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Author Topic: Modify $20 ebay active RF probe for high voltage  (Read 325 times)

WA7EM

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Modify $20 ebay active RF probe for high voltage
« on: November 18, 2021, 08:05:48 PM »

I bought one of the Ukraine $20 active RF probes. I want to use it with my Heathkit tube gear (HW16, HW100, ...). But, the probe says its good for about 300 volts DC. That is set by the blocking capacitors that are the 1st components in the chain. Can I put a 1KV .01uf cap at the front of this chain to allow it to work with the 300-800 volts present in old tube gear? If yes, what specific type of cap.

SMD would be pretty much a waste of time and money for me. I don't have equipment or eyesight to solder stuff the size of a pencil point. But, I don't intend to observe any RF above maybe 30Mhz. So, messing up the frequency response at 100+Mhz and above is not a concern.

thanks in advance for your help

Ed
WA7EM
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G3RZP

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Re: Modify $20 ebay active RF probe for high voltage
« Reply #1 on: November 19, 2021, 01:01:33 AM »

It depends on the probe circuit and what the first device is. This is because the capacitor has to charge when first switched on, and that can give a large transient into the first device. The other point is if the rest of  the probe circuit can stand the RF peak voltages you can find in old tube gear. Some old VTVM RF probes ( eg Marconi Instruments TF1041 series) were rated up to 300 v RMS below about 150 MHz, dropping  off as you went higher. That used a pretty well now 'unobtainium' wire ended disc seal thermionic diode.
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VK6HP

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Re: Modify $20 ebay active RF probe for high voltage
« Reply #2 on: November 19, 2021, 08:13:27 PM »

Judging from the published circuit diagram on the eBay listing, the active RF probe you refer to is most likely a rip-off of this one:

http://www.seekic.com/blog/project_solutions/2011/08/03/Poor_Man%E2%80%99s_1_GHz_Active_Probe__DIY_Saves_a_Pretty_Penny_%EF%BC%881%EF%BC%89.html

The input capacitance is 0.5 pF and was originally made using patches on a two-sided PCB.  I can't quite tell if the Ukrainian unit uses that construction but, either way, an easy way to replace the input capacitor is to replace the original capacitor with a "gimmick" capacitor made out of twisted wire having sufficient insulation to achieve the voltage rating you're looking for.  Note the comments in the article referenced regarding the capacitive divider effect with the MOSFET input impedance.

There are also some tests of the Ukrainian unit in various blogs which show it falls well short of a flat response to 1.5 GHz but, for general use below 100 MHz, it's OK for a few dollars worth of parts.

Re the comments by G3ZRP, some of the older high voltage RF probes (which were often rectifying types rather than the pre-amp type mentioned by the OP) achieved excellent performance in terms of signal handling and frequency response.  I recently restored an HP410C solid state voltmeter (c. 1970) and managed to obtain a replacement EA53 acorn diode for the matching 11036A RF probe from Electols in Brussels.  Measurements show a +/- 1 dB response to well beyond 700 MHz and a usable response to 1.5 GHz.

73, Peter.



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G3RZP

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Re: Modify $20 ebay active RF probe for high voltage
« Reply #3 on: November 20, 2021, 03:36:38 AM »

WA7EM  said he wants it for work on old tube gear like an HW 16 and HW100.  How well is the BF998 going to put up with say the 30 volts RMS drive to a pair of PA tubes? Let alone the 1400 volts pk to pk at the plates?

Even with an EA73, you really want a capacitive divider before measuring RF plate volts.
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VK6HP

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Re: Modify $20 ebay active RF probe for high voltage
« Reply #4 on: November 20, 2021, 05:43:39 AM »

But hold on...

If he has a scope - which is presumably why he needs this type of probe - he'll use the scope's x10 or x100 probe for the high-level RF.  I credit him with a modicum of sense and assume that he wants an amplified probe for low-level RF applications.  In my own case, I tend to use my home-brew high impedance (solid state) amplified probe with a spectrum analyser for very low-level work.

