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Design >> RF Design >> Confusion about the Transmit spectrum mask of Zigbee
https://designers-guide.org/forum/YaBB.pl?num=1248631842

Message started by AnalogAroma on Jul 26^th, 2009, 11:10am

Title: Confusion about the Transmit spectrum mask of Zigbee
Post by AnalogAroma on Jul 26^th, 2009, 11:10am

According to IEEE-Std 802.15.4-2006 spec ( Zigbee ) ,p 299-300
The out of band spurious emission for European 868 Mhz ISM band at 300 Khz offset from fc = 868.3 Mhz are

-36 dBm for 100 Khz detector BW or resolution BW in active mode . That is , -86 dBm /Hz.

-57 dBm for 100 Khz detector BW or resolution BW in standby mode. . That is , -107 dBm /Hz.

The spur level is measured with the transmitter outputting an unmodulated carrier and resolution bandwidth ( RBW ) used on the spectrum analyzer is 100 Khz. The output power at fc is at least -3 dBm /Hz and at most 14 dBm/ Hz. So, signal attenuation at 300KHz offset should be at least 83 dB/ Hz,

Here I am confused about the issue of the unmodulated carrier. When baseband signal will modulate the carrier, its spur -energy will spread. Right ? But, what about the baseband spectrum ?

The baseband signal I have got from other digital guys have an interesting spectrum which is attached herewith. In this spectrum, the signal level at dc is -19.68 dB and at 300 Khz is -68.12 dB.

So, signal attenuation is 48.44 dB which is much less than 83 dB !!!

Then , after upconversion and power amplification , the transmitted signal will violate the Transmit spectrum mask. Am I right ?

Title: Re: Confusion about the Transmit spectrum mask of Zigbee
Post by RFICDUDE on Aug 12^th, 2009, 7:55pm

Double check the actual frequency resolution. This plot does not look like a true dBc/Hz resolution, so it may be that the actual dBc/Hz value is closer to the specification after accounting for the resolution.

Title: Re: Confusion about the Transmit spectrum mask of Zigbee
Post by AnalogAroma on Aug 13^th, 2009, 7:15am

@ RFICDude,

Thanks for your response. Well I have used MATLAB finction "psd" which gives dB/ Hz values , not integrated power over a band. So, frequency resolution is not needed for this graph. Am I right ?

I have used the following MATLAB code to plot the spectrum.
------------------------------
% x is the data vector.
% Fs1 is sampling frequency.

h2 = spectrum.welch('Hann',512,50);
hopts = psdopts(h2,x); % Default options
set(hopts,'Fs',Fs1,'NFFT',4096,'SpectrumType','twosided','centerdc',true);

hpsd = psd(h2,x,hopts);
dataplot6 = figure;
plot(hpsd);
-------------------------------------------------------------------

The following info is taken from MATLAB help.

-------------------
The power spectral density is intended for continuous spectra. Note
that unlike the mean-squared spectrum (MSS), in this case the peaks in
the spectra do not reflect the power at a given frequency. Instead,
the integral of the PSD over a given frequency band computes the
average power in the signal over such frequency band. See the help on
AVGPOWER for more information.

The Hpsd object also contains a vector of normalized frequencies W at
which the PSD is estimated. W has units of rad/sample. For real
signals, W spans the interval [0,Pi] when NFFT is even and [0,Pi) when
NFFT is odd. For complex signals, W always spans the interval
[0,2*Pi).
------------------------------------------