Author Topic: High-Z wideband buffer amp for 50 ohm CRO inputs  (Read 21086 times)

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Offline tggzzz

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #75 on: June 12, 2016, 08:03:36 am »
For 200+ MHz designs, Tektronix did not use relays to switch their high impedance attenuators until they were part of a hybrid with one exception and in both cases, they used a slightly different topology.  Instead of using a relay to bypass each attenuator, they used it to short the upper section and disconnect the lower section from ground.
This is the front end of a Tek 485, a 50ohm/1Mohm 350MHz scope with useful response to >500MHz.

The BNC input, J1, is at the lower left. To its right are three horizontal sections, the 50ohm attenuator chain, the rotary switch positions, and the high impedance chain.

K1S1 is a relay used to disconnect the internal 50ohm termination (so the input is 1Mohm) when there's an overload.

The switches are exposed flexible fingers on the PCB which are lifted 0.5mm by cams on the rotary switch.

The complete manual is available at various places around the web.
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Online joeqsmith

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #76 on: June 12, 2016, 03:36:19 pm »
All of my scopes are LeCroy and are all fairly old.  Shown is a 4GHz plugin from my 7200A.   

Let me know if I can be of more help.   :-DD

You have a spare Lecroy to donate?  ;D
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Online joeqsmith

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #77 on: June 12, 2016, 03:44:29 pm »
A few of the 64Xi's relays.   
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Offline GK

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #78 on: June 13, 2016, 02:52:03 am »
A few of the 64Xi's relays.   


Very pretty. What model is that from and what is the bandwidth?
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Offline David Hess

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #79 on: June 13, 2016, 08:08:27 am »
For 200+ MHz designs, Tektronix did not use relays to switch their high impedance attenuators until they were part of a hybrid with one exception and in both cases, they used a slightly different topology.  Instead of using a relay to bypass each attenuator, they used it to short the upper section and disconnect the lower section from ground.
This is the front end of a Tek 485, a 50ohm/1Mohm 350MHz scope with useful response to >500MHz.

...

The 485 was the first design I checked but it uses the custom cam switches Tektronix was so fond of instead of relays and there is no detail in the schematic about the hybrid attenuators although there may be in a different service manual.

Tektronix was making their own relays (and selling them to others) at this time but I could not find any examples where they used them above 100 MHz except at low impedances; the optional X-Y horizontal phase compensation in the 400 MHz and 500 MHz 7000 mainframes used them.  The relays were expensive and so were replaced by cam switches early in the 7000 series products up to at least 400 MHz and later improved cam switches were good to 1 GHz.  I do not know which cam switches the 600 MHz 7A19 used; they may have been improved from the original 400 MHz design.  The two examples I gave used different higher performance relays.

It sounds like GK found the problem though and it was not the relays.  We had a discussion over on the Yahoo Tektronix list not long ago about sweeping oscilloscope inputs to look for high frequency resonances but I did not expect such a good example of the problem.
 

Offline tggzzz

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #80 on: June 13, 2016, 08:25:28 am »
For 200+ MHz designs, Tektronix did not use relays to switch their high impedance attenuators until they were part of a hybrid with one exception and in both cases, they used a slightly different topology.  Instead of using a relay to bypass each attenuator, they used it to short the upper section and disconnect the lower section from ground.
This is the front end of a Tek 485, a 50ohm/1Mohm 350MHz scope with useful response to >500MHz.

...

The 485 was the first design I checked but it uses the custom cam switches Tektronix was so fond of instead of relays and there is no detail in the schematic about the hybrid attenuators although there may be in a different service manual.

Tektronix was making their own relays (and selling them to others) at this time but I could not find any examples where they used them above 100 MHz except at low impedances; the optional X-Y horizontal phase compensation in the 400 MHz and 500 MHz 7000 mainframes used them.  The relays were expensive and so were replaced by cam switches early in the 7000 series products up to at least 400 MHz and later improved cam switches were good to 1 GHz.  I do not know which cam switches the 600 MHz 7A19 used; they may have been improved from the original 400 MHz design.  The two examples I gave used different higher performance relays.

It sounds like GK found the problem though and it was not the relays.  We had a discussion over on the Yahoo Tektronix list not long ago about sweeping oscilloscope inputs to look for high frequency resonances but I did not expect such a good example of the problem.

:) Good pedagogical examples are worth remembering.

On the principle that some performance improvements have been made in the past 46 years, searching Mouser for "RF Relays" leads to http://www.mouser.co.uk/Electromechanical/Relays/High-Frequency-RF-Relays/_/N-5g33/ That contains 1059 items with frequencies up to 8GHz. Looking at the datasheet for one of those indicates pulse risetimes  <50ps, VSWR <1.1 f<1GHz, VSWR <1.05 f<0.5GHz.

There may be some standard cost-performance engineering tradeoffs to be made :)
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Online joeqsmith

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #81 on: June 13, 2016, 11:22:00 am »
The 64Xi has a bandwidth of 600 MHz.   So many relays, yet so fast.  :-DD   I've never looked in the 8500A but with the DC coupled, 50 ohm inputs, I would guess the relay count is much less.   If you can't tell, these are a OMRON G6KU-2F-Y.     

