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This paper presents a signal injection technology showing significant reductions in both 3rd-order and 5th-order intermodulation distortions (IMD3 and IMD5) in space traveling wave tube (STWT). By applying the IMD3 to the IMD5 ratio (TFR) as measures of location, the simultaneous suppressions of IMD3 and IMD5 in TWT are achieved by second harmonic distortion (2HD) and IMD3 injection. According to the research on theoretical analysis and computer simulation, the optimum amplitude and phase parameters of the injected signal for maximum simultaneous suppressions are obtained. Then an experiment system is established based on vector network analyzer, optimum TFR are 2.1 dB and 12.5 dB, respectively, by second harmonic and IM3 injection, and the output powers of IMD3 and IMD5 were decreased. TFR with IMD3 injection is smaller than that with second harmonic injection in STWT, and the experiment system is more straightforward and easy to operate. Thus, the IMD3 injection performs better than that of second harmonic injection to suppress IMD5s for the narrow-band STWT.

Traveling wave tube (TWT) amplifier is widely used in the fields of communication, signal processing, radar, electronic warfare, and so forth. Space TWT is the core component of satellite communication system, and it plays a crucial role in power amplification in the field of space technology, high gain, and excellent linearity is an essential prerequisite for its normal operation. Unfortunately, TWT exhibits nonlinear characteristic and therefore generates unwanted intermodulation distortion (IMD) and second harmonic distortion (2HD). Among various distortions, harmonics can be eliminated by the filter, but the 3rd-order (IMD3) and 5th-order intermodulation (IMD5) fall on the main signal band and cannot be easily filtered out. Meanwhile, the IMD signal severely limits the fundamental energy of TWT and reduces the output power; thus it causes lower overall system performance. Earlier articles show that the mechanism of distortion suppression by signal injection is the destructive interference of the injected signal with nonlinearly generated distortion product [

The MUSE theoretical model [

In recent years, there are some new research results about signal injection including the improvement of the fundamental signal power of TWT by harmonic injection [

First of all, many abbreviations in this paper are explained. FUN is the abbreviation of fundamental signal, and FUN− and FUN+ represent lower and upper fundamental, respectively, in two-tone signal. IMD3− and IMD3+ are lower and upper third-order intermodulation distortion components, and IMD5− and IMD5+ are lower and upper fifth-order intermodulation distortion components. Initial power and initial fundamental frequency represent input power and frequency of two-tone fundamental signal, they are variable, and the initial power and frequency variation of fundamental signal can influence output power of FUN, IMD3, and IMD5 under optimum injection and without signal injection.

We use the proposed nonlinear model on analog amplifier to analyze the suppression in IMD3 and IMD5 theoretically. The amplifier nonlinearity can be expressed with a Taylor series equation [

The input signal (two-tone fundamental signal) consists of two tones given by expression

A IM3 signal of amplitude

The IMD3 output power (

The IMD5 output power (

All parameters in (

It can be seen from (

When the optimal operating point of the injected amplitude and phase is found where

Serious nonlinear distortion components appear at the output port after STWT begin to work on saturation state. The third-order intermodulation frequencies

The study on signal injection to simultaneously suppress the IMD3 and IMD5 has been held all along to improve linearity performance of STWT significantly. Wöhlbier et al. study comprehensively physical principles of signal injection technique for TWT [

The 3D plots of the output gain and power of fundamental signal according to the fundamental frequency and initial power are shown in Figure

(a) 3D plot of output gain; (b) 3D plot of output power according to the fundamental frequency and initial power.

The FUN− in two-tone frequencies is set to 1.5 GHz; frequency spacing is increased from 0 to 200 MHz; the 3D AM/AM curve of the two-tone fundamental signal according to the increasing fundamental frequency spacing is shown in Figure

The 3D AM/AM curves of FUN− and FUN+ according to the increasing fundamental frequency spacing.

It can be seen from Figure

(a) 3D plot of fundamental differentiation; (b) 3D plot of IMD3 differentiation according to initial power and power difference.

On the basis of the previous research achievements on fundamental signal parameters in STWT, the initial two-tone fundamental frequencies

The 3D plot of the output power at IMD3 and IMD5 according to the phase and amplitude of injected IM3+ signal.

The amplitude and phase of the injected IM3 signal are kept at −21 dBm, 312°, which produce maximum suppression for IMD3. Initial power of fundamental increases from −12 to 0 dBm and phase is kept at 0°. The comparison of computation results of output powers at the fundamental, IMD3, and IMD5 under optimum IM3 injection and without IM3 injection is completed in this paper. It can be seen from Figure

The output power of FUN, IMD3, and IMD5 according to initial power variation under optimum IM3 injection and without IM3 injection.

The optimal signal injection conditions for IMD3 and IMD5 are not consistent. So we need to find the phase and amplitude of injected signal that ensure the minimum relative power of IMD3 to IMD5, so that the simultaneous suppression for the IMD3 and IMD5 is observed by signal injection.

Based on simulation results, IMD3 has larger output power level compared to IMD5 and more close to fundamental frequency, so IMD3 suppression is a more important work for improving the performance of STWT. IMD3 to IMD5 ratio (TFR) represents relative output power of IMD3 to IMD5 and is set as the parameter to evaluate the performance of signal injection technique.

