Spectral Masks for Transmitters - don't build a jammer!

Transmitters should,  ideally,  transmit all their power within their own frequency allocation.  Unfortunately,  even without phase noise,  this is not possible,  so we need to understand the effects of transmitting outside out band and the effects on other radio equipment.

There are two main effects of phase noise on transmitter output spectrum:

• the close-in phase noise often impacts on the spectral mask the transmitter must meet

• the wideband phase noise floor - gives co-siting difficulties with receivers

Close-in Phase Noise

Most of the phase noise energy is concentrated close to the carrier.   That within our channel usually only impacts on our own system,  essentially we transmit noise along with the signal,  this is covered on the next page.   The phase noise adjacent to our channel can result in the radiation of significant energy into the adjacent channel.   This transmitter can jam nearby receivers operating on the adjacent channel.

Example: The allowable spectral mask for a transmission is given in Figure 3-1.   The specified mask comes with the additional information that your signal must meet the mask when measured in a 3kHz bandwidth,  using a peak detector.   The 25kHz wide mask is to be centered on your channel.   Determine the phase noise requirement to meet this mask.

Figure 3.1 - Simple transmission mask

To determine a phase noise specification to meet this mask,  we note that the critical point is at an offset frequency of 12.5kHz,  the power must be 70dB down relative to P_max.  Via simulation or analysis,  you must relate P_max to the carrier level.   We will assume that the modulation is narrow-band FM,  and so P_max equals the carrier level.   To be 70dB down at 12.5kHz in a 3kHz bandwidth requires an SSB phase noise at 12.5kHz of 

-70dBc/Hz - 10*log₁₀(3kHz) = -104.8dBc/Hz. 

However this will make the average power in the 3kHz channel be 70dB down.   To make the peak value be at this level needs about another 10dB (a typical number for the difference between the peak and average power readings for noise on a spectrum analyzer).   Thus to arrange the phase noise to meet this mask requires an SSB phase noise of -115dBc/Hz at 12.5kHz.   

Of course,  if the modulation produces significant power at these offset frequencies then this will have to be included in the analysis.

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Wideband Phase Noise

As all amplifiers produce come broadband noise,  so do most transmitters.   This broadband noise is at a significantly lower level than the close-in phase noise,  but it may be of sufficient level to deafen a co-located receiver.

Example: A 100mW transmitter is operating full duplex with a receiver which has a 3dB noise figure.   The diplexer which connects both devices to the one antenna provides 40dB of isolation from the transmitter output to the receiver.   What wideband phase noise floor is needed to prevent the receiver noise figure exceeding 4dB.    

A receiver with a 3dB noise figure is effectively receiving -174dBm/Hz,  for the noise figure to be 4dB requires an additional  -177dBm / Hz of input noise at the receiver.   The transmitter produces +20dBm,  so allowing for the 40dB of isolation,  the wideband phase noise floor of the transmitter must be no greater than -157dBc/Hz.   

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