A few of the 64Xi's relays.   

Very pretty. What model is that from and what is the bandwidth?
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Offline David Hess

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #82 on: June 14, 2016, 02:09:02 am »
:) Good pedagogical examples are worth remembering.

Check off your $100 word for this week. :)

Quote
On the principle that some performance improvements have been made in the past 46 years,

The way I understand it, Tektronix started making these relays because nothing of similar performance was available at the time and they actually ended up OEMing them for others.  The claim to fame for the relays was how they were constructed with the switch sections immediately adjacent to the base for minimum parasitics; modern high performance small signal "telecom" relays use the same configuration and have similar performance.

Where Tektronix could, I think they replaced them simply for cost reasons either with their high performance cam operated switchs or by altering the circuit designs.  Early 7000 plug-ins all used the relays but were quickly replaced by newer plug-ins which did not; the best example might be the 7A16 with 7 relays being replace by the 7A16A with no relays.  Similarly the 7A12 was replaced by the 7A18.

In one way however, modern relays are worse.  The Tektronix relays used a clever symmetrical pinout so the DPDT models can be turned 180 degrees and function exactly the same.  Modern relays use an asymmetrical pinout as I discovered when trying to find replacements for the Tektronix ones.

Quote
searching Mouser for "RF Relays" leads to http://www.mouser.co.uk/Electromechanical/Relays/High-Frequency-RF-Relays/_/N-5g33/ That contains 1059 items with frequencies up to 8GHz. Looking at the datasheet for one of those indicates pulse risetimes  <50ps, VSWR <1.1 f<1GHz, VSWR <1.05 f<0.5GHz.

There may be some standard cost-performance engineering tradeoffs to be made :)

That is definitely the case and more.  RF relays are a lot more expensive than "telecom" relays and being designed for a constant impedance environment, may actually work worse in something like a high impedance attenuator.  I suspect miniature RF relays as we know them were also not available back then either; Tektronix used reed relays with a split coaxial shield instead and they perform well into the multi-GHz range.  There are some good photos of them here:

http://w140.com/tekwiki/wiki/7T11

 

Offline tggzzz

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #83 on: June 14, 2016, 07:31:47 am »
RF relays are a lot more expensive than "telecom" relays and being designed for a constant impedance environment, may actually work worse in something like a high impedance attenuator.

Just so.

I can't imagine using a relay for a high-frequency application without seeing "RF specs" such as VSWR, or at the very least some frequency-dependent specs.
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Offline David Hess

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #84 on: June 15, 2016, 01:33:49 am »
RF relays are a lot more expensive than "telecom" relays and being designed for a constant impedance environment, may actually work worse in something like a high impedance attenuator.

Just so.

I can't imagine using a relay for a high-frequency application without seeing "RF specs" such as VSWR, or at the very least some frequency-dependent specs.

It would be nice to have detailed specifications but telecom relays are not controlled impedance devices so VSWR and most RF attributes have little meaning.  They sometimes have an ambiguous frequency specification of roughly 1 GHz and depending on the design are good up to at least 500 MHz if the circuit is laid out properly as GK showed.

I used Panasonic TN2-12V relays (since discontinued) to replace a couple of Tektronix relays and they have no RF or parasitic specifications at all but worked great.
 

Offline Cerebus

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #85 on: June 16, 2016, 01:00:57 am »
I used Panasonic TN2-12V relays (since discontinued) to replace a couple of Tektronix relays and they have no RF or parasitic specifications at all but worked great.

If I recall the Panasonic specs correctly, NEC have exact equivalents available. Whether their HF performance is the same is another question.
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Offline tggzzz

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #86 on: June 16, 2016, 08:02:24 am »
I used Panasonic TN2-12V relays (since discontinued) to replace a couple of Tektronix relays and they have no RF or parasitic specifications at all but worked great.

Many years ago, just as optical fibres were arriving, telecos transmitted 2MB/s down audio grade twisted pair cables specified at 1.6kHz. The telcos couldn't ask for the same cable's specs at different useful frequency, because that would allow the cable companies to increase the price (not cost!).

So they developed test equipment to measure the NEXT and FEXT of the cables, and allow individual pairs to be selected.
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Offline tautech

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #87 on: June 16, 2016, 08:10:06 am »
I used Panasonic TN2-12V relays (since discontinued) to replace a couple of Tektronix relays and they have no RF or parasitic specifications at all but worked great.

Many years ago, just as optical fibres were arriving, telecos transmitted 2MB/s down audio grade twisted pair cables specified at 1.6kHz. The telcos couldn't ask for the same cable's specs at different useful frequency, because that would allow the cable companies to increase the price (not cost!).

So they developed test equipment to measure the NEXT and FEXT of the cables, and allow individual pairs to be selected.

And in some parts of NZ it's still here.  :palm:
Still today telcos try to a sell a shite connection like that to us ^^^^^and have the cheek to call it a fast broadband service.  :bullshit:
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Offline Cerebus

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #88 on: June 16, 2016, 12:08:23 pm »
I used Panasonic TN2-12V relays (since discontinued) to replace a couple of Tektronix relays and they have no RF or parasitic specifications at all but worked great.

