Boonton 93A True RMS Voltmeter

I have a 93A, but not the external probe. Assuming that it works like a 10x scope probe, it can easily have less capacitance than the meter itself. (This isn't mysterious, it's just how voltage dividers act.)

Measurements, aka Boonton, made a lot of good gizmos. I'm a particular fan of their 72BD digital capacitance meter and their model 59 grid dip meter. (I have two of each.) I also used one of their Q meters long ago, but don't have one myself.

Cheers

Phil Hobbs

That went over his head like a Scud missile.

.... Phil

"amdx"

Then use it on the ohms range to verify that 2Mohm figure.

If it is, then add another 2M in parallel to get 1M.

Now you can use any standard 1:1 or 10:1 scope probe.

Shakes head....

.... Phil

I have decided it is not a misprint, because I noticed it says the the same 2Mohm 25pf at the input connector. Although I don't know why it measured as close (9%) as it did with that mismatch. More checking today. Mikek

Mikek, welcome back from the show. Phil, kudos on your suggestion!

that 9% sounds like a 'conversion' error. near to the difference between AVERAGE and RMS.

take a waveform with a peak of 1.414 and the rms is 1, but the average is sqrt(2)*2/pi about 0.9; a difference of 9-10%

or, your meter could just be broken.

The show was a bit of a bust, it rained Fri. and Saturday, I only got about 6 hrs of total sell time. Sunday was nice, but the crowd was thin and most vendors packed up by 1pm. I didn't sell my two scopes and spectrum analyzer. I don't really want to ship those but I might try Ebay.

Phil, kudos on your suggestion!

Except it has a 0.033uf input capacitor. And I see on the schematic, depending on the scale, either two 1 Meg series resistors or a 1.98 Meg and some parallel capacitance across the input. So, the 2Meg question is answered.

I don't have a reference voltage so I'm comparing my signal generator meter reading with the 93A meter. Measurements were made with a 50 ohm termination at 6 MHz.

I measured on 6 different ranges and got 11% to 12% error vs the signal generator, using a 10 to 1 probe. With the 1 to 1 probe it consistently read 42% low over 8 ranges.

I would think with a 10 to 1 probe designed for a 1M input the 93A would read almost 2 x high with its 2M input.

Can anyone help me decide if my 93A reads 50% low accounting for the probe mismatch or is the probe ok and the meter just reads 12% low.

I have a higher quality Tek probe, I'll go try it and see if it reads different than my $13 Chinese probe.

Thanks, Mikek

------------- Just to add to my confusion checking with a scope and dividing the pp scope reading by 2.828, the scope reads about 4% low compared to the Signal generator.

check your scope spec, they're usually 3% instruements. great for 'relative' measurements, though.

if you use a 10x probe designed for 25pF or a 10x probe designed for 40pF you'll see a big difference.

What is the break point? something as low as 16kHz, or such. hwhen the probe goes from looking like a resistor to looking like a capacitor.

Unless your generator is something beautiful like a HP 3325, its amplitude calibration is suspect. (Signal generator folks seem to think that +-0.25 dB is a good accuracy spec.)

The 93A is specified at 2% relative or 1% FS. I'd set the generator to some frequency where the 93A and a nice shiny true-RMS DVM both work well (e.g. 100 Hz), or just use a transformer on the AC line, where you know that the DVM will do its thing properly. The 93A is fully specified down to 10 Hz.

Cheers

Phil Hobbs

I have adjusted the probe cap just to see if it would make up for the error, it does affect the reading, but not 11%. But maybe my probe capacitance adjustment doesn't go high enough for the input of the 93A.

Yes, I understand that, it was added just as another reference, which may or not be as good as the sig gen meter.

Thanks, Mikek

I did some more manual reading :-0. The >optional< probe is for high impedance measurements. So, the 93A was designed for a direct input connection. I did that, now I'm down to a 5.4% to 6% error over 8 ranges vs the (unknown calibration) of the signal generator.

I'll check my DMM for its frequency range and try that check at 60hz or higher if my DMM is up to it. With a good reference and a little calibration, I think I got my $5.00 worth. Very surprised it even worked. Now, the Ballantine 323 repair. :-) Thanks guys, Mikek

"RobertMacy"

See that screw head thingy ??

Ever read the instruction sheet ??

You have hours of fun just waiting.

.... Phil

"RobertMacy" Phil Allison

.... Phil

I still use an "OLD" HP with tubes and chopper motor in the back for an RF millivolt meter!

Have two of them, both work great, that was after I did some retooling in there! :)

Jamie