Simple log power meter

[neazoi]

Hi in this simple HF qrp power meter (and load combination), can I use germanium diodes instead, which should in theory increase the sensitivity even more (or won't they?)

Also, I guess if I use a 20uA meter the sensitivity will be 5 times more?

**broken link removed**[/URL]

 

[FvM]

No valid link...

 

[neazoi]

This is the non-translated version https://www.sm7ucz.se/QRP_Meter/QRP_Meter.htm

 

[betwixt]

Strange design!
I wonder why the voltage doubler then that particular opto-coupler. It seems to be a conventional RF voltmeter design with some current diverted through the transistor. It is very expensive device but has no characteristics making it especially suitable in this application.

Small signal Schottky diodes generally have a lower Vf than germanium diodes so there probably wouldn't be much advantage in using them. If you do use a different type beware of the PIV rating. According to the schematic there could be 63V p-p at the input so potentially twice that across the diodes although there would be some sharing between halves of the doubler circuit.

And changing the meter will not work in practice, it will make it more sensitive to low power levels but the effects of the 'logarithmic' part will be less because it will reach FSD before the optocoupler has become conductive.

Brian.

 

[neazoi]

I think he used this opto because he had some for selling
I also thought of using a transistor like the BC549 in the buffer of this design **broken link removed** as a variable resistance device, but I do not know about it's log response.

Anyway can I use 1n5711 diodes, will they be ok?

For the 20uA meter, I really think of adding a momentary switch anyway, when pressed it will change the meter resistor/s to less values so that it sill be very sensitive in this pressing. The log scale is good but QRPers should see more details in the 1-2W region, so I think it is ok to relax the scale a bit.
After all the sensitivity of the opto is controlled by the led variable resistor.
What do you think?

 

[vfone]

In most of the cases, used as detectors, point contact Ge diodes works better than Schottky diodes, with comparable results if both are DC biased:
**broken link removed**

I never saw a 20uA (full scale) galvanometer. The lowest that I ever seen was 50uA.

 

[neazoi]

Thanks.
There are, see here for example https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313&_nkw=20uA+meter&_sacat=0 some of them are pretty cheap.
I will try the circuit with Ge diodes and a 20uA meter. R2 scaling should be enough for using the new meter, whereas the action of the opto won't be affected much I think.
There will be a switch that will select a 10W and 1W level, by changing the P1 value or P1 and P2 at the same time. This should increase the resolution of the meter much on the lower end, so that even low output oscillators can be used.
There will also be a switch to switch out the dummy load. If the equipment to be measured has already a 50R shunt at the output the load of the meter should not be used. Switching off the load will also help in detecting RF (but not correctly measure it's level) in unbuffered oscillators. Because some of them, if loaded to 50R, won't even start.

How do you find my mods?