Many years ago, just as optical fibres were arriving, telecos transmitted 2MB/s down audio grade twisted pair cables specified at 1.6kHz. The telcos couldn't ask for the same cable's specs at different useful frequency, because that would allow the cable companies to increase the price (not cost!).

So they developed test equipment to measure the NEXT and FEXT of the cables, and allow individual pairs to be selected.

And in some parts of NZ it's still here.  :palm:
Still today telcos try to a sell a shite connection like that to us ^^^^^and have the cheek to call it a fast broadband service.  :bullshit:

That 2Mb/s wouldn't have been a consumer connection. It was either a fast (and expensive, think £1000's a month) commercial data link or a voice trunk circuit from a street cabinet multiplexer back to the switch. Back in the day (1995) I used to run a whole ISP on a 2Mb circuit that was split 348 kb/s transit and the balance peering traffic. You could get a lot of dialup users into that 2 Mb.
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Offline tggzzz

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #89 on: June 16, 2016, 02:08:38 pm »
I used Panasonic TN2-12V relays (since discontinued) to replace a couple of Tektronix relays and they have no RF or parasitic specifications at all but worked great.

Many years ago, just as optical fibres were arriving, telecos transmitted 2MB/s down audio grade twisted pair cables specified at 1.6kHz. The telcos couldn't ask for the same cable's specs at different useful frequency, because that would allow the cable companies to increase the price (not cost!).

So they developed test equipment to measure the NEXT and FEXT of the cables, and allow individual pairs to be selected.

And in some parts of NZ it's still here.  :palm:
Still today telcos try to a sell a shite connection like that to us ^^^^^and have the cheek to call it a fast broadband service.  :bullshit:

That 2Mb/s wouldn't have been a consumer connection. It was either a fast (and expensive, think £1000's a month) commercial data link or a voice trunk circuit from a street cabinet multiplexer back to the switch. Back in the day (1995) I used to run a whole ISP on a 2Mb circuit that was split 348 kb/s transit and the balance peering traffic. You could get a lot of dialup users into that 2 Mb.

Neither. Remember I wrote "just as optical fibres were arriving". Multimode fibres were just being installed, and single mode fibres were still in the labs.

They were 2048Mb/s links between exchanges, transmitted down some of the quad twisted pairs in a big cable. Only some pairs had the bandwidth, and only some avoided crosstalk.

My second product was an optical attenuation test set.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline David Hess

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #90 on: June 16, 2016, 02:10:28 pm »
I used Panasonic TN2-12V relays (since discontinued) to replace a couple of Tektronix relays and they have no RF or parasitic specifications at all but worked great.

If I recall the Panasonic specs correctly, NEC have exact equivalents available. Whether their HF performance is the same is another question.

I did not find any equivalents but I was not really looking; the Panasonic relays were discontinued after I finished the repairs.  The unique thing about the TN2-12V was that it has the exact same pin spacing as the Tektronix relays and while its pinout is wrong, the coil connections are in the right place and both poles can be combined to replace a SPDT Tektronix relay without changing the physical circuit.

As far as RF performance, these relays all perform about the same if they use the layout with the switching section directly adjacent to the header.
 

Offline David Hess

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #91 on: June 16, 2016, 02:35:19 pm »
Many years ago, just as optical fibres were arriving, telecos transmitted 2MB/s down audio grade twisted pair cables specified at 1.6kHz. The telcos couldn't ask for the same cable's specs at different useful frequency, because that would allow the cable companies to increase the price (not cost!).

So they developed test equipment to measure the NEXT and FEXT of the cables, and allow individual pairs to be selected.

This crops up with tested specifications in electronic parts all the time; it is sometimes more economical to do your own grading with your own specialized test than for the manufacturer to do it but you run the risk that your source of "good" parts may dry up.

Usually I see it with leakage in diodes or bias current in operational amplifiers.  Leakage tests at the 50 to 100 nanoamp level are fast making them cheap so that is what small signal transistors are tested at but if you want a low leakage diode, you can grade them yourself and get down to the picoamp range.  The difference between a National LMC6001 and LMC6081 is that the former spent more time (5 dollars worth?) on the automatic tester to guarantee a maximum 25 femtoamp input bias current.

The noise difference between the TL071 and TL081 also came about because of test time although I assume that is no longer the case since they now cost the same; maybe they test them all now and mark TL081s as TL071s as needed.  Low frequency noise by definition costs a lot of test time making it expensive if it has to be tested for.  Popcorn noise used to be the same way until they solved it.
 

Online joeqsmith

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #92 on: July 08, 2016, 02:36:08 am »
A few of the 64Xi's relays.   
Very pretty. What model is that from and what is the bandwidth?

Give up? 
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Offline GK

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Re: High-Z wideband buffer amp for 50 ohm CRO inputs
« Reply #93 on: July 08, 2016, 07:42:05 am »
No, just busy with other stuff.
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