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Which to believe

 

I have an Elecraft XG3 signal source that I use for troubleshooting
radios. It outputs a square wave full of harmonics that can be set for
-107dBm,-73dBm, or -33dBm. I have attached some screenshots from my
Oscilliscope and the tinySA. When I was using the tinySA with the
windows app the app measurement did not agree with what I see on the
screen.

So basically none of my readings agree. The scope measurements kind of
bother me because they are pretty far off. This is not a lab quality
scope (Hantek DS02D15) but I did not think it would be this far off.
Just wanting some input on how to resolve these differences.

thanks.

dso_01_01_06_48_22.bmp
tinysa_LOGMAG_S11_2025-02-01_17-17-42.png

 

On Sun, Feb 2, 2025 at 08:09 AM, Rick Stanback wrote:
-33dBm
First question: did you use a 50 ohm termination on the scope? It appears you didn't. So the generator is putting out 10 mV into the 10 meg or so scope.
If you had a 50 ohm termination that would halve to 5 mV (the generator has an output impedance of 50 Ohms so the voltage would halve).
5 mV in 50 Ohms is .5 microwatt is -63 dBW is -33 dBm. That is spot-on.
?
You are measuring a square wave on the scope. A square wave can be seen as the sum of infinite odd harmonics of a sinewave. The SA is only measuring the fundamental? sinewave.
Than the power in that sine is .707*5 mV == -66 dBW or -36 dBm. Or in a quick manner: -33 - 3 == -36 . -37 is well within the specs of the SA in your settings.
?
Your measurements are correct.
?
regards
?
Henk
?
?
 

Just fiy:
?
Nice Youtube video on terminators:
?
For low frequencies you could also use a BNC-T terminated with 50 Ohm on one side.
?
?
 

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Thanks for the quick reply. ?You were spot on. ?I made the bad assumption that the scope input was 50ohms. ?I used a tee with a 50 ohm terminator and the readings are correct now.

On Sun, 2025-02-02 at 01:15 -0800, pa3cqn via groups.io wrote:

Just fiy:
?
Nice Youtube video on terminators:
?
For low frequencies you could also use a BNC-T terminated with 50 Ohm on one side.
?
?
 

When measuring a signal you need to ask yourself what you are measuring. Voltage or power? And when measuring voltage you can distinguish between RMS and amplitude.
A square wave is good starting point as it is easy to make an accurate level signal. The relationship between the amplitude of a square versus the amplitude of the fundamental is 4/pi (2.1dB).
To convert a sine wave amplitude to RMS -3dB is taken into account.
To convert the RMS voltage to power +13dB is used.
?
In your example you have 5mV amplitude (into 50 Ohm):
db20(5mV)+2.1-3=-46.9dBV. The tinySA can report in mV (dBmV), so +13.1dBmV
If you measure power in dBm you should see -33.9dBm. You seem to measure -37.8dBm. That's outside the +/-2dB of the tinySA.

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