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Old 4th Dec 2016, 1:28 pm   #1
Skywave
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Arrow R.F. voltmeter: your thoughts, please

Found on the 'Net: original source unknown, probably QST.

Two aspects puzzle me: (a) the range switching, in particular the 'gain trimmers' arrangement: I would have expected separate pots. for each range and (b) there are 6 switchable ranges, yet only 4 shown on the meter scale. Finally, would anyone like to estimate the freq. range of this proposed instrument?

Your thoughts & comments will be appreciated: thank you.

Al.
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Old 4th Dec 2016, 2:32 pm   #2
David G4EBT
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Default Re: R.F. voltmeter: your thoughts, please

As to the meter scale, to keep it uncluttered, as is commonplace, for the additional two scales not shown on the dial, the two top ranges would be maked on the switch x 10. Hence, the 0-30 RF Volts on the range switch would be read on the 0-3V range, so 30V would be FSD and 0-100 RF Volts on the range switch would be read on the 0-10V range. However, I wouldn't want to suggest that the diodes in the probe circuit or the transistors would stand that sort of Voltage, and couldn't say what frequency it would operate to. I assume that it's meant for tweaking homebrew QRP transmitter stages with just a few Volts of RF, up to say 30MHz.

Just my thoughts, for what they're worth.
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Old 4th Dec 2016, 2:41 pm   #3
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Default Re: R.F. voltmeter: your thoughts, please

At first glance it looks like this meter uses a remote head for measurement with temperature compensating diodes.

The diode detector will operate up into the linear region on the top two scales on the dial so the ranges 4, 5 and 6 will probably be 10V, 30V and 100V. So you only need 4 scales for 6 ranges? I haven't checked the resistor dividers to see if this is the case though... The diode PIV rating will limit the voltage that can be measured without damage...
The JFETs are drawn as N but I think they should be P.

I would have expected to see three gain trimming pots. One for the bottom range (close to square law?) one for range two (transition region ?) and then one for the top ranges that will all be linear.

I think that the diodes will limit the frequency range and I'd expect it to work OK over the HF range but the accuracy would begin to suffer in the VHF range. A modern version would use Schottky diodes (eg 1N5711) but these diodes have a lower PIV rating. But the probe would then work with a flat response up into the UHF region if the head was built with good RF layout etc.

The temperature compensation would be (most) needed for the bottom two ranges.
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Old 4th Dec 2016, 2:44 pm   #4
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Default Re: R.F. voltmeter: your thoughts, please

Looks like I was writing my post at the same time as David
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Old 4th Dec 2016, 3:22 pm   #5
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Default Re: R.F. voltmeter: your thoughts, please

Freq range will depend on the construction of the head and probe. 1N914 suggests no more than HF. Could go lots higher with schottky diodes and careful construction.

If building, an opamp would be far better and the right one would make the offset adjustment unnecessary.

David
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Old 4th Dec 2016, 4:50 pm   #6
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Default Re: R.F. voltmeter: your thoughts, please

1n270 (bitsbox/farnell) has PIV of 100v, 0.8pF capacitance and is good to 200MHz-ish. Also being Ge based the forward drop and linear region is closer to zero than an Si or Schottky diode.

I've built an RF voltmeter. I won't detail it here as the schematic is long lost so I will summarise that I decided to avoid all linearity problems by using a basic attenuator, then a FET follower (J113) followed by a switchable 40dB amplifier, followed only then by detector and then the meter driver. Peak is nearly always in the linear region and it doesn't load the circuit due to the FET follower. This drives a £2.68 DMM from CPC as a readout
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Old 4th Dec 2016, 5:32 pm   #7
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Default Re: R.F. voltmeter: your thoughts, please

Isn't there a chip that does all of this with a logarithmic scale (saves all that switching) up to a few hundred MHz?
 
Old 4th Dec 2016, 5:54 pm   #8
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Default Re: R.F. voltmeter: your thoughts, please

Hi Merlin,
I think AnalogDev is hes producer...
Karl

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Old 4th Dec 2016, 7:11 pm   #9
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Thumbs up Re: R.F. voltmeter: your thoughts, please

Thank you, gentlemen. Good answers to all my questions. Plenty of things there for further thoughts.

Al.
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Old 4th Dec 2016, 7:20 pm   #10
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Default Re: R.F. voltmeter: your thoughts, please

Quote:
Originally Posted by merlinmaxwell View Post
Isn't there a chip that does all of this with a logarithmic scale (saves all that switching) up to a few hundred MHz?
Expensive AFAIK.
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Old 4th Dec 2016, 8:22 pm   #11
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Default Re: R.F. voltmeter: your thoughts, please

Try Analog Devices AD8357 500MHz 92dB dynamic range. £8 at farnell in singles. I use them rather a lot. Easy to use and very dependable. Much more expensive than a vanilla opamp and a diode, but a good price for what it does.

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Old 4th Dec 2016, 8:33 pm   #12
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Default Re: R.F. voltmeter: your thoughts, please

Speaking of chips, attached is a Power Meter circuit from QST which seems to have attracted attention over the years, with a couple refinements being published subsequently. Wide range of input levels and good to 500MHz...apparently. Perhaps of interest?

