Fig.8. Reference source assembly with vibration isolators Fig.9. PSD
damping effect of mechanical stabilization
b). Electrical Stabilization
Apart from mechanical compensation electrical stabilization is also
required which can be achieved by taking following point into
consideration while designing PLL system.
Decoupling: Use capacitors on supply voltage and charge pump supply
lines because these are most vulnerable to noisy signals, Use of
tantalum capacitors of 10uF, 1uF, 0.1uF, 100pF and 1kpF at the supply
to the reference source.
- Use Electrolytic capacitor in UUT Power supply for filtering.
- Use DC-DC converters for better phase noise performance making sure
their switching frequencies are not lying within vibration profile
spectrum.
- Use Low noise Voltage Regulators for better phase noise performance.
Make sure their ripples are not lying within vibration profile
spectrum.
- Layout design: Protect charge pump supply lines and VCO tuning voltage
lines from noisy signals (can be done by making these traces short
(max 6 mm length) and as close as possible to the PLL chip).
- Use isolated routing of reference source output to PLL IC with shield
protected co-axial cable so that conducted and radiated pickup during
random vibration testing can be minimized.
- Avoid the effect of acoustic noise and external vibration in the test
chamber.
- Secure the RF and power supply cables to minimize bending and strain
due to vibration. It is also important to shield the power connectors
of UUT for avoiding electromagnetic interference.
- Ground loops interact with magnetic and electric field generated by
the vibrating actuator. Minimizing the ground loops is of utmost
importance for accurate measurements [2].