[HBQRP] Test equipment projects -VNA ect - Sean

[Sean Pepin]

Hello Arnie and the group!

Interesting stuff. That VNA project is awesome. It's a standalone add-on 
for the N2PK VNA that allows it to be used without a computer. I've got 
the boards and the parts together to construct the N2PK VNA, I've just 
been putting it off due to recurrent distractions. Hopefully I'll be 
able to find some time and peace in July, after I'm finished with our 
move at work (again). I won't name names, but there's another of us 
who's also got the parts and has been waiting for time. :)

As to the ESR meter, I built one just about a year ago from plans that I 
can no longer find. I remember that I had the article printed out, but I 
think I gave it to someone at one of the meetings.

The one I built has a few differences. It uses an diode regulated opamp 
based Wien bridge oscillator to generate the test signal and has a 
buzzer with it's level settable by a pot. The oscillator will output a 
fairly clean sine wave at close to 100kHz, which is useful for switching 
power supply capacitors that are operated at high frequency. I read 
during my research of these meters that caps will more often have a 
100kHz ESR listed on their datasheets than any other arbitrary values, 
but I don't know if it's true. I'm also not sure if it really matters 
that it's a sine wave, as the harmonic energy would probably be 
attenuated heavily and likely wouldn't contribute much to measurement, 
but to be honest I don't really know.

Either way, it's good to know what the waveform looks like on a scope, 
because I've had use for that information at least once already to 
evaluate a scope I was testing! Turned out the scope was ok, but the 
leads to the calibrator were corroded so it couldn't be verified beyond 
picking up buzz from touching the probe end. Not really 
earth-shattering, as we were twenty feet from a signal generator, but it 
got an 'oooh, neat' anyway.

There are two opamps and some gain in the active rectifier bringing the 
signal back up, all told around 30dB, so the injected signal level is 
divided down below 15mV. This pretty much guarantees that I won't 
trigger any active devices with the signal, but really helps with 
switching supply caps. It may seem a bit of overkill in that regard, but 
those switching supply rectifiers are often Schottky units with a low 
forward voltage and this helps avoid passing a bad filter cap when 
testing in circuit.

Much of the rest of the project is the same, and I think I even used the 
same housing from Radio Shack! This meter has seen an awful lot of use 
given my day job.

Thanks for the heads up on these, Arnie. I'll bring my meter along to 
the next meeting in case anyone wants to see it.