Transient Power Requirements
Engineers should also be aware of a new specification in the European regulations that imposes restrictions on transient power, which is defined as the power falling into the adjacent spectrum when the transmitter is switched on and off during normal operation. This limit has been added to the latest regulations to prevent spectral splatter.
As the current to the power amplifier (PA) increases or decreases, the load seen by the voltage-controlled oscillator (VCO) changes, causing the phase-locked loop (PLL) to unlock for an instant and producing spurious emissions (or spectral splatter) while the loop seeks to reacquire lock. In systems where a unit only transmits at intervals, the splatter can significantly increase the power falling into neighboring channels.
Figure 4 highlights the problem of spectral splatter. The yellow trace shows the PA output spectrum from an ADF7020 transmitter when the PA is turned on and off once every 100 ms while the spectrum analyzer is kept on maximum hold. It is evident that significant power is falling into channels on either side of the carrier. The blue trace shows the PA output being ramped on and off in 64 steps every 100 ms, and indicates a considerable reduction of the power falling into the neighboring channels. Specification 8.5 of the latest EN 300 220 regulation establishes a limit on the amount of power falling into these adjacent channels.
4. ADF7020 output spectrum for the test described in the text.
The measurement procedure requires that the transmitter be turned on and off five times at maximum output power; and that the power falling into the second, fourth, and 10th channels on either side of the carrier be measured.
The simplest way to ensure compliance with this specification is to ramp the PA gradually off-to-on or on-to-off. This is normally accomplished by using the microcontroller to turn the PA on/off in stages. With the ADF7020 transceiver, the PA can be stepped from off to +14 dBm in a maximum of 63 steps. A faster and simpler approach is to use a transceiver with an automatic PA ramp such as ADF7021. The device has a programmable ramp for which both the number of steps and duration of each step can be set by the user.
Communication Protocol Considerations
5. Star network topology.
The communications protocol intended for use in worldwide sub-1-GHz bands is based on a star-type network (with up to 255 endpoints), as shown in Figure 5.
At the heart of the protocol is a nonslotted, nonpersistent carrier-sense multiple-access scheme with collision avoidance (CSMA-CA). The endpoint (EP) listens to the channel before transmitting (LBT), thereby avoiding collisions.
The nonslotted aspect of the protocol means that EPs can transmit as soon as they have data, subject to first performing a listen-before-talk operation. This approach also ensures that no synchronization is required. If an EP senses the channel is busy, it backs off for a random period before performing another LBT. The number of times this back-off can occur is limited, hence the nonpersistent nature of the protocol. In FHSS mode, the protocol uses this CSMA-CA system on each hopping channel, thus fulfilling the LBT requirement for the new European regulations.
The new European regulations impose very specific requirements for over-the-air protocols in the 863-MHz-to-870-MHz band. Specific rules must be observed for systems using single-channel, FHSS, or DSSS configurations. Fortunately, however, the new ETSI regulations mirror FCC Part 15.247 regulations in many aspects, thus simplifying the design of a protocol intended for multiregional use.
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