Hi Z probe for 50 ohm spectrum analyzer

[joeqsmith]

Many years ago, I picked up an old (at that time) Tektronix P6202 active probe.  It can drive both 1M and 50ohms and had an optional attenuator and DC block.   

The newer version's manual can be found here:
https://www.tek.com/p6202a-manual/p6202a-instruction-manual

This is the one I have:
https://www.ebay.com/itm/Tektronix-P6202-500-MHz-10X-FET-Pobe-Kit-Full-Accys-Manual-Case-Tested/324172342811?epid=1718864835&hash=item4b7a2d661b:g:zC8AAOSw01lexYjt

Looks like you can find them for $70 on eBay.

I thought about a nano but then I have to set it up, learn, and waste a bunch of time.
With my Siglent SSA 3021X  good to 50VDC  and a probe running at 5vdc it should be plug and play with no pray and pay.
That's a busy screen, pretty nice, which model is it?
Hank

Now days you have access to what I understand it some very good open source software for the NanoVNA.  For the filters, you would be using the Nano's port 1,  so there would be very little risk of damage.  Of course, if you wanted to play with something with gain....
 

That particular one is an old Agilent PNA that I bought it to experiment above 2GHz.    The hardware is really nice but the software looks to me like Agilent fired every senior software developer they had, hired a bunch of freshout Windows CS majors and then managed the group with people who had no idea what a VNA was. 

[Kosmic]

Maybe this is suitable for your application.  It is somewhere between DIY and a packaged solution.
"RF Active Probe 0.1-1500 MHz-1.5 GHz analyzer oscilloscope" from ebay seller 60dbmco in Ukraine
https://www.ebay.com/itm/332393676082

Nice find.  Have you tried one to see how well it performs?

I have one of those. Look like the seller tried to implement the design published in Elektor magazine a while ago. On his ebay listing he even use some schematic from the article.

Most of the problem come from C1. For some reason (limit attenuation between 0-500Mhz  ), he decided to use 2 x 2nF in series. Was supposed to be a 0.75pF PCB cap created with 2 planes on each side of the PCB.



This is what the original probe look like.



This is the frequency response from 50 to 1500Mhz.



Now a bodge to try to fix the probe. I relocated the pogopin on the other side of the board. It was soldered on a piece of copper tape. I added some super glue to make sure the copper tape and pogopin will stay in place. The Kapton tape was just added to isolate the ground pin.





Now the frequency response of the fixed probe.




For reference, the Elektor article about the active probe is in attachment: https://www.eevblog.com/forum/testgear/hi-z-probe-for-50-ohm-spectrum-analyzer/?action=dlattach;attach=1060862

[Mechatrommer]

This is the frequency response from 50 to 1500Mhz.


Now a bodge to try to fix the probe. I relocated the pogopin on the other side of the board. It was soldered on a piece of copper tape. I added some super glue to make sure the copper tape and pogopin will stay in place. The Kapton tape was just added to isolate the ground pin.

Now the frequency response of the fixed probe.

try to change the tip setup again, say using 50mm of wire betwen probe tip and pogo pin and plot the respond again. i bet you'll get another entirely different respond. i experienced this while testing an active probe in just hundreds of MHz range not long a go, its all over the place i'm not sure whats going on, my best guess is... reflection...

[Kosmic]

try to change the tip setup again, say using 50mm of wire betwen probe tip and pogo pin and plot the respond again. i bet you'll get another entirely different respond. i experienced this while testing an active probe in just hundreds of MHz range not long a go, its all over the place i'm not sure whats going on, my best guess is... reflection...

It has 1nF input impedance, not 0.75pF like the probe described in the Elektor article. Sure my, probing technique is probably ideal, and my bodged input cap is probably also not ideal but, I'm getting a lot closer to the specified -20db and the response is a lot more linear than the original one.

[2N3055]

It has 1nF input capacitor connected to what? To mosfet gate and loaded with 10M resistor

Input capacitance of that probe is 1 nF input capacitor in series with gate capacitance of that mosfet ( Which is typically around 2 pF), that is in parallel with parasitic capacitance of that 10M resistor .

So how much of that capacitive load is presented to DUT?
1nF or something about  2pF?

