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Siglent SDG6000X series 200-500 MHz AWG's

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Performa01:
Analog (especially RF) circuit designers like to characterize their product’s distortion performance by means of the classic dual tone intermodulation test. Since this test requires two signal sources operating at very similar frequencies and identical amplitude, two expensive signal generators would usually be required. Up to 500 MHz, a dual channel SDG6052X can serve that task and provide perfect results – if used correctly.

The main requirement for this test is a clean dual tone signal and the challenge is to combine two signals in a way that the output stages of the generators do not produce intermodulation distortion products by themselves.

If we want to utilize the full available frequency range, we can consider three different setups suitable for the full frequency range of the SDG6052X. Consequently, the following test compares three configurations:

1.   Resistive power combiner with two attenuators in the source paths from the generators.
2.   Resistive power combiner with a single attenuator at its output.
3.   Internal “Wave Combine” function with external attenuator.

The attenuators in this test are 20dB. The big advantage of this arrangement is that we need not touch the generator settings (except for frequency) and get the exact same output levels. As a consequence, the spectrum analyzer always sees the same signal level. All in all, the three tests are perfectly comparable over the entire frequency range.

I’ve tested both 4 MHz (low frequency) and 400 MHz (high frequency), as we can expect the generator performance to degrade at higher frequencies. The 2nd tone is just 10 kHz above the first one. An automatic Intermodulation measurement has been used to get the results quickly and accurately.

Let’s start with the textbook approach: resistive power combiner with a 20 dB attenuator in each source path. As expected, the resulting signal is near perfect and we can measure the intermodulation performance of the spectrum analyzer, which is better than -87 dBc when both input signals are at -30 dBm level at the analyzer input.

No point in showing the 4 MHz result, the first screenshot is for 400 MHz:

SDG6052X_Ext_2x20dB_400MHz_-4dBm

Now we remove the attenuator in the source links and place a single one at the output. This way, the levels remain unchanged, but the isolation between the two generator outputs is now only 6 dB, whereas it was 46 dB before.

Not a problem at 4 MHz:

SDG6052X_Ext_1x20dB_4MHz_-4dBm

But look at 400 MHz!

SDG6052X_Ext_1x20dB_400MHz_-4dBm

The Intermodulation products are very significant at only -58 dBc now.

Finally, we can try the internal wave combine function, 4 MHz at first:

SDG6052X_Int_1x20dB_4MHz_-4dBm

Well, it works, but distortion is already significantly higher than with the external splitter. -82 dBc might be good enough for many tasks, yet it is about 6 dB worse than the worst result with external power combiner, which in turn quite likely marked the limits of the SA.

The true disaster becomes apparent if we try to use the “Wave Combine” at high frequencies – no wonder, now we have both frequencies in a single amplifier without any isolation at all. That means, we are measuring the linearity of the output buffer in the SDG6052X now, and we cannot expect it to be highly linear at 400 MHz anymore.…

SDG6052X_Int_1x20dB_400MHz_-6dBm

With only -38 dBc, this is unusable even for the most undemanding tasks. I had to reduce the output level to -6 dBm, because the SDG6052X cannot maintain the -4 dBm per channel in wave combine mode at frequencies that high. The result is valid and fully comparable nevertheless.

Verdict: As tempting the internal “Wave Combine” feature might look, its use has to be restricted to low frequencies.

Martin72:

--- Quote ---1.   Resistive power combiner with two attenuators in the source paths from the generators.
2.   Resistive power combiner with a single attenuator at its output.
--- End quote ---

Even though I might not have this effect because my generator only goes up to 120Mhz anyway (I don't have a second SML01):
What do these attenuators/combiners look like that you used ?

Performa01:

--- Quote from: Martin72 on October 04, 2023, 08:22:17 pm ---
--- Quote ---1.   Resistive power combiner with two attenuators in the source paths from the generators.
2.   Resistive power combiner with a single attenuator at its output.
--- End quote ---

Even though I might not have this effect because my generator only goes up to 120Mhz anyway (I don't have a second SML01):
What do these attenuators/combiners look like that you used ?

--- End quote ---
As I've demonstrated, even at only 4 MHz the internal (numerical) combiner is inferior in the SDS6052X.
Chances are, that the output stage of an SDG2000X might be less powerful, hence also less linear at higher frequencies.
In any case you shouldn't take a chance and make sure you actually measure the distortion of the DUT and not your generator.

I use a fancy Suhner 4901.19A power splitter/combiner, usable up to 12.4 GHz:

https://www.distrelec.nl/en/resistive-power-divider-huber-suhner-4901-19/p/14645419

But for frequencies up to a couple hundred MHz you should be able to find other (cheaper) ones or you could build one yourself. Or stick with Mini-Circuits and get their still not cheap BNC model with 2 GHz bandwidth:

https://www.minicircuits.com/WebStore/dashboard.html?model=ZFRSC-2050%2B

The inline attenuators are Mini-Circuits VAT-20A+ (20 dB) and VAT-10A+ (10 dB). I don't think there would be good alternatves to them:

https://www.minicircuits.com/WebStore/dashboard.html?model=VAT-20A%2B

For the BNC solution, there aren't many high quality inline attenuators available, but once again Mini-Circuists can help you out. They have about 14 different 2 GHz variants from 3 dB to 20 dB available, here's the 20 dB one:

https://www.minicircuits.com/WebStore/dashboard.html?model=HAT-20A%2B

Finally, I use appropriate cables - something like this:

https://www.mouser.at/ProductDetail/Amphenol-RF/095-902-451-004?qs=3lX5AGUhm28OKKH1LFEK5A%3D%3D

And then I need some SMA (f) to BNC (m) adapters of course.

If you stick to BNC, then you should use high quality low loss cables with good (double) shielding, so to avoid crosstalk and the scope or SA picking up enironmental noise, which could be rather bad if we try to measure signals down at -80 dBc or even lower. Cheap RG58 is a no-go here.

I am using Hyperflex 5 cable in such cases, specified up to 6 GHz and >105 dB screening at 2 GHz.

A splitter/combiner is also very useful for the Bode Plot in Siglent DSOs - and to get good acuracy at low levels, good cables with high shielding are important there as well.
 





Performa01:

--- Quote from: Martin72 on October 04, 2023, 08:22:17 pm ---... my generator only goes up to 120Mhz anyway ...

--- End quote ---
Well, here is a test at just 100 MHz, and the result is still highly inadequate even with the 500 MHz SDG6052X. The SDG2000X will probably be the same, if not, it could only be worse.

Se attached screenshot: only -60 dBc!

gammaburst:
Hi Performa01,

I tried your most recent test on my SDG6022X (liberated), 100.00 + 100.01 MHz using CH1+CH2 Wave Combine, both set to -30 dBm. I'm seeing about -74 dBc IMD on my old Advantest R3265M (calibrated long ago).

On my SDG2042X (liberated) I'm seeing about -54 dBc IMD. Interestingly, if I increase its two amplitudes to -20 dBm, IMD improves to about -63 dBc. I think that's because the instrument seems to use 8 dB attenuator/relay steps, the DAC provides the smaller amplitude steps, and the amplifier distortion grows quickly with amplitude. At -30 dBm the DAC happens to be near max amplitude (higher amplifier distortion), and at -20 dBm the DAC happens to be near min amplitude (lower amplifier distortion). Just my guesses. I haven't noticed that behavior with the SDG6022X.

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