# What is impedance 'masking' and how it is related to impedance 'matching'?

While reading ECSS-E-ST-20C Standard, I have encountered with a concept named 'impedance mask'. However, I could not find any detailed definition or explanation related to it.

In the corresponding article, it is said that :

"At the point of regulation, the impedance mask of a fully regulated bus, operating with one source shall be below the impedance mask shown in Figure 5‐1."

I would like to know what impedance masking is and how it is related to -if so- impedance matching.

I think it's talking about a DC power distribution bus and basically it's saying that the source of the regulated voltage on that bus must have a very low output impedance. If it didn't have a low output impedance it wouldn't regulate very well.

The term "mask" just refers to the allowable "top-limit" of impedance that is acceptable and, as far as I can tell has no bearing on such concepts as impedance matching.

So, take the example of a 5V bus capable of putting out 100 watts. At low frequencies (0.01 Hz or less) the output impedance can be calculated from the graph as: -

0.002 x voltage$^2$ / power = 0.4 milli ohms

OK I found ECSS-E-ST-20C and it says (regarding this test): -

NOTE 2 Rationale for the impedance mask: It translates requirement 5.7.2i.1 of 1 % voltage change for 50 % load change in a domain of regulation up to 10 kHz bandwidth. In DC the integrator in the control loop is designed to ensure no static error, in higher frequency, between 10 kHz and 100 kHz it is likely that the inductance effect of the components and connections are seen and the impedance rise not always making feasible to respect the ideal impedance mask.

This proves to me it is about power sources feeding a power bus.

• Thanks for the answer @Andyaka. Here we are talking about ripple frequency on the bus, right? Also, I cannot say that I clearly understand why it wouldn't regulate well if I didn't keep the output impedance of the source low. Commented Feb 5, 2016 at 8:04
• No, ripple frequency is something else. If a battery has a zero output impedance then any change of load will produce ZERO effect on the battery voltage. If on the other hand it has an output impedance of 1 ohm then clearly it cannot keep pure regulation when load current is changing. Commented Feb 5, 2016 at 9:33

It means the impedance needs to be below the line.

If you measure the impedance over frequency with a network analyzer, you get a single trace. Placing the mask over the trace would hide it completely for a conforming bus.

• Thanks for the answer @SimonRichter. What do you mean by 'placing the mask' exactly? Commented Feb 5, 2016 at 8:14
• You are ignoring any measurement points below the given line, so these are "masked out", as if a piece of cardboard was placed over the display. Any points not hidden are violations of the specification. Commented Feb 5, 2016 at 17:28