[Kosmic]

It has 1nF input capacitor connected to what? To mosfet gate and loaded with 10M resistor

Input capacitance of that probe is 1 nF input capacitor in series with gate capacitance of that mosfet ( Which is typically around 2 pF), that is in parallel with parasitic capacitance of that 10M resistor .

So how much of that capacitive load is presented to DUT?
1nF or something about  2pF?

From the article:

Capacitor C1 has a value of about
0.5 pF, and is made by patches of copper on each side of the board. The gain
of the buffer itself is a little less than
one, but because of the voltage divider
action of C1 and the input capacitance
of T1, the overall loss of the probe is
approximately 20 dB

So no, I don't think substituting the 0.5pF cap by 2 x 2nF cap will really give the same input capacitance of 0.75pF. Unfortunately I'm not equipped to measure the input capacitance of the probe so I can't validate.

[2N3055]

No, input capacitance will be somewhere in 2 pf region... But it would have better low frequency response...

[Kosmic]

No, input capacitance will be somewhere in 2 pf region... But it would have better low frequency response...

Yes it should be sold as a 0-500Mhz probe. In that range it's OK-ish.

[Kosmic]

There's also this one on ebay for 30$. A bit more expensive but specs might be authentic since they didn't simply copy the specs from the Elektor article like the other one.

https://www.ebay.com/itm/RF-Active-Probe-DC-Out-100KHz-1GHz-HF-VHF-UHF-SHF-analyzer-oscilloscope-SDR/184372665711

[joeqsmith]

I have one of those. Look like the seller tried to implement the design published in Elektor magazine a while ago. On his ebay listing he even use some schematic from the article.

Most of the problem come from C1. For some reason (limit attenuation between 0-500Mhz  ), he decided to use 2 x 2nF in series. Was supposed to be a 0.75pF PCB cap created with 2 planes on each side of the PCB.
.....

I remember seeing that article or at least the first picture from the article.   -20dB loss and +/-2.5dB flatness.  They talk about using a scalpel to trim the PCB which I think we all have all found to be common practice.  The lack of a DC coupling is a deal breaker for me.   

As others have said, I too find it odd not to see the ground on these probes.  That alone tells me the person was just trying to follow someone else's design without understanding it.    Your addition of this is a big step in the right direction.    For comparison, attached are a few pictures of one of my vintage LeCroy probes.   The probe comes with a test PCB.  Sadly this particular probe met it's match and required repair.  The two drops of glue to help stablise the needles after reattachment is my work.   
 
Many years ago Pease showed a simple FET probe.  I found his old book where he shows it (link below).  He shows the BW is 90MHz and a slew rate of 300V/us.  This probe is DC coupled.   I still have my original copy of this book and highly recommend it.  It was very practical. 
http://www.introni.it/pdf/Bob%20Pease%20Lab%20Notes%20Part%208.pdf

While trying to find RAP article on-line, I came across this old video on scope probes....
   

[mawyatt]

I have one of those. Look like the seller tried to implement the design published in Elektor magazine a while ago. On his ebay listing he even use some schematic from the article.

Most of the problem come from C1. For some reason (limit attenuation between 0-500Mhz  ), he decided to use 2 x 2nF in series. Was supposed to be a 0.75pF PCB cap created with 2 planes on each side of the PCB.
.....

Many years ago Pease showed a simple FET probe.  I found his old book where he shows it (link below).  He shows the BW is 90MHz and a slew rate of 300V/us.  This probe is DC coupled.   I still have my original copy of this book and highly recommend it.  It was very practical. 
http://www.introni.it/pdf/Bob%20Pease%20Lab%20Notes%20Part%208.pdf

While trying to find RAP article on-line, I came across this old video on scope probes....
   

That book along with the one from Jim Williams are masterpieces for analog-types, I have both somewhere

I think William's book discusses the Ross "T" network that Tektronix kept as a trade secret, and the Ft doubler amplifier that they patented, with these it took HP almost 20 years to duplicate the 1GHz real time scope Tektronix had in 1969!!

