## 1 Introduction  
Microwave converters, which are used to perform frequency conversion including upconversion and downconversion, are a fundamental part of transceivers and can be applied in many fields, such as satellite communication, radars, and microwave signal processing [1]. The upconversion has the process of increasing the intermediate frequency (IF) to the specified radio frequency (RF) point, and it is used to achieve high-gain launching and transmission. Meanwhile, the downconversion moves the spatial RF signal to a fixed IF point through conversion and filtering, which is convenient for the signal processing circuit to sample and analyze. Conventionally, the technology of the microwave electrical converter is mature and has high conversion accuracy [2, 3]. However, it suffers from low LO/RF isolation (15 dB) and high LO drive power (10 dBm) [4, 5]. Furthermore, bandwidth limitation and large insertion loss also restrict the application of the electrical converter [6]. Over the past 30 years, microwave photonic technology [7] received extensive attention due to its advantages such as wide bandwidth and the immunity to electromagnetic interference. This means it can be used in many systems, for example, microwave photonic filters [8, 9], beamforming [10, 11], etc. Therefore, a frequency converter based on microwave photonic links (MPLs) is considered as an effective method to solve the above problems. Microwave waveform generators based on microwave photonics are presented in many papers [12; 13; 14], and can be used for synthetic aperture radar (SAR) payload systems and show the launch capability of the Ka band [15]. The configuration of a microwave photonic converter system is similar to that of the photonic microwave waveform generator. Therefore, it is very significant to study the function and mechanism of microwave photonic converters in signal transmission and processing. In recent years, all kinds of microwave photonic converters have been proposed and demonstrated. A pair of Mach-Zehnder modulators (MZMs) in cascade can bring about infinite isolation theoretically, and the location of two MZMs can be exchanged at will to realize the input of two signals that are not in the same site [16]. However, the system is interfered with by lots of mixing spurs because the second MZM could re-modulate the sidebands achieved by the first MZM. Thus, it severely affects the performance of the converter. In order to improve the efficiency and linearity of the frequency conversion, a microwave photonic converter based on parallel MZMs is presented [17]. The RF signal and local oscillator (LO) signal are modulated separately so that only the 1 st-order sidebands of the RF signal and LO signal exist in the frequency components involved in conversion. Nevertheless, the interference produced by the instability of parallel path difference can reduce the mixing capability of the converter. One of the solutions is to utilize MZMs with an integrated parallel structure such as dual-parallel Mach-Zehnder modulators (DPMZMs) [18]. Compared to the cascade and parallel MZMs, the microwave photonic converter with a single MZM, which has received extensive attention recently, enjoys high conversion efficiency and excellent stability [19]. In [20], the MZMs are both biased at quadrature point, and undesirable spectral components are generated by beating between RF/LO sidebands and an optical carrier. As a result, the carrier-suppressed single-sideband (CS-SSB) modulation technology is applied in [21] so as to effectively amplify the sideband signals with an erbium-doped fiber amplifier (EDFA). Although CS-SSB modulation offers a purer IF signal by beat frequency between RF and LO sidebands, only the single sideband is used in the frequency conversion, resulting in a waste of spectrum resources. As for carrier-suppressed double-sideband (CS-DSB) modulation [22], both the upper and lower sidebands are utilized, therefore the spectrum utilization is improved.