FCC and ETSI requirements for ASK modulation/short-range UHF transmitters

For remote control key (RKE), home automation, home security systems and other unlicensed transmitters for wireless control devices, United States FCC and ETSI in Europe have provided their transmit power level limit. These power requirements on those wishing to launch devices and unintentional or spurious emissions shall apply. In conjunction with these power limits and follow the test procedure, to determine the power developed by the radiation are in conformity with the specifications. Test equipment set up and sent radiation characteristics on the relationship between the test result (passed or failed) can have a big impact.

This application note describes the amplitude shift keying (ASK) modulation spectrum, signal transmitter and the transmitter VCO phase noise transient frequency pulling effect on test results.

ASK modulation spectrum

Can understand ASK modulation spectrum, that is, with a periodic square wave signal to modulating an RF carrier signal, and then the spectral lines are "modified" to illustrate the randomness of the data flow. First of all, cycle square wave signals as is the data rate of 2T-1/T 1010 ... Do not return to zero (NRZ) data series, as shown in Figure 1. The square wave power spectra as shown in Figure 2, including zero frequency indicates the carrier frequency, F0. At this point, the carrier spectral line and F0 be normalized (1/2T) composed of an odd number of times the line frequency. Each spectral line of power and carrier (zero frequency) power ratio of the spectral lines from the bottom gives:

Cycle for 2T of square wave

Cycle for 2T of square wave power spectra

When you ASK modulated signals are real data, the randomness of the data correction for half a week sine wave power spectrum spectrum. Normalized to the carrier frequency power spectral density of a mathematical expression is:

Each side-lobe spectral density peaks and spectral density ratio of carrier frequency by type 1 strike.

The MAX1472 ASK transmitter with 4kHz square (8kbps data rate) of modulation spectrum. We can see that the sidelobe peaks are located in the 4kHz (that is, half the data rate) of an odd number of multiplier points. Note that the ASK modulated signal carrier line (or lobe) power and no modulation (CW) relationship between the carrier power. This is important, because sometimes the FCC and ETSI specification applies to relative power, sometimes applies to absolute power. If the transmitter stable radiation (no modulation) P0 Watts power and duty cycle for 50% ASK modulation data stream, the modulation of radiation after halving the total power, is P0/2. Moreover, the modulation sideband can form, so the main lobe of the frequency spectrum (carrier) only contain half of ASK modulated signal power. Thus, modulation sidelobe power and transmitter CW power compared to that in power ratio in Equation 1 minus 6dB (CW power and after ASK modulated carrier spectral lobe power ratio).

For example, a 315MHz of radiation not modulated carrier power 10mW transmitter after ASK modulated radiation only 5mW of power. 5mW, contained in 2.5mW only exist in waves in the flap, next to the 2.5mW scattered throughout the remaining flap. Therefore, when the data rate is 8kbps (see 1), next to the 101th flap (distance carrier frequency 404kHz) power: visible sidelobe power is not only better than wave ASK modulated signals contained in flap small 44dB, but smaller than the unmodulated CW carrier power 50dB.