IEEE Circuits and Systems Magazine - Q3 2020 - 41

the dielectric constant variations along the line since it
passes over fiber and epoxy. Alternatively, an isotropic
substrate material (such as TMM from Rogers) can eliminate this problem, but at a higher cost.
V. Bypass Capacitors
Bypass capacitors are used to suppress voltage ripples on
supply rails. The origin of the ripple could be from the voltage source itself, or a coupled noise from another part
of the PCB. Regardless of the source of the ripple, a sufficiently low ac impedance to ground guarantees it would
not disturb the operation of the powered circuit [4], [31].
Due to nonidealities in capacitor, and finite impedance of PCB traces, providing a low-impedance voltage rail cannot be made with a single large capacitor.
Fig. 11(a) shows the model of an off-the-shelf capacitor.
This model is typically simplified to a capacitor with an
Equivalent Series Resistance (ESR) and an Equivalent
Series Inductance (ESL), as shown in Fig. 11(b).
The presence of ESR means that the capacitor has
a finite rise time to compensate for any ripple. Also, ESL
means that the capacitor has a finite self resonant frequency, beyond which, the capacitor behaves like an inductor.

Typically, within the same technology, ESR is inversely proportional to the capacitance. This can be seen in
Fig. 11(b), where the bottom of the curve (which equals
ESR), is higher for small capacitors. On the other hand,
large capacitors are not sufficient to suppress all ripples.
Since large capacitors have a low self-resonance, they
are preferred for low-frequency ripple, and vice versa
for small capacitors. As a result, a number of incrementally sized capacitors are required to have a wide band
voltage ripple suppression.
While adding several bypass capacitors right next
to the power pin gives optimal results, this option is
usually not available due to limited available space.
For the example discussed below, we will assume that
the available space near the power pin fits only one bypass capacitor.
In Fig. 11(c), the power supply is connected to the
load using a microstrip line. The bypass capacitors are
distributed throughout the line. This gives better results
than dividing the capacitors between the supply and the
load. At low frequencies, longer lines still appear as short
circuit. As a result, large bypass capacitors, which suppress low-frequency ripples, can be placed further from

Straight
Transmission S21 (dB)

Square 90

Mitered

Straight

-2
Mitered

-4

Rounded
-6

Square 90
0

Curved

Measured

Reflection S11 (dB)

Double 45

Rounded

-20

-40

Curved
Straight

Double 45
-60

(a)

Simulated

Rounded

15
20
Frequency (GHz)
(b)

15
20
Frequency (GHz)
(c)

Figure 8. (a) Fabricated PCB to test various 90° turn types, in addition to their measured and simulated results for the transmission
(b) and reflection (c). Simulations at high frequency are done using Keysight Advanced Design System (ADS).

THIRD QUARTER 2020

IEEE CIRCUITS AND SYSTEMS MAGAZINE

IEEE Circuits and Systems Magazine - Q3 2020

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