Re: Power Meter

Nick Kennedy

Well, I did go and try the circuit described in the video. I wanted to see
that feedback circuit work to control output level. I had to fool with some
components to get the loop to control. I was only going to build enough to
see that part in action but wound up adding the LPF and attenuator

I cheated on the LPF by using molded chokes instead of toroids. The
performance (using PHSNA, of course) looked decent though. For others
experimenting with this circuit, my changes included:

CA3160 for CA3140
74HC86 for 74AC86
6.2 k for 12 k from MOSFET gate to ground
Added a second diode in series with 5 V to lower the reference voltage to
the op-amp
LPF used molded chokes 1.8, 2.2, 1.8 uH for the LPF
Used a 7.030 crystal
Didn't want to fight with building shielding so I just did the 0 dBm output
My attenuator is a tee, 200 ohms, 27 ohms, 25 ohms
(Attenuator shows 220 ohms to the amplifier and 50 ohms toward the load)

It hit 0 dBm right on the money the first time - I was amazed.

I was intrigued by that integrating capacitor feedback. What's its
function? Is it needed? Well, it seems to work better with than without. I
guess it has a stabilizing effect.

It was interesting that when I'd experiment with lowering supply voltage,
the mosfet drain current would take off going to over 100 mA versus typical
35 mA when voltage went below 7.5 or so.

I could not get the "battery low" LED scheme to work. My LEDs may be too
sensitive, if that's the word for it.

Thanks again Hans for an informative video.


Nick, WA5BDU

On Fri, Sep 13, 2019 at 8:52 AM Nick Kennedy <kennnick@...> wrote:

Very nice circuit Hans, and a great video to describe it in detail. I'd
be tempted to try to duplicate it but I've already got several "watches"
that give me a well defined RF level.


Nick, WA5BDU

On Thu, Sep 12, 2019 at 7:46 AM Hans Summers <hans.summers@...>

Hi JoergU

There is a very nice power calibrator design available from a German
Just google for "dl7av calibration". Use a translator, to translate the
German text.
It creates a 0 dBm sine output. I built it myself (cost approx. 10 EUR)
and the
harmonics are >30 dB suppressed.
Many thanks for this. Very rare for this to happen, but it happened to be
exactly WHAT I needed and WHEN I needed it... so I googled DL7AV
calibration and found several pages, of which the main one I followed was

So I built my own version from entirely junkbox parts yesterday afternoon
and I have made a YouTube video about it see

This will be very useful in my case - thanks again!

73 Hans G0UPL

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