I was recently making performance measurements of the DÆ Output Board v1.0 and recorded the following distortion test.
Distortion Measurement of DÆ Output Board v1.0 at the RCA jack. What is all the “grass” above about 2kHz.
The distortion is low at -126.7 dB as expected but what is all the “grass” above about 2 kHz. The grass consists of numerous low level spectral peaks below -130 dBr that look awful! The grass is not coming from the DÆ Output Board v1.0 but instead it is interference picked-up by the measurements system. How do you get rid of it? How do you cut the grass?
Here is an illustration of the measurement chain. A QuantAsylum QA480 Oscillator provides a 1 kHz 1 Volt RMS test signal. Cable 1 connects the oscillator output to the input of the Device Under Test (DUT) which is the DÆ Output Board v1.0. Cable 2 connects the output of the DUT to the input of the DÆ LM4562 Differential Input Instrument. Differential is a misnomer in this setup as the LM4562 Instrument also has a single-ended RCA input which is used here. The LM4562 Instrument is connected back to the notch filter input of the QA480. Finally, the notch filter output of the QA480 is connected to the input of the QuantAsylum QA401 Analyzer.
Measurement Setup
The grass or interference in the measurement is caused by interference picked-up in the loop shown as Area A in the figure below. A ground loop is being formed by the shields of the cables and the metal enclosures in the loop surrounding Area A. The shields of the input/output cables are connected to the metal chassis of each piece of equipment in the chain. I believe this is normally the correct approach for a single-ended interconnects so that the shield is unbroken over the whole measurement chain.
Diagram of measurement chain.
Problem is that this measurement chain begins and ends with the same instrument, the QA480, and the shielding forms a ground loop which acts as an antenna and collects interference.
You would not normally see this interference unless you are making measurements down to the microvolt level. You certainly will not hear it. None-the-less it looks ugly in the measured spectrum.
To reduce the interference pick-up, cables 1 and 2 were bundled closely together greatly reducing the area of the ground loop (Area A) and the results are shown on the figure below.
Distortion Measurement of DÆ Output Board v1.0 at the RCA jack after the grass was cut simply by bundling the the cables to an from the DÆ Output Board v1.0 to reduce the open area for noise pick-up.
Ah - much better. That is more like it.
Another way to break the ground loop forming Area A is to make a custom RCA to XLR cable with the cable shield disconnected at the RCA cable end. See the figure below.
XLR to RCA cable with cable shield disconncted at the end with the RCA connector.
The XLR connector is connected to the DÆ LM4562 Differential Input Instrument XLR input. This time the measurement is truly differential with the +ve and -ve inputs of the differential instrument sampling the voltages at the two terminals of the RCA connector. The shield extends over the length of the cable but does not form the ground loop because the shield is only connected at one end - the XLR end. The measurement results are shown below.
Distortion Measurement of DÆ Output Board v1.0 at the RCA jack with custom XLR to RCA interconnect cable with the cable shield left disconnected at the RCA plug to avoid forming the ground loop.
Now that looks like a clean measurement. Even the 60 Hz power line component is below the noise floor. In fact there aren’t any spectral components above the background floor at any frequency below the fundamental of the test signal at 1 kHz.