Hi all,
I have some inconvinience witht he M8190A AWG. I am trying to generate a multi-tone output. Therefore I use the Soft Front Panel for the M8190A. When setting the Tone Distribution to "Arbitrary", you could define the tones yourself, including amplitude and phase. I am wondering how the amplitude given in this field relates to the output amplitude. The User Guide states this is the "its relative amplitude (in dB)" (page 61, http://literature.cdn.keysight.com/litweb/pdf/M8190-91030.pdf). However it does not state relative to what. This value seems to be related to the Output amplitude given via the tab "Output" in the Soft Front Panel, but I am not certain how exactly. Could someone help me with this?
Another strange thing I noticed is, when I (in my case) provide only one tone at 20 MHz 0 dB (in this Arbitrary Tone Distrubtion Multi Tone form), the output voltage of the tone at the "Direct output" will be 100 mV (half of the given 200 mV as Amplitude at the output tab -> but that's not that weird because the 200 mV is probably the differential amplitude). However if I change the amplitude of this tone to, for example, -20 dB, the output amplitude stays 100 mV.
When I add another tone (lets say 40 MHz at -10 dB), the 40 MHz tone is 10 dB lower in power than the 20 MHz tone. However if I now also change the amplitude of the 20 MHz tone to -10 dB, both tones will get the same amplitude (luckily) but also get an 100 mV amplitude again.
Could it be that the tone with the highest amplitude always have an amplitude equal to that of the amplitude given at the "Output" tab (even if it is set to +/- x dB) and that all other tones are relative to that tone? So if the second largest tone has an given amplitude of y, the output power/voltage of this tone will be x-y dB lower than the largest tone (where tone x will have an amplitude of, in my case, 100 mV).
Just for convinience:
- AWG: Keysight M8190A
- Scope: Keysight InfiniiVision MSOX3054T
- Driver/Soft Front Panel version: 5.4.5.1
- Windows 7 Enterprise
Thanks in advance!
Hi,
I don't have access to an M8190A Arb at the moment but I think I can answer your questions.
First off the multitoned power in dB is relative to other tones in the same waveform.
If you create a two tones waveform, you can set the relative power between the two tones very accurately.
If you only create a single tone my guess is the software will optimize the waveform to use the full range of the DAC.
As you have already seen with a single tone the Arb output power doesn't seem to change no matter what relative power you specify. The single tone waveform is always optimized to use DAC full scale.
Finally relating the tone relative power to the power specified on the Amplitude tab is an equation that changes with the multitoned signal. Lets determine the actual output power for a single tone. Generate a single tone and measure the actual RMS output power. This can be done with a power meter or with a signal analyzer. We'll call this Max Power.
Now you can relate the output power to the Amplitude setting in dB using the following equation.
Output Power (dBm) = Max Power (dBm) + Amplitude Change (dB).
I don't have the M8190A in front of me so I don't know what units the Amplitude is specified in.
If the Amplitude is specified as a voltage you can calculate the change in dB using
20 * log10 ( New Amplitude / Max Power Amplitude)
This is all great for a single tone but what happens when you have multiple tones? The Max Power value is reduced by the waveform crest factor. Crest factor is the peak power to average power ratio of the waveform. Say we generate a waveform with 6 tones of equal amplitude. The crest factor can vary from 6 dB to more than 20 dB. The actual value is determined by the number of tones and the phase relation ship between all the tones. You will get the highest crest factor if all the tones are generated with the same relative phase. Random phase results in about 13.2 dB as the number of tones becomes large.
A common way to show the peak to average ratio is using a CCDF Curve. See the N7621B Signal Studio for Multitone Distortion to plot some CCDF curves. Change the signal phase distribution from random to constant to parabolic.
Getting back to our 6 tone example the power is
Output Power (dBm) = Max Power (dBm) - Crest Factor(dB) + Amplitude Change (dB)
Finally, the power per tone is determined by converting the Output Power to a linear value and dividing by 6. Convert the linear power per tone back to dB.
Hope this helps
Pete
Here is a figure showing a single tone (blue) verses two signals (red, green) of 6 tones each at different frequencies. The crest factor of the 6 tone signals causes the average power to be much less than the single tone signal.