Hi @Var, crosstalk is trickier than it seems because there are at least two mechanisms at play. The first is that the output somehow couples into the input. And this mechanism is sometimes a physical coupling between traces. For example, the left trace runs adjacent the right trace, and some of the left signal is coupled into the some of the right signal. That is what most of us probably think of for crosstalk. But a second mechanism that is more pervasive is coupling via the supply. For example, in a power amp, if you drive the left channel hard, at some point, you’ll see a bit of the left channel show up on the power supply rails as the current draw causes the supply to sag just a bit. When this is happening, you can probe a rail and see the 1 kHz or whatever on the rail. And then, it couples into the other channel. In an opamp, you have the PSRR help quash this. But in older amps that are open loop, or without much feedback, there’s not much PSRR to help.
In the QA40x hardware, you can see this as you crank the amplitude. For example, here’s the QA403 in single-ended loopback at 0 dBV, and the right channel input is shorted. Here we could say the cross talk is about -160 dB.
Now, let’s “unshort” the right channel inputs. This results in the default 100K to ground, and these now high-z inputs will pick up a lot of stuff, including 60 Hz and the 1 kHz. Now, we might say the cross talk is about -105 dB. Clearly, the output is being picked up by the high-z input. And this is all relative insensitive to the full scale input settings.
OK, so let’s go back and short the right channel inputs. Now, I’ll set the full scale input to +24 dBV and I’ll set the output to +18 dBV. we know the left channel is +18, and here we see the right channel is -98. So, our 1 kHz at this point is about 98+18=-116 dB below the left channel.
This is a substantial degradation from the shorted input -160 we saw before. And yet, the inputs are still shorted! So, we can surmise this is probably coming from the internal power supply. That is, the left and right channels share the regulated LDO output rails, and the regulation of the LDO under the high currents of the left channel driving into the lower-value internal resistors that are throughout the DAC side signal path are the culprit.
And we can see that too in the current: At 18 dBV out in single-ended loopback the USB draw is about 950 mA. And at 0 dBV out in single-ended loopback (where we got the killer -160 dB figure) the USB current is about 840 mA.
Additionally, you can see the 2H starts to come up too, and that is likely from LDO trying it’s best to keep the voltage constant, and doing better on some frequencies than others.
So, to back up a bit, what seems like a simple measurement can actually get fairly complicated. Now, the good news is that testing power amps that have around 25 dB of gain, you are probably around the 0 to 6 dBV level out of the QA40x, and at that level the QA40x cross-talk is still phenomenal.
Probably what would be most useful here is a new plugin, where you’d specify a chirp over a range of levels, and then the crosstalk would be graphed relative to the output signal. I think for the best accuracy, you’d want to prompt the user to short the left DUT input, drive the right, and the prompt the user to short the right DUT input, and drive the left.
And then maybe show the group of the left channels as a single color (since they’d ideally be around the same level) and ditto for the right (except do that in another color).
This would let you quickly see crosstalk versus freq and level, and you could tailer the output range based on the amp you are evaluating.
What do you think?
PS. Are you generally shorting the unused input when making a crosstalk measurement?
