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Old 1st Feb 2019, 4:34 pm   #1
IsquaredR
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Default Looking to build RF detector probe for low frequency

Would anyone know of a schematic for an RF detector probe that is capable of linearity down to 50 khz? If so, I would appreciate knowing where to find the schematic. I do a lot of low frequency IF work and need something for my oscilloscope that will accurately detect passband waveforms.
The upper frequency limit is not critical as I rarely have need of anything above 3 or 4 mhz.
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Old 1st Feb 2019, 7:40 pm   #2
Bazz4CQJ
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Default Re: Looking to build RF detector probe for low frequency

There is a 1955 book called "Probes", author Zucconi, Library of Congress #54-13280. It's old, but there's about 220 pages - worth taking a look at
http://www.tubebooks.org/books/zucconi_probes.pdf

On this forum, the discussion over recent years has tended to be concerned with achieving the high sensitivity, which people like Boonton and Marconi have done with RF Voltmeters, and what kind of "magic" diodes they are using!

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Old 2nd Feb 2019, 5:04 pm   #3
David G4EBT
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Default Re: Looking to build RF detector probe for low frequency

The was the one designed by RF Haigh in Radio Bygones No 83, back in June/July 2003 as a follow-up to the Wobbulator which featured in the preceding issue. The probe was built on a small PCB and housed in a small plastic probe box 2.5cms square x 12cms long, along with a PP3 battery and two slide switches - one for on/off. the other for high/low sensitivity. All a bit of a tight squeeze in the box.

The article ran to five pages - I've attached page one, which shows the circuit and may enable you to decide if the design would meet your needs.

Hope that may be of interest.
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Old 2nd Feb 2019, 5:12 pm   #4
David G4EBT
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Default Re: Looking to build RF detector probe for low frequency

The layout of the probe is attached below.
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Old 2nd Feb 2019, 10:02 pm   #5
IsquaredR
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Default Re: Looking to build RF detector probe for low frequency

David, thank you very much for the probe circuit. I'm going to construct one but had to place an order for a J310 since none were in my junk box. In the interim I have time to try to acquire the whole article that was in Radio Bygones as it looks quite interesting. I so appreciate your sharing that circuit with me.

Bazz4CQJ: I downloaded and saved the entire 200+ pages of the Zucconi book and am spending this afternoon enjoying its content. I thought I was pretty familiar with most of Gernsback's books (my library is full of them) but some how I missed this one. I am grateful for your bringing it to my attention.
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Old 3rd Feb 2019, 12:42 am   #6
Julesomega
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Default Re: Looking to build RF detector probe for low frequency

When you say "linearity" do you mean flat frequency response, or amplitude linearity? Frequency response is really no problem, but a linear amplitude response will be difficult to achieve with a diode detector at signal levels below about .5V P-P. If you want accurate voltage measurements at lower levels you should look out for an AC millivoltmeter, which all the major manufacturers made. Mark Hennessy listed some buget DVMs in another thread recently, where he lists some currently available that measure accurately to below 10mV RMS. I am rather fond of the "Advac" range, and Farnell and Philips models.
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Old 4th Feb 2019, 10:03 pm   #7
IsquaredR
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Default Re: Looking to build RF detector probe for low frequency

Thank you Julian. I am more interested in flat frequency response so the germanium detector David suggested should work out well. I will take a look at the thread you mentioned about AC millivoltmeters, however, as I can foresee future need of one on my bench.

Getting back to the Raymond Haigh RF probe brought to my attention by David
(thank you again, David!): In using it to primarily look at I.F. bandpass waveforms, does any one see any pitfalls if I constructed it somewhat differently difficulty; instead of constructing the whole probe in a single box containing a probe tip, building it in a metal box fitted with a BNC connector for attaching a standard 10X oscilloscope probe to sense the RF waveform and another BNC connector for the cable that goes to the oscilloscope input?
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Old 5th Feb 2019, 10:23 am   #8
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Default Re: Looking to build RF detector probe for low frequency

Just a couple of warnings here as I've gone down this route too and burned myself a couple of times. Any measurement device will load the circuit in some way. In the case of the probe there, any stray capacitance will drag your IF measurement off and/or reduce the Q of the circuit being measured. This may or may not be an issue but it's worth remembering it.

Since discovering this I tend to adjust IF shape by injecting noise into the front end of the receiver from a noise source and using an audio FFT on my smartphone to shape it. Surprisingly effective!

I am using a clone of the noise source here, without the bridge (everything after C4 removed) to do the job: https://qrpguys.com/wp-content/uploa...ide_120616.pdf ... this is loosely coupled to the antenna with a bit of wire wrapped around it usually.

