. Thanks. Roy Roy Morgan K1LKY since 1958 k1lky68@...
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My dial caliper's jaws won't reach down inside the
connectors I have, but I can estimate the pin diameter by laying them
across the connector and using some magnification to line up the jaws
with the two sides of the pin. I also looked around for N connectors with
mechanical drawings; it's quite surprising how few of them specify the
center pin diameter, but I at least was able to find one for a 50 ohm N
connector.
Long story short, the 50 ohm version has a center pin diameter of 65 mils
or 1.65mm. The 75 ohm connector I measured looks to have a center pin
diameter in the 35 to 40 mil range. Definitely enough to make a
difference mechanically. And I wholeheartedly agree with the earlier
comment that they should NEVER have made two connectors so similar but
yet with potential to damage one another.
Steve Hendrix
P.S. My regular business email has a problem with sending, so I'm using
my personal email while i get htat sorted. I can still receive just fine
at SteveHx@...
At 2019-02-07 11:05 AM, Roy Morgan wrote:
toggle quoted message
Show quoted text
> On Feb 6, 2019, at 4:21 PM,
Steve Hendrix <SteveHx@...> wrote:
> ...
>> Be aware that the 75 ohm Type N are different from the 50 ohm
Type N and you could damage them by attaching the wrong one. Someone else
may have a better explanation but 75 ohms seems to explain the different
dimensions.
>
> Yes, someone else already pointed that out to me, and an article on
N connectors on Wikipedia that explains the differences quite well¡.
I? think I'd better make some time to apply my micrometer to my
collection of various N connectors and adapters, to be sure what's
what.
Steve,
Perhaps you would report any connector numbers you find and which is
which.
I have a dial calibers here that would let me figure our what connectors
I have here.
Thanks.
Roy
Roy Morgan
K1LKY since 1958
k1lky68@...
|
As a reference, from the Maury Microwave precision calibration catalog the dimensions given for the two impedance N connector center pins are:
50 ohm: 0.0650¡± +/- 0.005¡±
75 ohm: 0.0355¡± +/- 0.005¡±
For commercial-grade field-replaceable N connectors (the ones most people are used to) it has been found that these dimensions may vary a thousandth or two.? From the above you can see why damage will result if the two different impedance connectors are mis-mated.
?
I sometimes receive emergency requests from engineers who do not have a 75 ohm connector immediately available so will take a 50 ohm center pin and turn it down in a jewelers lathe to the proper diameter for 75 ohms use.? Such an action is usually done when the need is imminent and any variations in the physical dimensions of the connector that may affect minute impedance variations is not important.
If you are really determined to find out what the pin diameter of an assembled connector is, I recommend obtaining an inexpensive hand-held optical comparator and an insertable reticle that gives diameter measurements in the range needed.? That way you can simply place the comparator over the end of the connector and get at least a close estimate.
Greg
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https://www.keysight.com/upload/cmc_upload/ck/zz-migration/images/AMER_9_WN_1.jpg
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An easy way to tell N male 75 ohm and N male 50 ohm.
The center pins on 75 ohm N connectors draw blood, the 50 ohm ones do not!
-- Sam Reaves
ARS W3OHM
Owner and Moderator of: LeCroy Owners Group on Groups.io (Current and Future Group)
LeCroy_Owners_Group on Yahoo! Groups (Legacy Group) Electronics and Mechanical Hardware Design Engineering Manager
Andritz Rolls Global Research Center
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Just going through my old emails. You may find the attached useful. Don't even think about getting into the bridge to change the impedance. HP uses stripline on a saphire substrate (on the 85046A anyway) and the 75 ohm is a different dimension. PeterB
On Tue, Feb 5, 2019 at 1:55 PM Steve Hendrix < SteveHx@...> wrote:
A fellow engineer raised a question re the 75 ohm
characteristic impedance of the HP8714ET I just purchased off eBay.
Although I have minimum-loss matching pads ordered and enroute, does
anyone here know just what would be involved if I were to open up the
unit and try to change the impedance with some part swaps? The firmware
already supports a choice of 75 ohm or 50 ohms as the impedance, for
Smith charts, etc. I suspect that before I do that, I had better get the
service manuals, but that's for the future - thus far I'm less than 1/4
of the way thru reading the user manual, and I have a bunch of other
documentation to read as well. I'm very pleased with my purchase, if only
for the education I'm getting!
Steve Hendrix
|
bownes
Sam Reaves