Harmonics

Jim,

I just trying to learn so please do not blame me.

In conclusion what you are seeing is normal. How far down from the fundamental the orders of harmonics are is a function of the quality of the sine wave source and how the wave is created. The difference between the level of the fundamental frequency and the harmonics are generally measured in db, a ratio. There is no good or magic number. The acceptable limit is a function of what the signal will be used for.

You have a 50Ω system designed to work into a 50 ohm load. Your fundamental looks like this:

Vpk = 4.05 Volts
Vrms = 2.864 Volts
dbm = 22.15
dbv = 9.139

Your 2nd order harmonic looks like this:

Vpk = .053 Volt
Vrms = .03748 Volt
dbm = -15.51
dbv = -28.52

The dbv difference between -28.52 and 9.139 = 37.7 dbv

This is the same as saying dbv = 20 Log10 of Vlarge/Vsmall where 4.05 / .053 = 76.415 so you get the log of 76.415 = 1.8831 * 20 = 37.663 dbv.

Now when you got your measurements were you terminating the source into 50Ω as it is designed to work into?

Anyway, the numbers you have are not bad. Just a matter of what you want or need.

Ron

Ron,
What leads you to think that the harmonic component voltages are express as peak voltage, rather than RMS voltage?

JimB

JimB said:

Ron,
What leads you to think that the harmonic component voltages are express as peak voltage, rather than RMS voltage?

JimB

Click to expand...

Hi Ya Jim

Good question and actually, I don't have a clue?

OK, seriously it was a somewhat educated guess. Most devices like this that I have seen display a peak value of the signal. Additionally I did notice that in the controls this device does have a "Peak Find" function. That is what primarily led me to assume (I hate that word) the display was a peak value and the subsequent voltages listed were peak values. This is not to say my guess is actually factual.

Ron

JimB said:

Ron,
What leads you to think that the harmonic component voltages are express as peak voltage, rather than RMS voltage?

JimB

Click to expand...

What is the difference of expressing the Harmonic components by the "peak" value or by "RMS" value?

Ron
I think you will find that the "Peak Find" function relates to the peak of the response of the spectrum component.
The "peak" of the signal is the maximum response on the screen, this does not mean that the peak voltage (ie 1.414 x RMS) is being measured, any more than an ordinary meter will be measuring the peak voltage just because someone has adjusted something for the maximum (ie peak) reading on the meter.

drp
Oscilloscopes measure peak (to peak) voltages very easily.
Spectrum Analysers on the other hand tend to be more concerned with the power of a signal and use the RMS values.

JimB

Spectrum analyzers do measure the RMS voltage, but whether it measures peak or RMS, the ratio of the harmonics to the fundamental would remain the same.

Hi Jim & Carl

When I mentioned peak what I had in mind can be seen in the attached image. What I believe here is that the top of the waveform is a peak value. As to the original posted image I assumed the waveform present was a peak. I also assumed that the numeric values shown on the display for voltages were peak values. My years of swept spectrum analysis were long ago and I have never used FFT methods. Today my only use for spectrum analysis consist of doing some audio testing (acoustic noise testing) on an older HP system. Even that is outside my department.

Carl makes the same point we have been over as to be it dbv or dbm the numbers come out the same in that the 2nd order harmonic is about 37.7 db down from the fundamental. That shows up in my earlier post.

Considering the function generator in question and in comparison to function generators costing much more that number seems realistic to me? Even though the specification sheet for the function generator in question does not spell the numbers out like the unit I linked to earlier. To me it seems a reasonable number.

It would likely help if the OP could provide exactly what he is using for the measurement and any supporting data like the manual? As I said, I pretty much took a somewhat educated guess as to the numbers on the screen and assume they are peak values and not RMS. I just wish I kknew what the instrument was.

Almost forgot, the analyzer I do work with on occasion uses a "peak find" function and when used the cursor is placed at the peak on the screen. Actually it is as I recall a tiny cross hair. Monday I'll have to see what those old units are and get more info. Additionally there is some software I use at home with a data logger. When I record and open the file it also has a peak find function. When used it places the cursor at the highest peak of a waveform on the screen.

Ron

The top of each peak is the peak of the RMS value.

crutschow said:

The top of each peak is the peak of the RMS value.

Click to expand...

I thought peak value but if you say the RMS value, then good with me. Thanks Carl. I was seeing the top as a peak value.

Thanks
Ron

JimB said:

Harmonic content:

.
Running the oscillator or function generator with the output unloaded probably would not cause any problems like increasing the harmonic content of the signal.
However, overloading the input to the spectrum analyser will certainly make the harmonic content look far worse.

Happily there is a very easy way to tell if the analyser input is being overloaded, reduce the input to the analyser by 10db, the fundamental and all the harmonics should reduce by 10db.
If the analyser is overloaded, the fundamental will reduce by 10db and the harmonics will reduce by a lot more than 10db.

JimB

Click to expand...

Good point, the OP may be exceeding the P1dB (**broken link removed**) of the analyzer input. Having the analyzer spec sheet would help.

JimB said:

Try some simple calculations:

If there is 1 volt across a 1ohm resistor what is the power dissipated in the resistor (answer 1watt).

If there is 10volts across a 1ohm resistorwhat is the power dissipated in the resistor (answer 100watts).

Now calculate the dbs for 1 volt and 10 volts
and
calculate the dbs for 1 watt and 100 watts.
(answer 20db in BOTH cases)


JimB

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I forgot to mention that your above calculation seems to be valid JUST when the load is equal to "1".
So how much of usefulness is it??!

By the way Why we use the RMS values when we are treating the POWER? Why we limit ourselves to RMS values when POWER comes up?
Is that due to this fact that it is the RMS of the power that does "work"? If so that fact is true when treating the Voltage too!

dr.power said:

I forgot to mention that your above calculation seems to be valid JUST when the load is equal to "1".
So how much of usefulness is it??!

Click to expand...

Pick any value of resistance and voltages you like.
I just used 1 volt, 10 volts and 1ohm to keep the numbers simple.

JimB

dr.power said:

By the way Why we use the RMS values when we are treating the POWER? Why we limit ourselves to RMS values when POWER comes up?
Is that due to this fact that it is the RMS of the power that does "work"? If so that fact is true when treating the Voltage too!

Click to expand...

It is the RMS VOLTAGE and CURRENT which is used in the calculation of power.
It is not RMS power, it is just power.

In a DC circuit, power = voltage x current or power = (voltage)²/resistance.

In an AC circuit we can use the same calculations IF and ONLY IF the voltage and current are expressed as their RMS values, not average value, not peak value, but RMS value.

JimB

JimB said:

Pick any value of resistance and voltages you like.
I just used 1 volt, 10 volts and 1ohm to keep the numbers simple.

JimB

Click to expand...

Yea you are right. I calculate it for different values And it is right.

Thanks a bunch dude.

Hewlett Packard first product HP200A, 1939. https://www.electro-tech-online.com/custompdfs/2011/08/002patent_tcm_245_921599.pdf

A few MBA's later, Hewlett Packard's last product, PC, 2012

Some amplifiers and speakers are rated in real Watts. Cheap ones are rated in phoney Whats.

Cheap amplifiers and speakers are rated in "maximum" or "peak" power which is simply the real RMS power number doubled.
VERY cheap amplifiers and speakers are rated in "Maximum Music Power" or "Peak Music Power" which is when they are fed with a short duration pulse so that they don't blow up. Their power number is imaginary.