The initial two-tone fundamental frequencies are set as 1.50 GHz and 1.52 GHz; amplitude and phase are 0.0°, −10.0 dBm. The amplitude and phase of the injected IM3+ signal are kept at −21 dBm, 312°; the minimum is −27.6 dB when the power of injected IM3+ increases from −28 dBm to −17 dBm and phase is kept from 270° to 310°. This condition represents the IMD3 to IMD5 relative power at corresponding frequency point (higher or lower order) achieving the minimum and making the optimum simultaneous suppression for both IMD3 and IMD5 of STWT; then the IMD3+ and IMD5+ output power is −9.6 dBm and 17.9 dBm, respectively.

Second harmonic (

Minimum of

The 3D plot of

The 3D plot of

Fundamental frequency spacing for simultaneous suppression in both IMD3 and IMD5 of STWT is studied through the simulation method under the optimal signal injection for

Suppression situation for IMD3+ and IMD5+ under optimal signal injection according to fundamental frequency spacing (1 MHz~100 MHz).

The vector network analyzer (VNA) can precisely describe the property of a device according to its amplitude/phase responses to frequency and power scan of measurement signals.

The rated output power of the STWT which is used in the experiment is 120 W (55 dBm), its gain is 50 dB, and working frequency band is 1.4–1.6 GHz. The experiment block diagram of simultaneous suppressions in both IMD3 and IMD5 is shown in Figure

The experiment block diagram of simultaneous suppressions in both IMD3 and IMD5.

The IMD3− (1.48 GHz) is introduced into the input port of STWT, and phase and amplitude are adjusted by signal generator and phase shifter. When the injected IM3− power is −5.86 dBm, the minimum value of

The output signal spectrum under optimal IMD3− injection of simultaneous suppression for IMD3 and IMD5.

Maximum suppression of 25.1 dB for IMD3− can be observed by vector network analyzer; then the suppression for IMD5− is 17.3 dB. The total output power of STWT is 121.4 W, and it also has good characteristics of gain. The experiment proves that IMD3 signal injection can make simultaneous suppression for the IMD3 and IMD5 at corresponding frequency of STWT.

Alternatively, 2HD (3.0 GHz) is introduced into the STWT; when the amplitudes are adjusted to 13 dBm, the minimum value of

The output signal spectrum under optimal signal injection for IMD3 and IMD5 of STWT is shown in Figure

The output power, suppression, and corresponding TFR under optimum IM3 injection and 2HD injection in simulation.

Mode | TFR | Output power | Suppression | |||
---|---|---|---|---|---|---|

IMD5 | IMD3 | FUN | IMD5 | IMD3 | ||

IM3 | | | | | | |

2HD | | | | | | |

The output power and corresponding TFR under IM3 injection and 2HD injection in experiment.

Mode | TFR | Output power | Suppression | |||
---|---|---|---|---|---|---|

IMD5 | IMD3 | FUN | IMD5 | IMD3 | ||

IM3 | 2.1 | 6.7 | 8.8 | 46.7 | 17.3 | 25.1 |

2HD | 12.5 | 8.6 | 21.1 | 46.4 | 15.4 | 12.7 |

The output signal spectrum under optimal 2HD injection of simultaneous suppression for IMD3 and IMD5.

The value of TFR and IMD suppression achieved experimentally is limited by resolution bandwidths of signal generator and phase shifter; in particular, the suppression of IMD is particularly sensitive to the phase adjustment of the phase shifter, and the amplitude and phase of injected signal can be tuned precisely to get simultaneous suppression.

Comparison of TFR and IMD suppression between simulation and experiment is shown in Tables

Comparison of TFR and IMD suppression between simulation and experiment.

Mode | Simulation | Experiment | ||||
---|---|---|---|---|---|---|

TFR | Suppression | TFR | Suppression | |||

IMD5 | IMD3 | IMD5 | IMD3 | |||

IM3 | | | | | | |

2HD | | | | | | |

This paper introduces the definition of IMD3 to the IMD5 ratio (TFR), by choosing the optimal operating point of the injected amplitude and phase where the TFR has minimum variations, and derives the signal injection conditions for simultaneous suppression in the IMD3 and IMD5. According to the research on theoretical analysis and computer simulation, the optimal amplitude and phase parameters of the injected 2HD and IMD3 for maximum simultaneous suppressions are obtained. Then an experiment system is established based on vector network analyzer; the optimal TFR are 2.1 dB and 12.5 dB, respectively, by 2HD and IM3 injection. The output power of IMD3 and IMD5 was decreased. TFR with IM3 injection is smaller than that with 2HD injection in STWT. The experiments system is more simple and easy to operate. Thus, the IM3 injection performs better than 2HD injection to suppress IMD5s for narrow-band STWT.

The corresponding author is Professor Kewen Xia.

The authors declare that there is no conflict of interests regarding the publication of this paper.

This work was supported by Hebei Province Natural Science Foundation (no. E2016202341), Hebei Province Foundation for Returned Scholars (no. C2012003038), and Research Project of Science and Technology for Hebei Province Higher Education Institutions (no. BJ20 1401).