On the probe front, your standard scope probe would have 15-20pF of capacitance across it. I would build the probe as is if you want to go down that route.
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Old 5th Feb 2019, 9:46 pm   #9
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Default Re: Looking to build RF detector probe for low frequency

Let's re-visit the thread which ran back in 2016 on the subject of RF Voltmeters https://www.vintage-radio.net/forum/...ltmeter&page=3. Inevitably, probes were a key issue in the discussion. IIRC, the objective was to come up with a design for an RF voltmeter which would operate over quite a range (1-150MHz ?) with a sensitivity of a few uV (as achieved by Marconi, Booton et al).

Quite a few published designs were found which claimed to achieve this requirement, but when a number of these were built and tested, serious disappointment was the main outcome. Of course, this is not quite the same as IsquaredR, is looking for, but beware...

If IsqaredR builds the Haigh design, I hope he let's us know what he thinks of it.

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Old 6th Feb 2019, 10:41 pm   #10
IsquaredR
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Default Re: Looking to build RF detector probe for low frequency

Thank you Mr. Bungle and Bazz4CQJ and your suggesting the reference to the 2016 thread which I will be most interested in reviewing momentarily. I appreciate your cautions and am aware of the possibility of inconsistency. As such I am attempting to maintain my component tolerances very closely because I know how "a little error here and a little error there" can compound, rendering the results of an otherwise good design disappointing. Indeed I have started constructing the Haigh probe. In fact, today I received one of the two remaining components not readily available in my junk-box that had to be ordered. Once I receive the last missing part (the J310) I'll be able to finish it, probably by this weekend and will certainly update this thread with my results and will include photos of my finished probe.
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Old 7th Feb 2019, 11:20 am   #11
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Default Re: Looking to build RF detector probe for low frequency

Quote:
Originally Posted by MrBungle View Post
Just a couple of warnings here as I've gone down this route too and burned myself a couple of times. Any measurement device will load the circuit in some way. In the case of the probe there, any stray capacitance will drag your IF measurement off and/or reduce the Q of the circuit being measured. This may or may not be an issue but it's worth remembering it.

Since discovering this I tend to adjust IF shape by injecting noise into the front end of the receiver from a noise source and using an audio FFT on my smartphone to shape it. Surprisingly effective!

I am using a clone of the noise source here, without the bridge (everything after C4 removed) to do the job: https://qrpguys.com/wp-content/uploa...ide_120616.pdf ... this is loosely coupled to the antenna with a bit of wire wrapped around it usually.

On the probe front, your standard scope probe would have 15-20pF of capacitance across it. I would build the probe as is if you want to go down that route.
I know nothing about noise bridges but am intrigued by this and may have a go at building one. However, reading the paper you reference it says to connect the antenna to one socket on the bridge and tune using the antenna coupler. Won't this just have the effect of transmitting white noise to all and sundry?

Kind regards

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Old 7th Feb 2019, 4:28 pm   #12
IsquaredR
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Default Re: Looking to build RF detector probe for low frequency

Quote:
Won't this just have the effect of transmitting white noise to all and sundry?
I think the answer to your question Robin is yes. The purpose of a noise bridge is to generate white noise. Having built several (although not the particular design referenced in the article here) it is a simple device to operate and use. You insert the noise bridge between a receiver's antenna input and the antenna coupler. You then adjust the antenna coupler's inductance and capacitance for minimum noise. When minimum noise is achieved you have the antenna coupler tuned to whatever frequency your receiver is tuned.

The writer of the article briefly mentions it but there is another very handy use for a noise bridge. I have frequently connected my noise bridge to a dead receiver. The white noise is broad-banded and can be used for finding circuit problems such as with a signal tracer.

Commercially-produced noise bridges (and many homemade ones) are roughly calibrated so that, once you have tuned to minimum noise you can "tweak" the bridge's variable resistor to "fine tune" and derive the impedance of an antenna.Omega-T and Palomar are two very popular versions.
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Old 7th Feb 2019, 4:41 pm   #13
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Default Re: Looking to build RF detector probe for low frequency

Quote:
Originally Posted by RobinBirch View Post
I know nothing about noise bridges but am intrigued by this and may have a go at building one. However, reading the paper you reference it says to connect the antenna to one socket on the bridge and tune using the antenna coupler. Won't this just have the effect of transmitting white noise to all and sundry?
They are extremely useful bits of gear if you have a receiver around.

