Simple RF probe for DMM

[MrBungle]

Due to the complete unavailability (for a reasonable price) of the RF probe for my Philips PM2425, I decided to build one last night and this morning. Details here. This is at the current time, uncalibrated and suitable for indication only but it does a rough conversion to RMS with a 4.3:10 divider against the meter's internal 10M impedance. Seems to be good to about 170MHz which was far more than expected. Above about 500mV it matches my scope's reading perfectly. Need a cal curve for anything under that which haven't bothered with

It is built on a hand carved piece of FR4, uses RG174 as the main lead (this must be shielded!), 2x 4mm banana plugs for termination and is built dead bug style. No enclosure yet. Waiting for some brass tube from ebay.

Schematic:



Whole probe:



Closeup:



BOM (from RS, Rapid, Spectrum Communications and bitsbox)

1x 1N270 (bitsbox)
1x 10n ceramic (bitsbox)
1x 4.3M resistor 1% (note 1)
1x piece of FR4 single sided offcut (rapid)
1x stretch of RG174 (spectrum communications)
1x header pin for the probe tip (bitsbox)
1x small length of 7/0.2 green wire (bitsbox)
2x banana plugs (bitsbox - note 2)
1x IC clip (bitsbox)
heatshrink

note 1: Vishay MBB0207 was originally selected. RS messed up the order for this so I had to substitute a hand selected a pair of 5% 0.25W carbon films of 3.9M+390K from the junk box that were within 1% of 4.3M.

note 2: banana plugs had the flanges removed by clamping in my drill's chuck, running it at full whack and attacking it with a file, then rubbing down with a kitchen sponge. This was so they actually fit in the meter!

Hope someone finds this useful.

[Diabolical Artificer]

This came out yesterday https://www.youtube.com/watch?v=uVkJqqZroN0 , it covers similar probes.

Andy.

[Bazz4CQJ]

There is a useful book called "Probes" which is mid-1950's USA, over 200 pages and all about the use of diode probes in servicing, probably of most interest to relative beginners. It's a free 43MB download from http://www.tubebooks.org/books/zucconi_probes.pdf.

It discusses both simple half-wave probes and "voltage doubler " probes, and the trade off between what works best on a meter and what is good for use with a scope. The term "vtvm" is used throughout but never defined; it refers to "vacuum tube voltmeter", but if you don't have one, modern digital multimeters will probably have a sufficiently high impedance to do the same job.

[Biggles]

A diode probe, just a small isolating capacitor in series with a germanium diode and a filter capacitor akin to a crystal set without any tuning components was an essential tool for rough alignment of transmitter stages on PMR equipment in the eighties. Fine tuning was then done by the book. I still have one somewhere housed in an old pen. It isn't used much nowadays though.

[Skywave]

As I have stated elsewhere, even the simplest item of test gear has its uses. The important factor is to be aware of its limitations when in use and in use with what.

Al.

[MrBungle]

Exactly. I've found the more I know the less kit I need as well which promoted this.

[techiesteve]

In previous employment in the 1980's I used to use a Boonton RF Millivoltmeter. One would be very useful. I've often seen them on eBay, but never with the probe. Has anyone ever concocted a probe to use with the older analog metered Boonton?

Another option is to use a simple probe and meter as here.

[Bazz4CQJ]

Originally Posted by techiesteve

In previous employment in the 1980's I used to use a Boonton RF Millivoltmeter. One would be very useful. .

There's a ton of stuff on Google about Boonton, which had several models. Looking briefly at the manual of the 92 model. it states that the probe employed fullwave rectification, but in the schematic, the probe is blanked out, with the comment that there are 'no user-serviceable parts' in there!

The meter itself uses a mountain of transistors!

B

[jimmc101]

A peek inside the inside of a couple of boonton probes here.

Jim

[MrBungle]

Very interesting read. Thanks for posting.

[Bazz4CQJ]

Yes, that is interesting; thanks for posting.

Right at the end of the article he seems to put the boot fairly hard into Ge point-contact diodes, but I'm sure I could find other articles where people are kinder to them, especially the gold-bonded ones. They may not be easy to find, unless fate has decreed you have some in your junk box these last ~50 years, in which case...

Given availabilty, the trade-off between different diodes is probably quite debateable, and dependent on the specific priorities for your probe. But as a probe can be put together in an evening, "suck it and see" may be a productive way forward?

B

[MrBungle]

I had the non scientific approach of picking a 1n270 because it was cheaper than a 1n34. Plus the breakdown voltage was pretty high.

They're pretty easy to get hold of: https://www.bitsbox.co.uk/index.php?...th=140_141_142

[Skywave]

Originally Posted by Bazz4CQJ

Right at the end of the article he seems to put the boot fairly hard into Ge point-contact diodes, but I'm sure I could find other articles where people are kinder to them, especially the gold-bonded ones.

The gold-bonded, germanium OA47 is a fast diode and with a low forward voltage drop: typically 0.3 v. @ 1mA; 100 mA max. fwd. current. Although for many applications, its low PIV of 25v. restricts its usage, over the years, I have seen the OA47 being used as a rectifier for H.F. voltages in several circuits.

Al.

[Radio Wrangler]

Probably the nicest RF voltmeter with probes is the HP "Vector Voltmeter"

Two high impedance probes with a kit or attenuators and rf connectors connected to a box with two massive meters. It will read down to sub-millivolt on either probe and accurately measure the phase difference between the signals on the probes.

