Products > Test Equipment
Siglent SDG6000X series 200-500 MHz AWG's
nctnico:
I guess the actual question is: how do you know the generator is calculating the waveforms correctly? From my experience with the SDG2000 series generator and judging from this thread, Siglent doesn't know the answer to this question for their own gear so it would be nice to be able to check for yourself what formulas are being used and which rounding errors there are.
maxwell3e10:
A bit off-topic, but I thought it would be interesting how this spectrum and distortion compares to old-school instruments: HP 3326A and HP 89410A. It's not so bad, the harmonic and intermodulation distortions are much better, that is what HP3326 is designed for. The close-in phase noise is also better, while wideband noise is somewhat worse, about -114dBc/Hz (partly limited by 89410). HP3326 can of course do phase modulation of the square wave since it is generated from a sine.
Detlev:
Hello everyone,
Today I did an FM modulation with a 100Hz sine wave. First I looked at what sidebands I would expect. To do this, I used the Bessel calculator
https://keisan.casio.com/exec/system/1180573474
assumed a modulation index of 0.01 and read the values for the 1-4 harmonics and converted them to dB.
So I have the following starting values:
Carrier: 10MHz / 0dBm
FM Frequency: sine 100Hz
frequency Dev.: 1Hz
Mod index: 0.01
Expected amplitudes:
1. 0dBm
2. -46.02dB
3. -98.06dB
4. -153dB
Measured amplitudes:
1. -0.33dBm
2. -46.08dB
3. -98.12dB
4. gets lost in the noise
I think the SDG6022X is doing very well here.
Attached are the unmodulated and modulated measurements and a photo of the settings of the SGD6022X
have a nice day
Detlev
Detlev:
Hi guys,
today I started with the modulation with a square wave. The parameters are identical to those of the sine modulation.
What would I expect?
If you now modulate with the square wave, then you modulate, if you use Fourier, with the fundamental wave and then with the harmonics of the square wave.
So I would await for the values of the fundamental from above and additionally the value of the 3rd harmonic of the square wave (I restrict to the 3rd harmonic since the next one would no longer be in the spectrum).
the 3rd harmonic is at 300Hz and has 1/3 of the amplitude of the fundamental of the square wave.
I enter this into the Bessel Calculator and get:
Order: 1
Initial value: 0
Increments: 0.0033333
Jv(x): 0.001666647685
with 1/3 the amplitude you get an amplitude of 1/3 Jv(x) and then you get 0.55555 x10^-3
Then convert to dB:
20log(0.55555x10^-3)= -65.11dB
So I would expect:
1. 0dBm
2. -46.08dB
3. -98.12dB
4. -65.11dB
Measured amplitudes:
1. 0.02dBm
2. -43.96dB
3. -92.67dB
4. -63.14dB
Here the measured values deviate from the expected ones. All of them are a bit too high.
If someone has an explanation for this, I would be very grateful. It's also possible that I've made a mistake in my reasoning, but I'm relatively sure that it fits.
It seems that the SDG does not calculate exactly during modulation and only interpolates with sufficient accuracy. This fits with sine but no longer exactly with square.
Attached is a photo of the SDG settings and a screenshot of the spectrum.
Greetings and good night
Detlev
gf:
The FFT spectrum of a numerically simulated FM gives me numbers close to your measured values.
(Used only a 10kHz carrier for convenience, but the result can be shifted to any other carrier frequency)
--- Code: ---fc = 10000 % carrier freq Hz (sine wave)
df = 1 % deviation Hz
fs = 100000 % sample rate Sa/s
fmod = 100
dp1 = 2*pi*(fc+df)/fs % phase increment per sample for fc+df
dp2 = 2*pi*(fc-df)/fs % phase increment per sample for fc-df
dp = [ones(1,fs/2/fmod)*dp1 ones(1,fs/2/fmod)*dp2];
dp = repmat(dp, 1, 10);
signal = sin(cumsum(dp));
% place frequency scale origin at fc
plot((0:length(signal)-1)*fs/length(signal)-fc,20*log10(abs(fft(signal)/length(signal)*2)))
grid on
ylim([-100 0])
xlim([-2000 2000])
--- End code ---
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version