UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Reply
 
Thread Tools
Old 19th Feb 2018, 1:57 pm   #1
MrBungle
Nonode
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 2,416
Default W7ZOI power meter (AD8307 based)

Needed a new power meter as I blew up the sensor on my HP one months ago. I can now understand why there are so many meters without power sensors. Not willing to pay the price for a replacement, I looked at EMRFD and found the W7ZOI power meter.

This particular unit is tested by me from -72 to +13dBm (35uV to 1V RMS) and 1-180MHz and is flat to within 1dBm. This is somewhat limited due to my signal generation and attenuation capability being relatively restricted. If anyone has a Marconi 2019A they want to get rid of, please let me know

Schematics and article here: http://www.qsl.net/sz1a/download/bui...er%20meter.pdf

I bought the AD8307's from Aliexpress from China for around 2.80 for 10. They are 10.15 each (!) on RS. Seem to work absolutely fine. Enclosure is a Pomona box from an ebay lot. I would use a die cast / eddystone box if there wasn't one on hand. Power is fed through one of the screw holes while I wait for some feed through capacitors to arrive.

Due to the linear response, if you have a DMM that can do math, you can hook this up and enter the M and C calibration constants (Y=MX+C). M is volts per decibel and C is the 0 dBm intersection voltage. This will then read dBm on the meter display!

Calibration is pretty simple. Stick a reference signal (I used 0dBm) on your generator and write down the voltage, then stick a 20dB attenuator inline with the PM and write down the voltage again. The M value is quotient of the difference of the two voltages and 20. The C value is derived by entering 0 to start with then applying 0dBm. The offset from zero is the C value. You can of course use a programmable calculator, graph or any other method to read the power with a normal DMM.

The buffer amplifier with LPF on the output is 100% necessary - the AD8307 output is quite sensitive so isolation is needed. The loop in the input resistor is actually a carefully (not) wound inductor which stops some UHF pick up. It was quite unstable without that little loop!

Construction is very ugly but it works.

Click image for larger version

Name:	powermeter2.jpg
Views:	153
Size:	111.0 KB
ID:	157750

Will be doing more SMD stuff. It's cheaper and not as hard as it looks. Tweezers and beer make it workable
MrBungle is offline   Reply With Quote
Old 19th Feb 2018, 4:34 pm   #2
Bazz4CQJ
Octode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 1,868
Default Re: W7ZOI power meter (AD8307 based)

That's interesting! I have a pack of similarly priced AD8307 which have been waiting for me to try them out for some time.

Now, a while ago you were talking about high-impedance RF millivoltmeters; is this power meter (with its 50R input) a stepping stone towards the millivoltmeter? Skywave described his (3 FET) wideband RF amp some time ago; what do you get if you put that (or some part of it) in front of the power meter?

B
__________________
We know there are known unknowns. But there are unknown unknowns ones we don't know we don't know.
Bazz4CQJ is offline   Reply With Quote
Old 19th Feb 2018, 5:28 pm   #3
MrBungle
Nonode
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 2,416
Default Re: W7ZOI power meter (AD8307 based)

Now that's a good question. Technically dBm can be converted to millivolts at a specific impedance. I think a better option for this would be to stick an Arduino or something similar on the output and do the conversion there. The AD8307's log output gives it a lot more dynamic range than other approaches.
MrBungle is offline   Reply With Quote
Old 19th Feb 2018, 6:30 pm   #4
Bazz4CQJ
Octode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 1,868
Default Re: W7ZOI power meter (AD8307 based)

But, isn't the quest for a high-impedance millivoltmeter driven by the desire not to load low-level circuits under test? So, even thought you could do number crunching and try get a figure with the power meter, the better approach there would probably be a probe which does not significantly affect the circuit. Of course, you'd still have the option to switch between the power meter at Z=50, or the voltmeter at Z>>>50. Side-steps the great unknown of what 'magic' diodes to use for sensitive RF probes...perhaps?

