Issue link: https://iconnect007.uberflip.com/i/1256432
JUNE 2020 I DESIGN007 MAGAZINE 41 GRM155R61H104ME14 0.1uF 0402-size X5R 50V for C2. These components have detailed simulation models [3] , including temperature and DC bias dependence, so that we can use a circuit simu- lator to predict what happens, or we can pur- chase the components, build the filter, and mea- sure it. If we have the time, we should do both. But before we do either simulation or measure- ment, we need to decide what to look at. For filters connecting a high-power noisy rail to a low-current sensitive consumer, the prop- er metric is the input-to-output voltage ratio [4] . In simulations, we can achieve this by using a voltage source for excitation and simulate the output voltage. In measurements, since hav- ing a source with zero internal impedance is not feasible, we need to use an instrument with one source and two inputs to measure complex voltage ratios. These instruments are commonly called frequency response analyz- ers, and they allow us to measure the ratio of input and output voltages of the filter. When we build and measure and simulate this filter's input-to-output voltage transfer function, we get the plots shown in Figure 3. There are two notable features of these plots. First, on the left plot, we see two lines—one for each condition of DC bias. The blue line shows what happens without input power when the input voltage is zero, and the red line shows the transfer function with 3.3V DC applied to the filter. The DC voltage across the capacitors and the load current consumed by the oscilla- tor may change the characteristics of the filter components. In general, we should not worry about the blue line because when there is no input DC voltage, the circuit does not work. However, the difference between the blue and red lines is a reminder that unless we take the biasing conditions of the DC voltage and DC current into account, we may get the wrong answer. For instance, with this filter at 200 kHz, we get only 20 dB attenuation when the circuit is pow- ered up. Without the DC bias, we would see attenuation of 30 dB. The simulated response with 3.3V DC bias is shown on the right. The second notable feature, which is equally troubling, is the peaking of the transfer func- tion at 57 kHz. The peak value is 14 dB, which translates to a five-times voltage amplification of the noise. Instead of making the noise volt- age smaller, this filter makes it five times big- ger at 57 kHz. We may argue that the purpose of this filter is to suppress the output ripple of a DC-DC converter, and these days, the typi- cal switching frequency is above 200 kHz. This is true, but wide-band noise of the converter and the noise generated by the loads connect- ed to the unfiltered rail still could produce sig- nificant energy at lower frequencies, includ- ing 57 kHz. The peaking of the filter behaves like a high-Q band-pass filter, and if we have Figure 3: Measured (L) and simulated (R) voltage transfer function of the filter circuit in Figure 2.