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Old 4th Dec 2016, 9:05 pm   #13
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Default Re: R.F. voltmeter: your thoughts, please

Quote:
Originally Posted by Radio Wrangler View Post
Much more expensive than a vanilla opamp and a diode, but a good price for what it does.
That's not terrible. RS were trying to mug me for £21 for one a while back.
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Old 8th Dec 2016, 5:53 pm   #14
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Default Re: R.F. voltmeter: your thoughts, please

Hi, this circuit was published in 73 Magazine, Dec/1973 issue, with a full explanation for construction. I have built one, it works very fine until 150MHz.

I downloaded the magazine from the web, it's free. I dont have the link, try with Google.

Happy new year.

Ruben.
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Old 9th Dec 2016, 1:03 pm   #15
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Arrow Re: R.F. voltmeter: your thoughts, please

Thank you, Ruben: your post refers to my opening post, q.v.: cct. of an R.F. voltmeter.
The article to which you refer is indeed in that issue of 73 magazine. It can be found here:

http://r.search.yahoo.com/_ylt=A7x9U...dA6JoJwxjbJx4-

I used the offered facility to download it as a .pdf file. There are many pages in that magazine issue, so it does take some searching to find it - but yes, it is there. Unlike in my OP, the '73 mag.' article goes into a lot of detail - which helps in not only fully understanding the cct. but also the construction of it. Then there is the fact that you state you have tried it with success: that accreditation is worth quite a lot.

Thanks again - and a merry Christmas and a prosperous New Year to you.
Al.

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Old 9th Dec 2016, 3:03 pm   #16
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Default Re: R.F. voltmeter: your thoughts, please

If the temperature compensation works well on the bottom (mV) range it might be worth making a series of external (Hi Z) attenuators for it because the bottom range looks like it will operate in square law. This means you could use it to measure the Vrms of various complex waveforms on this range and the attenuators would extend the range up for higher voltages than 20mV. On the upper ranges on the meter where the detector operates in the linear region you are kind of limited to measuring sine waves unless you can accept a fair bit of measurement uncertainty if there is much in the way of odd order harmonic distortion for example.

It might be worth having the zero control on the front panel if there is a bit of drift over temperature despite the temp comp that is already there. This could be useful on the bottom (mV) range.
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Old 9th Dec 2016, 11:45 pm   #17
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Arrow Re: R.F. voltmeter: your thoughts, please

I've now extracted the relevant pages from that magazine and stored them as .jpg files. If anyone needs a copy of them, please send me a P.M.

Incidentally, there is a drawing error on the cct. diag. The two FETs, Q1 and Q2 are drawn as N-channel types. However, Q1 and Q2 are 2N4360, which are P-channel types. That is confirmed since the source of Q1 and Q2 are commoned and via R12 go to the +ve rail - which is chassis / earth. The drains of Q1 and Q2 eventually go to the -ve supply rail.
OTOH, Q3 and Q4 - both NPN types - are drawn O.K.: their collectors (eventually) go to the +ve rail (chassis / earth); their emitters (eventually) go to the -ve supply rail.

Al.
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Old 10th Dec 2016, 2:57 pm   #18
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Default Re: R.F. voltmeter: your thoughts, please

Quote:
Incidentally, there is a drawing error on the cct. diag. The two FETs, Q1 and Q2 are drawn as N-channel types. However, Q1 and Q2 are 2N4360, which are P-channel types.
Yes, I spotted this and mentioned it in my first post in post #3 in the thread.
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Old 13th Dec 2016, 2:06 pm   #19
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Arrow Re: R.F. voltmeter: your thoughts, please

For those who are following this thread: an update.
On account of preparation for Christmas plus routine household tasks and chores, input to this thread from me has temporarily ceased. But now and again when time permits, 'behind the scenes', I am still investigating this circuit and others associated to it. It is my expectation and desire that when I have something useful to add to this thread, I will do so. Your patience is thus respectfully requested: thank you.

Al.
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Old 13th Dec 2016, 10:39 pm   #20
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Question Re: R.F. voltmeter: your thoughts, please

An idea: is what follows sound?

For various reasons, it would be convenient to replace the two P-channel FETs with N-channel types. To do this, the following changes will need to be made:

1. Replace both FETs with N-channel types; R12 stays same value
2. The -9v. rail becomes + 9v. But all the 0v. / gnd. connections stay the same.
3. CR5 and CR6 stay in the same position in the circuit, but they are reversed (the cathodes become the anodes, and vice-versa); but R11 stays same value.
4. The four rectifying diodes, CR1 - CR4 similarly will need anode and cathode 'swop'.
5. Q3 and Q4 (NPN) are replaced by PNP types.

And just as an after-thought, would replacing the FET's common source resistor, R12, with a constant-current sink, be worthwhile?

I cannot think of any other changes that may be necessary. And there are no polarized capacitors to consider!

Can anyone spot any flaws in my thinking and proposed changes here?

Thanks,
Al.
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