Experiments with NVE IL715 Isolator
A fellow diyer from Germany kindly sent me as a gift a PC board and other components for the IL715 isolator. I purchased the chip from Digikey. It came with very sophisticated packaging:
I configured the IL715 the “standard” way: both sides fully isolated including power and ground. Below is a photo of the mounted device including two .1 uF capacitor bypass for the power lines (forgive the messy soldering -I couldn’t find my bottle of flux 🙂 ).
Other projects have implemented the use of isolators and the NVE part is the favorite part. But at that time I did not find compelling reasons to use isolators. In addition, according to the IL715 datasheet, the part adds 100 ps of jitter. So lowering jitter is not a reason to use these isolators. In fact the amount of jitter is in the same order of magnitude as the jitter present in modern FPGA-based USB interfaces so the added jitter is a significant amount.
Note on Jitter: likely the figures quoted are “peak-to-peak” values. Clock jitter values are typcally specified as RMS jitter. The approximate conversion is 15 ps peak to peak ~ 1 ps RMS. Thus in RMS values, the added jitter by the isolator is about 7 psec RMS. Typical FPGA-based USB interfaces will add in the order of 150 psec peak-to-peak or 10 psec RMS [link]
In theory, the reason to use a signal isolator is to block any potential noise coming from the I2S lines including ground. I suppose one can find benefit if the detrimental effect of the noise is greater than the effect of the added jitter.
The power for the input side (the USB interface) is taken off the 3.3V supply inside the Musiland 03 US. The power for the output side is taken off the 3.3V digital supply in the BII DAC. Conveniently, the through holes for the Tridents provide the 3.3V. This way the two devices are completely isolated from one another. Neither GND or Power are shared.
Connection between the BII DAC and IL715 and Musiland USB interface. Only used 3 of the 4 inputs/outputs (no need for master clock as the Sabre/Buffalo is configured for asynchronous operation).
I ran my unlock tests with and without the IL715 and here are the results:
First couple of hours after start-up; 44.1KHz, DPLL set at LOWEST
The start up behavior looks similar with or without the IL715. It takes about 1 hour for the system to stabilize. With the IL715 effectively blocking any potential noise passing through the USB interface, this implies that what we are seeing is the “warm-up” characteristics of the Sabre32/Buffalo DAC itself.
After the warm-up (bottom graph), the IL715 seems to slightly increase the number of unlocks. If we compare the this test with what we obtained before without the IL715, the configuration with the IL715 does not look as good. A longer test may show similar results as without the IL715, but this at least hints that there is no need to use the isolator.
This test also seems to indicate that the remaining unlocks are not due to noise coming through the I2S inputs (through the PC and USB interface) since they are isolated, but are due to some other factor.
Increase the sample rate to 88.2KHz; DPLL still at LOWEST
Since the behavior of the DAC with or without the IL715 is about the same, it is hard to see any substantial differences.
It is well known that increasing the sample rate also requires increasing the DPLL setting. In this experiment we will increase the sample rate to 88.2 KHz but leave the DPLL with the setting set at LOWEST. We can expect a large number of unlocks and hopefully see how they IL715 compares with the direct connection. The sample rate will be increased at the Musiland control panel after it has stabilized at 44.1KHz.
The graph below compares the performance of connection through the IL715 vs direct connection. As expected we get lots and lots of unlocks. The tests were done at the same time frame: 10:00 PM to 12:00 AM an then again in the morning in order to eliminate other contributing factors.
The IL715 connection seems to be better (less unlocks) but neither connection really works to listen to music. It is interesting to note that there are intervals where there are no unlocks. This continues to hint at the fact that inherent jitter of the device (which in the case of adding the IL715 results in a total of ~300 ps) is not the factor causing the unlocks. There are other factors that contribute to the instability of the DPLL or clock. These other factors may also contribute to increased jitter causing the unlocks. I think it is worthwhile to focus on providing cleaner to the USB interface (the Musiland is currently fed by USB power) and also in preventing noise from entering through the power line.
Between midnight and around 8 AM I switched the DPLL setting to LOW. Here are the results.
As expected there are no unlocks if the DPLL bandwidth is increased to the next setting. Both interfacing with the IL715 and direct gives the same results. Notice that in the beginning when changing the DPLL setting from LOWEST to LOW, there are a bunch of unlocks. Seems the DPLL can become destabilized with a change of sample requiring some time for it to stabilize. The implication here is that low settings for DPLL may not work well when playing music of different sample rates.
As discussed earlier, at least in theory, the reason to use a signal isolator is to block any potential noise coming from the I2S lines including ground. If one cannot find other ways to eliminate noise from the source, and it is demonstrated that the noise is real and it is detrimental to the DAC, then one might consider trading off the added jitter with the elimination of noise.
However as shown by these experiment, there is no clear advantage of using the IL715. With “normal” operation (DPLL=LOWEST with 44.1K and DPLL=LOW with 88.2K), there is no real difference between using the IL715 or a direct connection. We know for a fact that there is added jitter. The data also shows that there is NO “noise issue” from the source.
Since I am a fan for minimal components in the data path, I would side towards NOT using an isolator.
I still have Ian’s FIFO jitter eliminator to test…