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Old 29th Oct 2018, 1:09 pm   #1
MrBungle
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Default Homebrew RF power meter mark 2

This one is a prototype. I screwed the board up on this cycle (SMA footprint) but it works and is dirt cheap (under 10!). Decided to put together a proper RF power meter as most of the stuff from China is junk. That doesn't mean the parts are junk but the way they put them together is.

This is an evolution of the W7ZOI power meter here: https://www.vintage-radio.net/forum/...d.php?t=144088 ...

The original one suffered from a few problems. That was mainly due to the layout. Secondly it was poorly decoupled. Thirdly it was badly constructed. Fourthly, the output transform circuit actually pretty much made it useless. It was a hack job.

Now we have version 2...

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Calibration is done as follows: Apply 0dBm. Record the voltage on the DMM here to get the 0dBm intercept. Stick a 20dB attenuator in line (or crank generator down by 20dBm) and record the difference in voltage. This gives you a y=mx+b transformation.

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Calibration is pretty much 100% linear and it has 60dB dynamic range which is good for most tasks with suitable attenuators and/or taps.

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Currently waiting for a suitable signal generator to appear so I can check it across a wider frequency range than my AWG kicks out. It's flat across 2-25MHz though.

The board is designed to fit inside a specific hammond enclosure with a pigtail on one end. Once I've fixed the boards and get hold of a signal generator, I may be offering this as a kit. Time will tell!

Edit: also to note, the output buffer and filter is designed to respond fast enough to allow a sweeping signal to be applied from the generator. This allows you to use this to do antenna (with return loss bridge) and filter sweeps with a sweeping generator and get a log-log output on a scope this turning it into a scalar network analyser too!

Last edited by MrBungle; 29th Oct 2018 at 1:25 pm.
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Old 30th Oct 2018, 11:57 am   #2
vosperd
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Default Re: Homebrew RF power meter mark 2

Hi, I built up one of these using strip board and through hole components. I can't say that I did any exhaustive tests or calibration but it seemed to give sensible readings with my FT817 on 80M.
if a kit ever becomes available I, for one, would be interested. Much tidier than strip board.
Don m5aky
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Old 30th Oct 2018, 7:59 pm   #3
Bazz4CQJ
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Default Re: Homebrew RF power meter mark 2

Mmm...maybe you could offer the board as an individual item?

B
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Old 31st Oct 2018, 8:42 am   #4
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Default Re: Homebrew RF power meter mark 2

That’s a possibility. They’re not exactly expensive to get done so I don’t mind ordering 100 or so (probably never get rid of that many!). Past the board and the AD8307 though, the entire parts inventory costs less than 1
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Old 31st Oct 2018, 9:43 am   #5
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Default Re: Homebrew RF power meter mark 2

I'd have a couple!

Gordon
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Old 31st Oct 2018, 4:07 pm   #6
Bazz4CQJ
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Default Re: Homebrew RF power meter mark 2

Yes, the whole kit makes sense, as does taking two (my SMD skills may need the first just for practice ). Keep us posted.

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Old 31st Oct 2018, 5:57 pm   #7
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Default Re: Homebrew RF power meter mark 2

Will do. Waiting for another board run now after I screwed the last one up

If your SMD skills are a bit iffy, it's worth hitting a cheap 1 SMD practice kit for aliexpress/ebay/banggood. The one with the circle in the middle. Buy two as they don't always get the bits right. Just need some fine solder, flux pen, tweezers and some isopropyl alcohol and a paint brush to clean it with and done. Can also recommend a pint down the pub first to iron out any jiggly hands

Without alcohol I can solder an 0603. With alcohol, 0402

Edit: I'll post some applications in here as well.
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Old 1st Nov 2018, 4:14 am   #8
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Default Re: Homebrew RF power meter mark 2

I can't even see 0402, alcohol or not

A friend sent me a kit to repair one of my radio transceivers which I dare not open (the kit, not the radio).

I can see a pcb and what looks like specs of fly dirt inside (which I'm reliably told are actually the components).
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Old 1st Nov 2018, 9:19 am   #9
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Default Re: Homebrew RF power meter mark 2

I will say that I lose 50% of the parts when I do it. They just ping off the tweezers never to be seen again, followed by some swearing.

For my hand soldered designs I only go down to 0805.

With a bit of practice I don’t want to go back to through hole now. So much easier to rework SMD stuff.
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Old 2nd Nov 2018, 10:24 am   #10
vosperd
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Default Re: Homebrew RF power meter mark 2

With smd I find complete kits are better for me than bare pcbs.
Thr ICs are easy enough to source but the resistors and capacitors etc for a one off can be a problem. I find all you need is never available from a single source. They are cheap enough to buy in quantity but sometimes you can be paying a couple of pounds to buy 10 or 100 when you only need one or two of a praticular value.
I can manage the AD8307 pin spacing but I think the next size down the pin spacing is half that which is a problem.
Don m5aky
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Old 2nd Nov 2018, 11:23 am   #11
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Default Re: Homebrew RF power meter mark 2

Quote:
Originally Posted by vosperd View Post
I can manage the AD8307 pin spacing but I think the next size down the pin spacing is half that which is a problem.
Don m5aky
As long as the pcb has a proper solder mask around the pads, it is not that hard - flood solder the pins and use flux and desolder wick to remove the excess.
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Old 2nd Nov 2018, 1:25 pm   #12
MrBungle
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Default Re: Homebrew RF power meter mark 2

Indeed. They're dead easy. I've done down to MSOP8 / 0603 now regularly. My eyesight isn't that great either. Years of avoiding it turned out to be a fuss over nothing.

In fact the best thing is that it's pretty easy to get the smaller parts off without trashing the board which is difficult on older PTH boards with some of the multi pin parts. I usually snipped off IC leads and then desoldered the pins individually to avoid this.

No need with these!

With respect to parts prices, I am using LCSC and they are incredibly cheap. I think I paid 10p for or so for 100 0805 1% resistors. Sometimes it's hard to get enough in the basket to get past the small order charge (which goes away at $10)
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