Wave Gen.I’ll start this review with some general thoughts.
As with most additional features that go beyond the core functionality of an instrument, we cannot expect them to replace dedicated gear, rather have to accept a bunch of limitations.
A DSO is just a DSO and just like the integrated trigger frequency measurement and the automatic measurements cannot replace (proper) frequency counter and multimeter, this is also true for the built-in waveform generator – but it could still be very useful when at a pinch.
The built-in Wave Gen of the SDS2000 is a single channel arbitrary waveform generator without sweep and modulation capabilities.Many dual channel arbitrary waveform generators have been introduced during the last decade and these can be incredibly useful. Before that time, however, we had to make do with just one channel and this is still enough for the majority of applications today.
Even the old analog function generators provide internal/external sweep capabilities, i.e. frequency modulation. Nowadays we expect a complete set of modulation capabilities together with an internal universal modulation source, which means there must be essentially another DDS-generator per channel. The SDS2000 obviously doesn’t have a spare DDS generator to be used as modulation source and it doesn’t have the connectors usually associated with a fully-fledged waveform generator, like trigger/gate in, modulation in/out, sync out, which is no surprise, considering it is just a DSO after all.
However, a sweep function could have been easily implemented just in software.
According to the data sheet, it is a 125 MSa/s, 14 bits system with 1µHz of frequency resolution.These specs sound good and would have been a big deal even just a decade ago. But the specs aren’t everything; what counts at the end of the day, is the actual signal quality.
The frequency range covers 25MHz for sine, 10MHz for square, 300kHz for ramps and 5MHz for arbitrary waveforms. This is not too bad either. In the good old days, analog function generators were usually limited to 2MHz, and most early DDS function generators could not exceed 10MHz.
The output impedance is proper 50 ohms, but the maximum signal level is limited to 3Vpp into 50 ohms (6Vpp open circuit).That’s a typical limitation of many built-in generators, as they lack a proper output amplifier that could provide 20Vpp open circuit and 10Vpp into 50 ohms. It should still be enough for a bunch of everyday tasks though.
Actual frequency resolution is just 2 or 4 digitsWhen reviewing the specs, the claimed frequency resolution of 1µHz is rather misleading. While the lowest frequency is indeed 1µHz, the default frequency resolution is exceptionally low at just 2 digits. This can be changed to ‘fine’ adjustment, where the resolution is 4 digits. That means that above 10MHz, the smallest frequency step is 1kHz, which might be too coarse for some measurements, such as determining the bandwidth of a 10.7MHz IF filter for instance. On the other hand, given the poor response characteristic of the select knob, this approach is still welcome as it would otherwise take hours to go through the entire frequency range.
The resolution for amplitude setting is 2 or 4 digits as well.WaveformsThe Wave Gen provides all the standard waveforms including DC and noise, plus a few built-in arbitrary waveforms and 4 memories for user defined waveforms (AWG_Gauss)
I don’t know how well it works to create a user defined waveform and then transfer it into the scope memory. The manual suggests to use EasyWave, which didn’t come with the scope (there are just a couple PDFs on the CD). We should be able to download it from Siglent’s website, but since I wasn’t particularly interested in arbitrary waveforms right now I couldn’t be bothered.
Btw, the datasheet claims a maximum of 16k for the arbitrary waveform length.
Now let’s take a closer look at some waveforms:
SineAmplitude accuracy appears quite good. At 1kHz, a 2.8Vpp signal measures 1.00864Vrms, which is not too far from the expected value of 0.99Vrms, indicating an error of +1.8%. This really is not bad at all, given the fact that the actual resistance of my pass-through terminator is a bit high at 50.259 ohms.
The same experiment without pass-through terminator results in an output of 1.00596Vrms and the error is +1.6%.
No matter how many bits and what the sample rate is, what really counts is the signal quality. I don’t have a proper spectrum analyser right now, but I can make accurate THD measurements up to 20kHz and will use a properly implemented FFT on another scope to look at the frequency range above.
At 2.8Vpp into 50 ohms, the measured distortion levels from 20Hz to 20kHz are generally below 0.01%, thus would be well suited for quality audio measurements (AWG_THD)
As already mentioned, for higher frequencies I have to use the FFT on another scope, which doesn’t provide the dynamic range to measure distortion figures below 0.1%. In order to check the limits of my test setup, I measured 10kHz once again (AWG_Spectrum_10kHz)
THD is measured as 0.066% now, whereas the true figure is an order of magnitude lower. This test setup will still be useful, as the distortion figure inevitably rises at higher frequencies, hence will eventually enter the dynamic range available.
The sinewave remains nice and clean up to 5MHz, where distortion is still below 0.1%, but non-harmonic spurs touch the -60dBc mark (AWG_Spectrum_5MHz)
Distortion and spurs keep rising with increasing frequency and at 10MHz, THD starts exceeding 0.1% (AWG_Spectrum_10MHz)
At 25MHz, the signal shows all kinds of harmonic and non-harmonic content and THD is measured as 0.355% (AWG_Spectrum_25MHz)
All in all these results aren’t bad at all – at least the real thing appears a lot better in terms of spectral purity than what the rather unexciting specs in the datasheet would suggest.
SquareI’ve checked the square wave across the entire frequency range and could not find any flaws. Overshoot is next to non-existent and there particularly is no ringing, provided the scope input is properly terminated at 50 ohms. Rise- and fall times are both 24ns, which is still fast enough to produce a nice looking squarewave at 1MHz (AWG_Square_1MHz_2800mVpp)
At the maximum frequency of 10MHz the output doesn’t looks very square anymore, but that’s to be expected (AWG_Square_10MHz_2800mVpp)
So no complaints here either. The rise- and fall times are exactly as specified, and overshoot is actually much better.
PulseMinimum pulse width is 48ns, rise- and fall times are the same as with square. Again, the pulse looks nice and clean (AWG_Pulse_48ns_2800mVpp)
RampRamps are limited to a maximum frequency of 300kHz but the linearity looks still pretty perfect at that frequency in exchange. I chose a symmetry of 0% and as a big surprise, the leading edge has a risetime of only 12ns. So ramp can do twice as good as square and pulse (AWG_Ramp_300kHz_2800mVpp)
NoiseNoise at the maximum standard deviation of 225mV provides a reasonable flat spectrum up to 10MHz and submerges in the noise floor of my test setup (some -98dBV) at about 115MHz (AWG_Noise_225mV_2800mVpp)
ConclusionThe Wave Gen option on the SDS2000 is actually quite a useful signal source. Without any bells and whistles, particularly no sweep capability and limited output level, it still does the job for many applications and performs significantly better than specified in terms of signal purity.