B
__________________
We know there are known unknowns. But there are unknown unknowns ones we don't know we don't know.
Bazz4CQJ is offline   Reply With Quote
Old 19th Feb 2018, 6:48 pm   #5
MrBungle
Nonode
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 2,416
Default Re: W7ZOI power meter (AD8307 based)

I played with that idea a few months ago and it seems impossible to measure things like this without having some kind of detrimental effect on the circuit. Sure we can have a probe with impedance of much larger than the system impedance, but that probe also has capacitance which is significant above a few MHz. I noted this effect when I built the FET probe from Troubleshooting Analogue Circuits. It could change filter response and VFO drift even when probing low impedance portions of circuits.

Fortunately I'm not limited to reverse engineered systems here where someone else decides the impedances so I'm basing my forward engineering work purely on 50 ohm blocks and substitution based measurements. Start in LTspice, prototype, build, test, integrate. This means that the measurement system has the same load on the system as normal duty and therefore no side effects (well not quite but it's far closer to ideal).

RF probes are horrid. That's what started me on this journey. They're good for finding if there is a signal but not how big it is. I'm not sure how you can do any reasonable receiver measurements with one.

Please note I'm drunkenly bumbling my way through this. I might be discovering obvious things on the way that professionals are already aware of. Or more likely completely missing the target
MrBungle is offline   Reply With Quote
Old 19th Feb 2018, 10:44 pm   #6
Bazz4CQJ
Octode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 1,868
Default Re: W7ZOI power meter (AD8307 based)

Well, you may it right; Skywave and myself built a number of published designs for RF millivoltmeters and none of them worked as had been claimed for them. The commercial ones are quite expensive for purely amateur use.

B
__________________
We know there are known unknowns. But there are unknown unknowns ones we don't know we don't know.
Bazz4CQJ is offline   Reply With Quote
Old 19th Feb 2018, 11:03 pm   #7
MrBungle
Nonode
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 2,416
Default Re: W7ZOI power meter (AD8307 based)

Yes indeed. Ridiculously expensive. They all seem to be based around diode probes with various bits of black magic around them. Even looking at the construction of some of the probes, they seem to have a lot of stray inductance and capacitance around them and a hefty coupling capacitor up front. I'm going to leave this one on a big shrug.

Perhaps there is some mileage in the Tektronix sampling and differential probe designs which can be leveraged. A project for another day I think
MrBungle is offline   Reply With Quote
Old 20th Feb 2018, 12:04 pm   #8
TrevorG3VLF
Octode
 
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,048
Default Re: W7ZOI power meter (AD8307 based)

Could two AD8307 devices be used to read the outputs of an SWR meter and then subtract the two voltages to give instantaneous log SWR.??
TrevorG3VLF is offline   Reply With Quote
Old 20th Feb 2018, 12:35 pm   #9
MrBungle
Nonode
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 2,416
Default Re: W7ZOI power meter (AD8307 based)

Yes. Someone has already done that here: https://sites.google.com/site/loftur...-and-swr-meter

I suspect that's how some of the commercial units work as well.
MrBungle is offline   Reply With Quote
Old 20th Feb 2018, 12:43 pm   #10
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 10,178
Default Re: W7ZOI power meter (AD8307 based)

Not strictly log VSWR, but a linear scale of decibels of return loss, which is actually better.

I made one with the AD640 predecessor chip in 1993. G3ROO made the boards. We didn't publish because it seemed a bit too much for most UK magazines of the time.

Oh, and I fed the limiter outputs to an CL phase detector, so I get return loss and phase.

A VECTOR, logarithmic power meter!

It's built into my 'biggest mistake' ATU.

DAvid
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now   Reply With Quote
Old 20th Feb 2018, 1:51 pm   #11
Argus25
Octode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 1,519
Default Re: W7ZOI power meter (AD8307 based)

It seems really odd that the designer wasted the opportunity to connect the unused 1/2 of the LM358 to the meter ground leg to improve the meter balance and stability, still in practice might not make a huge difference, but its just sitting there wasted.

from Mr Bungle post #5 :

"I played with that idea a few months ago and it seems impossible to measure things like this without having some kind of detrimental effect on the circuit".

Never a better word spoken.

This is really a macroscopic version of the uncertainty principle. Many don't realize that when you introduce test apparatus to a system, you modify the very system you are trying to measure by loading.

It is actually extremely difficult to get the loading low. I did some experiments a while back including things like loading a VFO with 6 cmos buffer gates in series, but with the output of the final one shorted, or not, it still pulled the VFO. In the end I had to resort to an optically coupled buffer for excellent isolation.

