Spurious Spectral Lines | ||||||||||||||||||||||
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Unwanted signals that are introduced by a non-perfect amplifier can be from various causes and each type of undesired signal needs to be uniquely specified for acceptable limits. The four types discussed here are: A. Carrier Modulation B. Carrier Harmonic C. Non-Carrier related D. Intermodulation Products A. Carrier Modulation These spurious signals are those that follow the carrier in frequency and amplitude and are a result of power supply noise and ripple. These signals come under the amplifier specifications for residual AM or FM. Since these are power supply induced, the range of test is usually from 10Hz to 50MHz from the carrier. The table below lists the typical Xicom specifications for these limits and these signals.
B. Carrier Harmonics These signals result from non-linearities in amplifiers and will show up at exact multiple frequencies of the carrier. These are usually worse at saturation power levels and are therefore measured at rated power. Typical C Band harmonic power can be from -3dBc to -6dBc. Most amplifiers include harmonic filters to meet a harmonic power specification of less than -60dBc. C. Non-Carrier related spurious Discrete signals which may occur at any RF frequency and may be the result of low-level RF oscillations. These signals are normally independent of carrier signal and may be out of band. These signals are specified to be 80dB below rated power and are not normally detected at all. The specific requirements usually come under the heading of spurious or spurious non-carrier related. Thermal Noise is also present in the output of a TWTA. The power density of the noise is the product of the noise figure of the tube and the tube gain. For example, if the noise figure is 30dB and the gain is 50dB then thermal noise is the -114 dBm/MHz + 30dB, + 50dB = -34 dBm/MHz. If the amplifier incorporates a solid state preamp, the output power typically rises a few dB. Since it sometimes occurs as an oversight not to have residual AM/FM (phase noise) as a separate call-out in a specification, "spurious" requirements are then sometimes leveled against all types of signals. This is inappropriate, as a -80dBc requirement cannot apply to residual modulation 10Hz from the carrier where -30dbc is an acceptable measure of value. D. Intermodulation products When operating a TWT amplifier with more than one carrier, Intermodulation signals (IMs) are generated. The amplitudes of these Intermod signals with respect to original pure signals will be small at low power and proportionally higher near saturation. The level of IM signals for two equal level signals can be estimated from the following formula: Where TOPB is the Total Output Power Backoff ratio (dB) relative to the single tone rated power and IMdBc is a ratio of the first IM to one of the two carriers. For example, if the rated power of the amplifier is 50dBm (100W), and the total power of the two 5W carriers is +40dBm (10W), then the TOPB is 50-40 or 10dB. Therefore, the IMdBc=(2*10+12)dBc=32dBc. Thus the first IM product will be 32 dB below each of the two 5W signals. Intermod values for typical Back off are as follow:
Summary Typical amplifier specifications for spurious lines are as follows: Harmonic content: >-60dBc Two-tone Intermodulation products (10MHz separation) -24 dBc @ 7dB Total Output Power backoff -18 dBc @ 4dB Total Output Power backoff Residual AM: As per Table 1 Spurious: Non Carrier Related: -80dBc Note: Phase noise requirements are defined per IESS-309, paragraph 4.3.3.1. Phase noise is specified in dBc/Hz and is primarily dependent upon the phase noise characteristics of the RF Source. The HPA itself has minimal and non-measurable contributions to the total system phase noise. |