One of the pinnacles of classic 1960s HP gear.

The probes are actually sampling heads, converting the signal down to a low IF. The oscillator in the main box phase locks onto the signal with an IF offset. 100kHz to 1GHz with good specified accuracy. You can use it as a vector network analyser working on equipment running on its own signals.

You can pick them up for a song because most people don't have a clue what they are or what they can do, and of the people who do know, rather a lot are scared to death of them.

David

[Skywave]

Ah yes! The HP VVM: I remember it well. I used to use one frequently way back in the early 1970s during the design of R.F filters and R.F. broadband amplifiers: a very useful piece of kit. A photo of the model I am referring to in this post is as per the photo at the end of this post, q.v.

Perhaps it's just me, but I have a bit of a job understanding your remarks:

"You can pick them up for a song because most people don't have a clue what they are or what they can do, and of the people who do know, rather a lot are scared to death of them."

A few moments looking at the front panel gives a good clue as to what they are designed to do. Obviously, that requires a basic understanding of amplitude and phase of a signal, but in a typical environment where one would be found, a knowledge of such fundamentals is surely a given? And what is there to be scared of? It's not as though it will expose the user to high voltages, ionizing radiations or emit high-powered laser light, to cite just three examples.
As for price on the S/H market, about 7 years ago I was tempted by one for sale at an auction site, but the price was in the £600 - £700 range. That price - and the huge amount of bench space it occupies - sadly caused me to walk away.
Wonderful piece of kit, though.

Al.

[G0HZU_JMR]

Yes, the HP8405A VVM is the classic 1-1000MHz VVM, and was a common sight in RF labs in the 70s and 80s. I was a regular user of these back in the late 1980s. A few years ago I bought one for home use and you can read about it here:

https://www.vintage-radio.net/forum/...d.php?t=102117

As you can see this was a very old example of an 8405A and it was littered with faults and some new faults happened during my long soak tests. Since then it has been reliable but I did have to strip the display meters a year or so ago to cure it of the dreaded 'dial peel' that happens to these old HP meters. The calibration on the dial is done with a thin transparency and this ages and becomes brittle and it peels/flakes away from the backing and jams the needle. Hopefully I've managed to cure this but it may happen again at some point.

However, I'm not sure I would recommend the 8405A VVM as a regular RF voltmeter because it is too easy to damage at 4Vpkpk damage level. Also it is mainly meant to be used in a very controlled lab environment where the meter is connected all the time in a system using a known locally generated signal source. So it can be set up to achieve lock and stay locked (and stay safe from overload damage).

If you used it as a regular VM it would need to acquire lock every time you probed something and you would need to know the frequency range of the signal. It's pretty quick at this but you can expect to be seeing the dial meters pinging back and forth a lot and you will be playing with the range switch a lot as you go from TP to TP. Plus you would have to make sure you don't damage it. There are divider heads available to increase the range/damage level but this meter is at its best in a controlled setup, maybe with the Channel A and B probes attached to a directional coupler or an impedance measuring bridge or each side of a filter or other DUT.

[G0HZU_JMR]

I've posted up this image on here before but here is one of my old RF probes. This is a Schottky diode based probe and I made the PCB into a probe shape. The probe has two ground options, either the croc lead or the little earth spike at the tip.

I have always used twisted pair wires for probes like this because it makes them easier to hold and move around and it doesn't seem to suffer from spurious pickup. Also, you can see that the probe handle is effectively insulated and I prefer this for a safety point of view. It could be built a lot better than this but I used this probe a fair bit a few years ago and I used a much uglier version when I was a student and this was made with a bit of veroboard. I don't really use it anymore as I have other options but a simple diode RF probe should be in every toolkit.

[Skywave]

Barrie - recently, progress has been slow - the 'Christmas effect'.

The G0WCA voltmeter is under construction in order to do a full performance evaluation, but in mine, initial tests show that the bandwidth of the FET buffer is inadequate. However, I do have ideas to improve that. The FET in use is a BF256. It was necessary to select-on-test to find the best performer. Doing ditto with 2N4416 did not show any worthwhile improvements.

I trust that your Christmas was fun for you & your family.

Al.

[Bazz4CQJ]

Thanks for those wishes, which are reciprocated. Had a good time and the soldering iron got very little use indeed!

It's curious that the 0CWA circuit is just one of many on the web where people report using 3819's in FSM's and all kinds of stuff and getting them to work up to ~150MHz. That's never been my experience with them, but they are comparatively ancient, and I'm not sure any of the manufacturers ever claimed they were too useful at those frequencies. Hopefully, the other FET's you mentioned will do a better job.

B

[Skywave]

Originally Posted by Bazz4CQJ

. . . using 3819's in FSM's and all kinds of stuff and getting them to work up to ~150MHz. That's never been my experience with them . . .

Ditto. Their main short-coming seems to be an inadequate gain-bandwidth product. However, at VHF I have had some success as oscillators and in simple super-regenerative receivers they have worked very well. The main attraction of any FET in most applications is its high-input Z, of course. Having said that, it should be possible to produce a high-input Z buffer using bi-polar transistors in a boot-strapped mode.
My main problem at the moment is simply that the ratio of 'things to investigate' to 'time available' is just too large.

Al.