Power Line Transceivers
FAQ

The maximum communication distance as well as the maximum number of usable transceivers depend on the specific power line network used.
The network impedance and noise as well as the signal distortion and attenuation are in fact strongly affected by the network topology and by the other devices connected to the same network at a given time.
As a consequence, the maximum communication distance can widely vary from one network to another or even from one instant to another on the same network.
For example, communication distances of 2 or 3 Km using ST7538 transceiver without adding any external power amplifier have been demonstrated in an outdoor undisturbed power line environment.
Concerning the maximum number of ST7538 transceivers connectable in a power line network, there is no limit but only that coming from the specific protocol used in the application.

In addition to ST7538 transceiver, what other components are required to build a typical power line communication node ?

ST7538 is a highly integrated solution with internal analog front end circuitry, power line driver, 5V linear voltage regulator and many other additional functions available "on demand" through an integrated control register. All these features allow to realize a complete power line communication node by adding only few inexpensive external components to implement the following sections:

•  the coupling interface (in general about 20 components, mainly resistors and capacitors and few diodes for burst and surge protections);
•  the power supply;
•  a quartz crystal with 2 capacitors allowing to realize a 16MHz oscillator by means of the internal ST7538 driver circuit;
•  a microcontroller to manage the upper layers of the used communication protocol and eventually to elaborate other signals related to the application.

The demo board provided by ST is a useful reference for the proper design of all these sections.

What do I get by ordering the ST7538 evaluation board?

The EVALST7538-2 is a fully functional demo board with related firmware loaded in the ST microprocessor and the suitable PC software to immediately start a power line communication application.

ST7538 transceiver integrates a driver circuit to realize a 16MHz oscillator: what else do I need for the oscillator implementation?

The ST7538 crystal oscillator driver circuit is realized with a MOS working in a sub threshold condition in order to get e very low current consumption.
Only an external quartz and two resonant capacitors are needed to realize a low power 16MHz oscillator. The oscillator circuit is able to drive a maximum load capacitance of 16pF with a typical quartz ESR of 40 ohm.
Concerning the quartz oscillator layout topology, standard considerations about noise immunity are always valid.
For example, it is recommended to keep the crystal quartz and the load capacitors as close as possible to the device and far away from noise sources such as:

    - power supply circuitry;
    - burst and surge protections;
    - modem coupling circuits to the mains.

To properly shield and separate the oscillator section from the rest of the board, it is recommended to use a ground plane, on both sides of the PCB, filling all the area below the crystal oscillator and its load capacitors. No tracks or conductive via holes, except for the crystal connections, should cross the ground plane.

I noted that during long transmission sessions, ST7538 transceiver becomes very hot: could it be a problem?

The band in use (BU) detection is a control on the presence in the programmed channel band of a power greater than a certain threshold. If band in use is not detected, the transceiver is allowed to transmit. This detection is particularly useful to satisfy a specific CENELEC normative to avoid transmission collisions.The carrier detection (CD) is a control on the presence of an FSK modulated signal on the channel band.
BU function and CD have different input sensibility and band pass filter selectivity. To prevent BU false transition, BU signal is conditioned to CD internal signal.
The preamble detection (PD) is a control on the demodulated signal. The condition is reached when the transitions between 0 and 1 on the demodulated signal are synchronous with the programmed baud rate (i.e. the internal PLL is locked).
Carrier and preamble detection is possible in conditioned or unconditioned mode:

- in the unconditioned mode carrier (preamble) detection, the modem ties the CD/PD line low and the clr/t lines are always running at the programmed baud rate even if no detection conditions are reached (in this case the RxD signal is tied to low).

- in the conditioned mode, the clr/t pin is driven only when a detection condition (preamble or carrier) has been reached, while the CD/PD line and the RxD line work as in the unconditioned mode.

ST7538 is a multi baud rate and a multi carrier powerline modem: is there any suggestion in the selection of these communication parameters?

In ST7538 transceiver, 8 programmable carrier frequencies from 60kHz to 132.5kHz and 4 selectable baud rates from 600 to 4800 bps are available. First of all, standard normative (CENELEC in Europe or FCC in USA for example) impose well defined constraints on usable frequencies, depending on the application. In the CENELEC domain, ST7538 offers 6 possible carriers in the metering band (A band), 1 carrier in the free consumer band (B band) and 1 carrier in the consumer band with required access protocol (C band). Once normative constrains are fulfilled, the frequency choice should be driven by network noise, attenuation and load characteristics whose frequency dependence is different from an application to another and should be fully analyzed. Usually, the maximum usable frequency is recommended, because, in general, the higher is the frequency, the lower is the noise. As the baud rate is concerned, the choice is mainly determined by the application performances needed and by the protocol used.

Is it possible to modify the output power level of the integrated power line transceiver to optimize the transmission?

In ST7538 transceiver, the transmitted voltage and current levels are controlled by two integrated control loops, whose characteristics can be adapted to the application by simply changing the value of the three external resistors present in the loop paths.

In particular, the Voltage Control Loop acts to keep the peak-to-peak voltage constant on the Vsense pin. The gain adjustment is related to the result of a peak detection between the voltage waveform on Vsense. 
The Current Control Loop instead acts to limit the maximum peak output current, by monitoring that the sensed current on CL pin does not exceed a certain limit value.
The Current Control Loop acts through the Voltage Control Loop decreasing the output peak-to-peak amplitude to reduce the current delivered by the PA.
The control loops logic flow chart and schematics are shown in the following picture:

click here to enlarge

Signal amplitude and load impedance are linked by the following control characteristics:

If the current limit threshold is high enough, the device operates according to the Voltage loop allowing to properly control the transmitted power by changing the value of R 20 and R 22 resistors, being the output peak voltage at the point linked to the voltage feedback divider (ATOP or ATO):

In terms of RMS values on ATOP pin, we can also write:

where -3dB and +3dB refer respectively to the case in which the voltage feedback divider is put on ATOP and on ATO pins.

For more details, please refer to:
- Application note AN1714 ST7538 FSK POWER-LINE TRANSCEIVER DEMO-KIT DESCRIPTION
- ST7538 Product Page.

Two possible receiving sensitivity modes (normal and high) are available in ST7538 transceiver: which do I have to use?

In an undisturbed network, when sensitivity is the limiting factor for the communication reliability, transceiver performances can be increased by setting the high sensitivity mode. In a noisy environment instead, to avoid that noise in the channel band induces a false carrier detection, it is better to use normal sensitivity and select a high detection time by properly programming the control register.