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Hello,

I was testing to see if my scope and TinySA Ultra agree regarding the
signals they see.

For that I set my signal generator (Siglent SDG 2042X) to output a 5MHz
sine wave at 1Vpp, used an SMA cable to connect the scope and the sig
gen, and added a 50 Ohm termination to my scope.

When measuring Vpp on my scope it does indeed show approximately 1 Vpp
and a sine wave of 5MHz.

When adding a 40dB attenuator, it shows 10.6mVpp, so still about the
right value, if not perfect.

When I then remove that SMA cable from the scope and attach it instead
to the TinySA Ultra (same setup, including the 40dB attenuator), it just
shows a value of 3.39mV at the marker exactly at 5MHz (searching for the
peak value the TinySA Ultra found the right frequency, so it's really
the peak).

My sweep settings are: 2MHz to 8MHz with a RBW of 30kHz. Lowering the
RWB to 10kHz results in an even lower value of about 3.19mV, though the
signal bandwidth gets narrower as you'd expect.

What does the TinySA Ultra measure when selecting Level/Unit/Volt in the
UI? And how does it compare to a Vpp value from a signal generator/scope?

Is it possible for the TinySA Ultra to display a reading like the Vpp
one for the signal generator/scope?

Best regards,
Ma?l H?rz

When I then remove that SMA cable from the scope and attach it instead
to the TinySA Ultra (same setup, including the 40dB attenuator), it just
shows a value of 3.39mV at the marker exactly at 5MHz (searching for the
peak value the TinySA Ultra found the right frequency, so it's really
the peak).

Just to clarify, though it's probably obvious, the 50 Ohm termination
from the scope was not moved to the TinySA Ultra when moving the cable
(and the attenuator) over to the TinySA Ultra, since it has already a 50
Ohm input.

Ma?l
Your scope seems to measure Vpp and the tinySA measures Vrms
Would that explain the difference?
--

Hi
it's Volts RMS (not Vp-p)

Thanks, it's better: 9.13mVrms measured at the TinySA Ultra for a sig
gen output of 1Vrms after a 40dB attenuator. The scope shows a value
that is closer, though, at 10.3mVrms.

Is there anything else I can do to make it closer in value? Also is it
possible to add a Vpp mode and rename the Volt entry to Vrms, for clarity?

Keep in mind the level resolution of the tinySA is 0.5 dB which is 6% in Volt.
The only thing you can do is to use CONFIG/MORE/LEVEL CORRECTION/INPUT LEVEL to set the correct input level in dBm

Renaming the V unit to Vrms in the menu makes sense.
Is Vpp more common when talking Volt?
--

in my opinion; RMS is common if talking about alternating current, - because DC equivalent on resistive load.

When comparing always the same quantities;
RMS to RMS
PP to PP...

- there is always trouble from mixing (It also possible if we recalculate; proportionality will not be clearly visible...)

Istvan

I'd prefer Vrms

Arie Kleingeld



Op 21-12-2023 om 08:22 schreef Erik Kaashoek:

Also be aware that 10mV = -27dBm, so very close to the top end for the TinySA best measurement accuracy.

10mV pk-pk = 3.535mV rms compared to the TinySA measurement of 3.39mV rms, so not too bad!!

73
Jeff G8HUL

OOPS! Ignore that I fell into the pk-pk vs rms trap.
10mV pk-pk =-36dBm not -27 so well within the range for the TinySa.

73
Jeff G8HUL

Renaming the V unit to Vrms in the menu makes sense.
Is Vpp more common when talking Volt?

At least when it comes to scopes and signal generators, these options
are listed first in my devices, and Vrms later.
For example on the signal generator the default is Vpp, and the buttons
to select Vpp come first, then Vrms.

Multimeters seem to like Vrms though and advertize it specially.

If possible, having both options, Vrms and Vpp, would be great :)

The maximum voltage is specified as 10Vpp and if you switch to Vrms it
is 3.535Vrms (and you cant go higher). So really Vpp is the more
basic/natural measure, regarding the signal generator.