So you have to be careful with any test instrument-probe, that the loading is low, but the higher the input impedance, the more difficult stray capacitance is and frequency compensation issues. At least Tek were the masters of this dilemma with the input circuits they created for scopes, so there are some good lessons there to look at their input circuitry, if you are planning to make an RF milli-volt meter, or similar.
Argus25 is offline   Reply With Quote
Old 20th Feb 2018, 2:23 pm   #12
G0HZU_JMR
Octode
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 1,377
Default Re: W7ZOI power meter (AD8307 based)

I made a similar power detector module when the AD8307 was first released many years ago. I still have it here and It looks quite similar to yours. We used the AD8307 in volumes of tens of thousands at work. Usually as a basic RF level sensor.

However, you do have to be careful if using it as a power meter. It is a log detector so can't measure average power correctly. This means you ideally have to use it with a clean sine wave. It won't cope well with modulated signals and it suffers high uncertainty in the presence of odd order harmonics. Best to keep these at -30dBc or better or the uncertainty creeps up rapidly.

I sacrificed some sensitivity with my version because I wanted a low VSWR input and mine has an input VSWR < 1.04:1 across LF through 500MHz. I also incorporated some frequency compensation into mine. The aim was to get it flat to 150MHz to about +/- 0.2dB at 0dBm and then allow it to roll off towards 500MHz by a few dB.

This way I didn't have to mess with correction factors vs frequency when using it up to 100MHz or so. At the time I also had an HP431C/HP478A thermistor power meter but this only works well down to 50MHz and has a limited range. So the A8307 sensor was used mainly below 100MHz.
__________________
Regards, Jeremy G0HZU
G0HZU_JMR is offline   Reply With Quote
Old 20th Feb 2018, 2:39 pm   #13
G0HZU_JMR
Octode
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 1,377
Default Re: W7ZOI power meter (AD8307 based)

Mine was at its best when measuring the loss of short RF cables or RF filters because if you know the mV/dB slope of the detector and the input VSWR of the detector and the RF source is low you can measure insertion loss quite accurately. Again, you do need to make sure the source has low harmonics. Even -40dBc harmonics can begin to affect a critical measurement like this if you use an AD8307 based sensor.
__________________
Regards, Jeremy G0HZU
G0HZU_JMR is offline   Reply With Quote
Old 20th Feb 2018, 3:20 pm   #14
MrBungle
Nonode
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 2,416
Default Re: W7ZOI power meter (AD8307 based)

This particular beast isn't going to see anything faster than about 30MHz. Plan is to use it for substitution tests on my never ending HF transceiver build. It's being driven from a Rigol AWG at the moment which has reasonable harmonic suppression. This is going soon (it has some real turds hiding in it) so will replace with a proper RF signal generator.

Also will reuse another one for the spectrum analyser project.

Got my feed through capacitors today so I can actually proceed with knocking up some decent RF tight modules for that and getting the DC in sorted on this.
MrBungle is offline   Reply With Quote
Old 23rd Feb 2018, 11:54 am   #15
MrBungle
Nonode
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 2,416
Default Re: W7ZOI power meter (AD8307 based)

Found a reference in "Arduino Projects for the Radio Amateur" for an AD8307 directional watt and SWR meter. I will make a little box to plug the above power sensor into and see if I can get a direct readout. Requires an Arduino and LCD so about a tenner from bitsbox.

If it works, I might actually build the SWR meter in there. I have enough bits already by the looks.
MrBungle is offline   Reply With Quote
Old 24th Feb 2018, 1:05 pm   #16
MrBungle
Nonode
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 2,416
Default Re: W7ZOI power meter (AD8307 based)

Good old bitsbox. Tenner spent yesterday, arrived an hour ago. Got it wired up and programmed.

Working on ADC and calibration side of things now. I will stick this in a separate box so there will be a power sensor head inside the pomona box and a readout box.

Click image for larger version

Name:	IMG_1522.jpg
Views:	29
Size:	99.7 KB
ID:	158158

Edit: this is the first time I've used an arduino properly - pretty good experience so far. Worried about there being no debugger.

Last edited by MrBungle; 24th Feb 2018 at 1:12 pm.
MrBungle is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 7:47 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.