@hpw:
I tried to replicate your tests but since I lack the mentioned, high-resolution audio gear that you utilized, I had to change the carrier frequency so I can comfortably measure it with my spectrum analyzer. The sweet spot regarding S/N ratio appeared to be around 1MHz, so that's what I uesd. These are my findings with a sine carrier of 1MHz, 0dBm, 1kHz sine modulation signal:
Below 0.01° phase shift, the sidebands @ 1kHz offset stay at ~ -84dBm, right down to 0° where they disappear in the noise floor all of a sudden.
Increasing the phase shift by 0.001° increments, not every increment has an effect, and only at 0.1° phase shift, the first order side bands reach the approx. correct level of -61.5dBm.
Moreover, as a reaction to
@blue's reports on his SDG2000X findings, today I took a closer look at the PSU of my SDG6000X due to the hot running MLCCs parallel to the switching MOSFETs. Other than I initially thought, the MLCC at the low-side switch wasn't cracked but there was just a speck of dust on it. I removed the PSU module altogether and took a closer look at both sides:
Obviously, the build quality is good and there's nothing to complain about the choice of components. All of the elecrolytics are Rubycon brand.
The controller is an ST L6599 resonant mode half-bridge specimen with integral gate drivers. A PFC circuit isn't utilized in the PSU. In order to understand better what's going on here, I decided to run the PSU separate from the AWG with a variable power supply and a DC load at its 6.5V output (that's the one used for regulation of the converter and designed for the highest output current). Here are some thermographs, taken at 115V:
It's quite peculiar to find that the MLCCs at the switches stay rather cool while a resistor in the current sensing circuitry get noticeably warm. Moreover, the three dropper resistors for the initial startup supply run at an acceptable temperature of some 70°C.
At 230V supply voltage, things change considerably:
The elevated temperature of both MLCCs at the switches as well as the unhealthy temperature of the dropper resistors lead me to the impertinent assumption that the PSU has been designed to be operated at a 115V mains and had only more or less "by coincidence" been found to be running off 230V as well. Anybody heard of the manufacturer "Unicorn" as revealed by the sticker at the solder side?
At a load current of 3A, the first smoothing cap at the scondary gets pretty warm as well. I may have already been overloading this output with the chosen settings, though.
Since the individual component temperatures appear to be as they are "by design", I decided to leave everything as it is and only take care of it if something should fail.
But since the AWG was apart anyway, I tore it down a little further to take some more photos of the things that make it tick...
Here the CPU module is shown. It contains a Xilinx Zynq 7010 ARM Cortex A9 / FPGA combo SoC, a total of 256MB DDR3 RAM and 256MB of NAND Flash memory. There's also other peripheral stuff present like power supplies, a LAN PHY, several clock generators and two USB transceivers. Altogether, this appears to be a pretty capable system core.
...tbc in next post...