The mentioned FET probe using the two JFETs actually came well before this book by Bob, we were using the concept back in ~1980. Think it originated from Tektronix and used in their scope front ends. With just 2 JFETs (2N4416, 2N4393) and a couple resistors you can have a really good high BW, high input Z buffer amp. The complementary emitter follows aren't necessary, you can take the output signal directly from the input JFET source for a low impedance (50 ohms with Rsource ~33 ohms & 2N4393), or from the bottom of the source resistor for a higher output impedance signal that is ~DC correct.

Anyway, great stuff

Best,

[joeqsmith]

I am interested in seeing the original article if you are able to find it. 

[Bud]

Hmmm.... Where did I see that ? 

[wd5jfr]

Many years ago, I picked up an old (at that time) Tektronix P6202 active probe.  It can drive both 1M and 50ohms and had an optional attenuator and DC block.   

The newer version's manual can be found here:
https://www.tek.com/p6202a-manual/p6202a-instruction-manual

What kind of a power  receptacle do I need for a P6202a and where do I find one?
Thanks
Hank

[joeqsmith]

Many years ago, I picked up an old (at that time) Tektronix P6202 active probe.  It can drive both 1M and 50ohms and had an optional attenuator and DC block.   

The newer version's manual can be found here:
https://www.tek.com/p6202a-manual/p6202a-instruction-manual

What kind of a power  receptacle do I need for a P6202a and where do I find one?
Thanks
Hank

I assume you were not able to download the manual and read it, so I have attached the page with with the power supply you would need to go with this probe.   

As far as finding one,  try eBay, salvage yards, other auctions, craigslist.

[wd5jfr]

Hey Joe
I guess I should have said I downloaded the manual but couldn't tell much about the power connector and after looking at 1101 power supplies I couldn't recognize the plug or like the prices..  And after that somehow I recalled that I had picked up a Tektronix probe power supply because it was such a good deal and looked nearly new.  So I dug around, found it and sure enough I had one and it's an 1103!  So now I'm looking for a good deal on a Tektronix P6205A. 

[wd5jfr]

   After scanning the pertinent info I could find online I'm considering a Tektronix P6202 500MHz that includes a line cord  power supply (Model # 119-07744-00) using Lemco connectors, around  $100.00.  I've not seen any individual line cord power supplies for sale and the Tek 1101probe  power supply is about $80.00.
   Since I have a tested working Tek 1103 probe power supply I'm considering a Tektronix P6205 750 MHz which plugs in directly.  (I made an error in the  previous post indicating a P6205A which doesn't exist.)  I see that there is a chance of getting a good P6205 for about $!00.00.
   From what I could tell from examining eBay listing pictures many of the P6202 have probe pins that are bent, broken, missing or a stub.  I'm guessing the pin is a straight wire (poorly selected metallurgy & form)  that can't withstand  the. stress of poking around and bends, finally breaking.  Replacement pins in packs of 10  and a special tool are listed in a Tektronix catalog.  I haven't seen any listed for sale.  This point is a huge negative for the P6202 and unless I find an exceptional deal it not a wise choice.  There's some pics on the old Yahoo Tektronix Groups site showing a tip replacement, not a task I want try. The P6202 also  has a manual switch to select high or low impedance, another failure point.

   The P6205 has a redesigned pin which is a conical, pointed tip being much thicker at the probe entry transition so that it can better withstand the stress of being poked around.  It doesn't have a manual switch to change impedance, the scopes that are designed for it  detect it and automatically switch to 50 Ω.  So I assume the P6205 is always 50 Ω.  If I'm wrong someone please correct me.  Also if there is something else I should consider please let me know.

   I'm not in a big rush as I'm still enjoying learning how to use the features of the Siglent SSA 3021X.  I'm also wondering what if any influence a DC block will have on results.
Thanks
Hank

[joeqsmith]

   After scanning the pertinent info I could find online I'm considering a Tektronix P6202 500MHz that includes a line cord  power supply (Model # 119-07744-00) using Lemco connectors, around  $100.00. 

...

   From what I could tell from examining eBay listing pictures many of the P6202 have probe pins that are bent, broken, missing or a stub.  I'm guessing the pin is a straight wire (poorly selected metallurgy & form)  that can't withstand  the. stress of poking around and bends, finally breaking.  Replacement pins in packs of 10  and a special tool are listed in a Tektronix catalog.  I haven't seen any listed for sale.  This point is a huge negative for the P6202 and unless I find an exceptional deal it not a wise choice.  There's some pics on the old Yahoo Tektronix Groups site showing a tip replacement, not a task I want try. The P6202 also  has a manual switch to select high or low impedance, another failure point.
....