Is Vpp more common when talking Volt?

Not in my experience. If unspecified, AC Voltage can always be assumed to be RMS. Sure, add ‘rms’ to clarify if you like, but IMO there’s no need to add a separate Vpp option (or Vavg for that matter).

Simply... A scope measures voltage in the time domain. A spectrum analyzer measures signal power (dBm) in the frequency domain. They are different measurements and different units are used for convenience. Signal power is based on RMS volts. It is difficult to "eyball" RMS on a scope, "smart scopes" notwithstanding. P-P is easy to measure and not dependent on waveform purity but does not translate easily to power in a scope if not a pure sine wave. With a spectrum analyzer you always have a sine wave at the fundamental frequency of a complex wave.

My RF signal generators are calibrated in RMS Volts or dBm by default.

73

-Jim
NU0C

On Thu, 21 Dec 2023 12:17:19 +0100
Ma?l H?rz via groups.io <mh-nexus@...> wrote:

Renaming the V unit to Vrms in the menu makes sense.
Is Vpp more common when talking Volt?

At least when it comes to scopes and signal generators, these options
are listed first in my devices, and Vrms later.
For example on the signal generator the default is Vpp, and the buttons
to select Vpp come first, then Vrms.

Multimeters seem to like Vrms though and advertize it specially.

If possible, having both options, Vrms and Vpp, would be great :)

The maximum voltage is specified as 10Vpp and if you switch to Vrms it
is 3.535Vrms (and you cant go higher). So really Vpp is the more
basic/natural measure, regarding the signal generator.

--

73

-Jim
NU0C

A scope is measuring the whole signal (all frequencies) in the time domain where the SA measures the power in a single frequency
With the SA you do not measure all harmonics so you do not know where the power is except for the frequency you measure.
So the best unit indication may be "Vrms into 50 ohm at the measured frequency"
I do not know how to convert a single frequency SA measurement into a meaningful Vpp
--

Not in my experience. If unspecified, AC Voltage can always be
assumed to be RMS. Sure, add ‘rms’ to clarify if you like, but IMO
there’s no need to add a separate Vpp option (or Vavg for that
matter).

I think Vpp would be a useful addition as explained in my other mail.

I do not know how to convert a single frequency SA measurement into a
meaningful Vpp

You could simply apply the formula that exists for converting between
Vrms and Vpp. The following link has the formulas:



Vpeak = Vrms * 1.414
Vpp = 2*Vpeak

The measures are shown on markers only, so you know it is meant for a
specific frequency.

So if you have a Vpp that is a pure sine wave on a scope, it's comparable.

Vpeak = Vrms * 1.414
Vpp = 2*Vpeak

Or more precisely Vpp = Vrms * 2 * sqrt(2) for a pure sine wave, as
mentioned in

Would that conversion factor be correct for any wave form? E.g. is the power of the fundamental of any wave form always only related to the Vpp as observed on a scope, or does it also depends on the shape of the wave form?
--

Would that conversion factor be correct for any wave form? E.g. is the
power of the fundamental of any wave form always only related to the Vpp
as observed on a scope, or does it also depends on the shape of the wave
form?

I have no definite answer to the question regarding the fundamental, but
at least for the non-decomposed signal, it depends on the waveform.
For example, for triangular waves, the conversion formula would be
different from sine waves:
Vpp = 2*sqrt(3)*Vrms
vs
Vpp = 2*sqrt(2)*Vrms

However many scopes measure Vrms simply by probing at 0.707 of the peak
voltage at some horizontal point/trigger point. So that would not be the
correct value for signals that aren't completely periodic, either.

Aside from scopes in FFT mode, I haven't seen them offering measurements
in dBm.

Given the non-perfect mapping of a scope to a spectrum analyzer, I think
it makes sense to assume you only compare pure sine waves, and as such
Vpp would be helpful.

Depends on the waveform.