73, Peter.
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AC2EU

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Re: Modify $20 ebay active RF probe for high voltage
« Reply #5 on: November 20, 2021, 07:37:35 AM »

I just bought one of these also. I was wondering what you decided to use for the coax for best physical flexibility and frequency response?

WA7EM

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Re: Modify $20 ebay active RF probe for high voltage
« Reply #6 on: November 20, 2021, 10:37:14 AM »

I am pretty much a newcomer to working on tube gear. I have a decent background in circuits,  but am pretty naïve about working around high voltage. "a modicum of sense", at best

VK2HP - thanks for the reference to the "Poor mans active probe" article. Yes, the Ukraine ebay item appears to be an implementation of the circuit in that article. Yes, the final stage of this probe appears to be a "gimmick capacitor" of pads on the 2 surfaces of a 2 sided PCB. That feeds into a couple SMT caps.

I did buy this thing after learning that an ordinary X10 scope probe stray capacitance, etc will affect the circuit being measured and confuse you as to what you are seeing being right or wrong. Yes, I have learned to stay away from the plates of the final and that you can typically view the action of the finals by looking at the cathode.

I do wonder what the Ukraine guy thought one was to do with his short probe cable. The best I have envisioned so far is use an SMA to BNC adapter and then use a longer bnc cable to reach the 'scope. Mechanically, its kinda a kludge. I guess I can buy a BNC to BNC cable made of that very thin coax.

Do you think the twisted pair "gimmick capacitor" would extend the voltage capability of this probe a lot more than the existing "pads on 2 sides" of the pcb does now. Ukraine guy seems to suggest 300V max.

thanks

Ed
WA7EM
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VK6HP

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Re: Modify $20 ebay active RF probe for high voltage
« Reply #7 on: November 20, 2021, 04:41:55 PM »

Ed,

You'd probably find different preferences according to what individuals have in their workshops but here's how I approach it.

For quite a bit of general RF work the few pF of loading by a good x10 or x100 scope probe is not significant at HF.  In situations where it is, I would tend to use a "light coupling" approach, such as a sniffer coil attached to my test instrument or receiver, knowing that absolute levels are rarely that significant, particularly in equipment of the vintage you mention.  I do find that a decent x100 passive probe is well worth having and, while the sky's the limit in cost and performance, the Hantek 3100 I bought off eBay for USD20 does a good job.  Just be sure to read the steady-state and pulse de-rating information (often contained partly in the form of a maximum ac voltage vs frequency plot) that accompanies the particular probe.

I would go ahead and package your active probe, then use it within the limits intended by the designer.  For the occasional low-level tube RF circuits where it's likely to be useful there's a good chance the RF and dc limits will be acceptable (e.g. low drive/output crystal oscillator).  It'll be somewhat more useful in solid stage environments but watch the ac levels you're dealing with.  Frankly, I regard my active probe as a "special" bit of equipment: 99% of all my RF work is done with the x10 or x100 passive probes.

Regarding the output connection, I would use a thin, flexible 50 ohm coax with an SMA male on one end and a BNC male on the other; you could just buy a Chinese pre-made jumper off eBay, keeping the cable length as short as conveniently possible. I'd look for a supplier that could sell me an el-cheapo 50 ohm BNC termination for the scope input as well.  When you're done, measure some known levels (e.g. from a signal generator) and make a note of the overall calibration factors.

73, Peter.
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WA7EM

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Re: Modify $20 ebay active RF probe for high voltage
« Reply #8 on: November 20, 2021, 05:43:51 PM »

Thank you Peter for your very thoughtful and helpful answer

Best Regards

Ed
WA7EM
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W6SSP

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Re: Modify $20 ebay active RF probe for high voltage
« Reply #9 on: January 04, 2022, 07:05:18 AM »

One of the things I worked on while at HP was the HP 85024A
active probe. Did a minor re-design of it. These were shipped with
two resistive attenuators of 3 dB and 10 dB, both having an input
impedance of 10 meg or more (its been a while...). Building a simple
resistor divider for these inexpensive active probes should make them
usable while probing high voltage circuits. Attach an attenuator to
the probe then use a blocking cap between it and the point you are
probing. These can be used with scopes, as well as spectrum and
network analyzers.
Steve W6SSP
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