The manual for my 6202 dates from 1976.  I would imagine that many scopes were still analog and most did not have 50 ohm terminations.   The 6202 was designed for a 50 ohm system and they included a switch selectable terminator rather than requiring an external one be used for the scopes of this era.

I've seen a lot of mangled probes over the years.   I am not sure why people want to drive over the cables with their chairs and use the tips for pry bars.  Still to this day I will see people who can't seem to reinstall the end caps.   
 
These were shipped with two replacement tips in case they were mishandled.  I bought mine at Dayton maybe 25 years or so ago and still have all of the original accessories, including the two replacement tips.   I'm not sure what the P6202 would have cost in today's dollars but I'm sure it wasn't cheap.  I would expect the people that used them would have taken care them (mine obviously has been), but then again, there's that LeCroy probe.

I've attached a few pictures of mine. 

[joeqsmith]

Using my NanoVNA, with my home made PCB standards to evaluate the P6202.     

After performing a SOLT,  an open test board was used to look at S11 using the probes IC cover and spring clip.  I measure roughly 2pF up to 300MHz.   At 500MHz, the Nano measures 3pF.    I don't have a lot of confidence in the Nano to make this measurement at these higher frequencies.  I suspect that the probe is performing better than what is shown.

Next I installed the home made T-Check (50 ohm thru) along with 2X10dB attenuators to simulate the 10X probes gain.   This seemed fine, so I installed the load standard and attached the probe with the external 50 ohm terminator selected.   The manual shows -3dB at 500MHz.  The Nano uses the harmonics for the source above 300MHz and we can see that.  Still it appears well within spec. 

****
The DC block was added as I wanted to see what effects if any, using the offset would have.   

I have also included a 20 minutes of data after power on.  I was expecting the probe would have some drift to it but it looks really stable. 

[BravoV]

Took me almost 4 years of lurking to score this one at "decent" price. 

HP 54701A, basically at 10:1 probe drives 50 ohm, at 2.5GHz with input capacitance 0.6pF and input resistance at 100kOhm.

[wd5jfr]

Hey Joe
It didn't take long!  Got offered this one  for $101.18 because I looked a couple times and it looks like it might have a good tip, at least it's got spares and doesn't look to beat up, tested with 30 day return, maybe not as complete as yours.  So now Ill have a dual purpose probe not sure how the higher impedance works and its uses.
Hank

https://www.ebay.com/itm/324172342811?boolp=5&ul_noapp=true

[joeqsmith]

It looks like it's all there, wasn't abused and the 30 day return is more than fair.  Hope it all works out. 

[wd5jfr]

Well I got  the P6202 on Saturday and is all there as described and not used much very clean maybe used a few times? Before using it on my Siglent SSA 3021X 50 ohms input I decided try it on my Tektronix 1012b using the calibrator and found that the impedance switch doesn't do anything to the trace except sometimes when moved from EXT to INT the trace flashes to about 1/2 size and returns to full scale. With my Fluke 189 I measured across the output BNC and it measures around 10 ohms regardless of the impedance switch position and I guess the Fluke is not quick enough to pick up the "flash".. The schematic shows that the INT switches in a 50 ohm resistor across the output and EXT disconnects it so a 10 ohm reading is quite puzzling. It says TESTED, who and how was it tested? I've  contacted the seller. I think it was a hangar queen for years because of this problem.  I wonder if someone with one can tell me what the resistance across the output BNC is in EXT & INT  positions?
Thanks
Hank

[joeqsmith]

It could be a user problem.   If the probe is powered up, I would expect you to get bad values.   After all the probe is driving the output. 

Assuming you had the probe powered off, I would expect it to be 50 ohms in internal and in external.   Certainly in the K ohm or higher.  Again, it not really a resistance you are measuring. 

*************
See attached

[wd5jfr]

I should have been clearer so I took some pics.  The  pics show power unplugged and IN and EXT switch exercised then firmly pressed into each position and it shows 8.83 ohms in both positions.  So I assume there's a problem with the 50 ohm resistor or the switch or both, it's puzzling.