They don't radiate anything of significance, certainly several orders of magnitude less than keying down and reading the SWR meter. Mine kicks out less than a couple of mV of noise.

The objective is to get an impedance match so the point at which there is the least noise returned back down the feed at the selected frequency on the receiver then the impedance of the source (receiver) and load (antenna) are matched. You can use them for tuning input and output matching networks to a specific impedance to.

If you ever get your hands on a spectrum analyser they can be used for measuring filter responses as well by injecting the noise in one end and looking at the response on the output (40m LPF here):

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That SA is now gone unfortunately. It caused marital problems
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Old 7th Feb 2019, 7:28 pm   #14
RobinBirch
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Default Re: Looking to build RF detector probe for low frequency

Ha, I'd love one of those and no marital problems . I have a mainframe here that it'd go very nicely into

Hmm, more research required then as this is a branch of setting up receivers that I have no experience of at all.

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Old 10th Feb 2019, 6:33 pm   #15
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Default Re: Looking to build RF detector probe for low frequency

I've just seen this thread after now joining the forum. I have a number of probes from Eico, and one Heathkit I picked up recently, boxed with instructions and all the parts as an unmade kit. I'm thinking I've got the ones built and all the circuits, and of course this Heathkit. Is anyone interested in this being put up on this thread?
Cheers and hello fellas, Ian Evans

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Old 11th Feb 2019, 5:07 am   #16
IsquaredR
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Default Re: Looking to build RF detector probe for low frequency

As promised I am pleased to report tonight I completed building the Haig RF detector probe. It seems to respond but I haven't done any seriously applicable tests, will do so in the next day or two and report finding here. The probe seems to be responding to some superficial juryrigged tests with a temporary "flying clip lead" ---which I will add permanently as soon as I can decide where I want it to egress from the box.
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Old 11th Feb 2019, 8:47 am   #17
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Default Re: Looking to build RF detector probe for low frequency

Looking really good
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Old 11th Feb 2019, 9:35 am   #18
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Default Re: Looking to build RF detector probe for low frequency

I have seen older RF probes using the following tubes: 6AL5, 955, 956,( Yes those are both triodes) 5642, and another "pencil" tube whose number I can't remember off the top of my head.
IIRC, Philco may have made one using a Loctal tube too.
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Old 11th Feb 2019, 12:21 pm   #19
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Default Re: Looking to build RF detector probe for low frequency

This is a high impedance probe, not unlike a normal 10:1 scope probe, and like the scope probe, it will be sensitive to the length of the earth lead.

On a scope, the effect of the earth lead is visible as distortion and ringing on fast edges. You can see the response change as you move the earth lead. On an RF voltmeter the uneven frequency response may not be obvious.

Decent scope probes have a removable earth lead so that you can use a lead appropriate to the job in hand, and for best performance you would use an earth spike attached at the probe tip which gives a “loop” with the probe tip itself of only half an inch or so across. You say you only need a response up to 3 to 4MHz, so you don’t need to go to such extremes, but it may be worth considering a removable earth lead so you can trade off performance with convenience.

It’s no use going to the trouble of minimising all the lead lengths on the signal input to the last fraction of an inch, and then adding a foot or more of earth lead.

Stuart
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Old 11th Feb 2019, 4:49 pm   #20
IsquaredR
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Default Re: Looking to build RF detector probe for low frequency

Thank you, MrBungle. And Frank I appreciate your information. Stuarth: How coincidental that you offered your posting and I am grateful for it. It probably sounds silly but last night as I was falling asleep I was weighing the options of where to locate the ground clip. I can make an argument for placing it on any side so the conundrum was where would it be most convenient? It is of minimal importance but one thing that is sometimes annoying about probe ground clips is that they get tangled-up while hanging in storage. Ironically I've been considering just putting a small jack on the side and using a removable ground clip. After reading your posting I think this would certainly be the way to go. It was also offer a "plus" because I could remove it for storage and vary it in length as my projects might require. So thanks for your guidance.

In a private message Dave G4EBT mused that if he were to build one again he would have used a bigger box because a small one became "fiddly." I'm with you, Dave. It was difficult to put it all together and make the logistics work out in my undersized box. I also was unable to use my metal box as the negative return because I feared hand capacity might affect sensing. So I also needed to fashion a probe tip and a BNC connector mounting that was insulated from the metal box. In his article, Mr. Haig recommended a plastic box but I did not have one on hand.

So I'm going to go with a jack on the side for a ground return and will post results here after I do some actual testing on a live circuit.

Thanks to all who have taken the time to make suggestions, provide information and lend a hand.
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