Summary

To being used in the key to remote control (RKE) Home automation, the intersection of family and security system and control to have the intersection of license and sender Apparatus while being wireless while being other, FCC and European ETSI of U.S.A. have all stipulated the restriction value of its transmitted power level. The power limitation is required intending to launch and have no intention to these apparatuses or stray emission is applicable. Combine the power limitation value and follow the corresponding test procedure, in order to confirm whether the power radiation of the Apparatus accords with norms made. The test instrument is set up and the relation between the sender radiation characteristics, to the test result (pass or not up to standard) Exert a tremendous influence.

Note of this application has explained amplitude shift keying (ASK) Modulation table, phase noise of sender and impact on test result of the sender VCO transient frequency pulling of the signal.

ASK modulation table

Can understand the ASK modulation table like this, modulate a RF carrier signal with a periodic square wave signal, then carry on the gamma line ” Revise ” In order to explain the random property of the data flow.

First of all, regard cycle as 1010 that the data rate is 1 / T for the square wave signal of 2T …Nonreturn-to-zero (NRZ) Data sequence, as shown in Fig. 1. The power spectrum of this rectangular wave is shown as in Fig. 2, among them zero frequency represents the carrier frequency, f0. At this moment, the carrier gamma line that f0 is normalized and in (1/2T) The gamma line of one time of frequency points of odd number makes up. Each table collinear power and signal carrier (zero frequency) The ratio of the collinear power of the table can be provided by the following formula:

;Fig. 1. Cycle is a rectangular wave of 2T

Fig. 2. Cycle is a power spectrum of the rectangular wave of 2T

When ASK modulation signal is a real-life data, the machine format random property makes the spectral gamma line of power revise as the half cycle sine wave. Normalize it until the power spectrum density mathematical expression of the carrier frequency is:

The crest value of spectral density of each side lobe and proportion of spectral density of the carrier frequency are still tried to get by the formula 1.

It is that MAX1472 ASK sender adopt 4kHz rectangular wave (equivalent to the data rate of 8kbps) that Fig. 3 shows Frequency spectrum while modulating. We can see, every side lobe crest value lies in 4kHz (namely half of the data rate) Odd number time frequency point.

Fig. 3. Signal frequency spectrum when MAX1472 ASK sender adopts 4kHz square wave modulation {{Paging }

Please look out for ASK modulated signal carrier gamma line (or the lobe) Power and unmodulated (CW) Relation between the carrier power. This is very important, because FCC and ETSI norm is suitable for the relative power sometimes, is suitable for the absolute power sometimes. If the radiation steadily of the sender is (unmodulated) P0 tile power, then adopt ASK data flow whose duty ratio is 50% to modulate, the radiative total power will be reduced by half after modulating, get P0/2. Moreover, modulating will form the side band, so spectral main lobe (carrier) Only include the power of ASK half the modulated signals. So, while modulating the power of side lobe and comparing with CW power of the sender, the power ratio in the equality 1 should deduct 6dB (namely the ratio of CW power to power of valve of the spectrum of year after the ASK modulation) .

For example, a 315MHz sender of radiation 10mW unmodulated carrier power passes only radiation 5mW power after the ASK modulation. In this 5mW, there is only 2.5m

W exists in the carrier lobe, other 2.5mW disperses in every side lobe. So as data rate until at the 8kbps (see the formula 1) ,101st side lobe (the carrier frequency 404kHz of distance) Power:It is obvious the intersection of side lobe and power not merely than the intersection of ASK and the intersection of carrier and the intersection of lobe and power little 44dB of modulated signal, and than the unmodulated CW carrier power small 50dB.

The intersection of FCC and the intersection of requirement for ASK sender and 15.231(c), FCC of transmitted bandwidth, The festival regulation, 0.25% that the transmitted bandwidth intended to send can’t exceed center frequency, among them the transmitted bandwidth is decided by frequency point less than modulated carrier power 20dB in the radiation spectrum. 315MHz and 433.92MHz are that 260MHz to 470MHz does not have the two kinds of most frequently used frequency in license frequency band, correspond to with it, maximum allow bandwidth 787.5kHz separately ’

- boncostick posted this