Note | PLL architecture | Added Features | Loop filter calculation | Recommended texts | RF layout |
RF layout for good results
Support provided through RF evaluation boards
In order to obtain a reliable and high performance RF design, several PCB layout guidelines must be followed. RF signal lines should be kept as short as possible, and where possible with 50 ohm terminations, this is relatively easy to realise when using a multilayer board.
Careful decoupling of the power supplies is also important. In addition to the usual large decoupling capacitors, smaller devices should also be placed as close as possible to the power supply pins. These devices act as a low impedance path to ground for any stray high frequency transients. Normally a value of around 100pF can be employed for this purpose.
The connections between the AC ground and the decoupling structures should be laid out as a ground plane in order to avoid the generation of stray inductance which can negate the effect of the capacitances, or worse still generate a resonant circuit which can lead to parasitic oscillations. Contacting to the ground plane from the bond pin, leads in general to an inductance of 1nH, therefore as many vias as possible should be employed. As a rule the synthesiser supply should be separated from any digital circuitry also present in the design.
Particularly sensitive is the ground connection to the loop filter. This ground should be properly decoupled from the charge pump output, and from the VCO. To achieve the best performance, the loop filter should be constructed from film capacitors, which exhibit a high Q factor, i.e. low series resistance.
Measurement hints
When measuring it is preferable to use a shielded chamber. A low noise signal generator or the actual TCXO from the system to be designed must be used for the OSCin signal. Ground loops can create 50Hz spurious and harmonics around the carrier signal. Some measurement instruments, especially those equipped with a cathode ray display, can generate higher frequency spurii. For a test setup, additional inductors in the power rails can improve the noise peformance. For phase noise measurements, all generators should be connected to the same reference timebase.
