Design of a reconfigurable upconverter with image suppression function

The upconverter is an important component of the RF transmitter, and its performance has a significant impact on the overall system. Image rejection is an important consideration for the upconverter. Without image rejection, the image signal will interfere with other systems. Therefore, many methods have been proposed to deal with the impact of image noise [1-3]. Among these methods, some methods are only applicable to RF integrated circuits and monolithic microwave integrated circuits and are not applicable to discrete circuits, such as using polyphase filters.
For discrete circuits, there are two ways to suppress image signals.

(1) Use an image rejection mixer to cancel the image frequency by using a specific phase relationship between the input and output signals. Image rejection mixers are popular because they can tolerate aliasing of the RF and LO bands and provide sufficient image rejection over a wide IF bandwidth (image rejection is usually limited by the hybrid coupler). However, due to the presence of multiple hybrid couplers and balanced and unbalanced transformers, the circuit becomes quite complex and large in size, which limits its application.

(2) Use filters to select the desired signal to suppress image noise. In a reconfigurable multi-standard multi-band system [4], the RF frequency of one standard and the LO frequency of another standard may alias. In addition, when different standards are activated, the RF and image frequencies may change. In these cases, conventional filters cannot suppress the image signal due to the frequency change. On the other hand, because mixers use nonlinearities to generate mixing products, some active mixers may encounter serious second harmonic problems. Therefore, parasitic responses should be suppressed.

This paper proposes a reconfigurable upconverter with image rejection and harmonic rejection. It consists of a mixer, VCO, a low-pass filter and an adjustable band-pass filter. By changing the output signal frequency of the VCO, the intermediate frequency signal can be converted into the desired RF signal. The change of the local oscillator frequency leads to the change of the RF, image and harmonic frequencies. In order to suppress the converted image and harmonic frequencies, an adjustable band-pass filter with harmonic suppression is proposed and adopted. Although the frequency is changing, the image frequency and harmonics are greatly reduced.

1 Design of up-converter with image suppression

1.1 Up-converter structure

The block diagram of the up-converter is shown in Figure 1. It contains 5 parts: low-pass filter, mixer, adjustable band-pass filter, voltage-controlled oscillator and attenuator. The core part is the mixer. Since the single-ended mixer is the most basic of all mixers, a single-ended active FET mixer is taken as an example. The mixer is excited by a voltage-controlled oscillator, and an attenuator is set between the two. The attenuator is used to control the output power of the voltage-controlled oscillator and provide a suitable excitation signal to ensure the best performance of the mixer. Both LO and IF are applied to the gate of the FET transistor. For this type of mixer, harmonic response and local oscillator leakage are important considerations. In order to prevent the local oscillator from leaking to the IF port, a low-pass filter is inserted between the IF and the mixer. In front of the RF output port, a tunable bandpass filter is used. When the output frequency of the voltage-controlled oscillator changes, the filter passband frequency also changes, so the desired signal can be selected and the image frequency and second-order harmonics can be suppressed. The tunable bandpass filter is based on the center-loaded resonator introduced in reference [5]. Regardless of the frequency change, the image frequency signal, LO leakage and parasitic response can be filtered out by the tunable filter. In this prototype of the frequency converter, the IF signal frequency is 400 MHz and the local oscillator frequency range is 1.3 GHz~1.7 GHz. The upper sideband is selected, so the RF signal frequency range is 1.7 GHz~2.1 GHz.

1.2 Mixer Design

The schematic diagram of the single-ended gate mixer is shown in Figure 2. The design uses the ATF-34143 transistor produced by Avago Technologies. The operating bias conditions of this transistor near the pinch-off region are: Vgg=-0.68 V, Vdd=3 V, Idd=8 mA. After the bias conditions are selected, the next step is to check the stability. Unfortunately, in this case, there is potential instability. Two measures can be taken to stabilize it. The first is to use series negative feedback. Connect two high-impedance transmission lines between each source terminal and ground, which are equivalent to inductors. The second method is to use L1 and C3 to improve low-frequency stability. Finally, design the matching network. Note that the low-pass filter and the adjustable bandpass filter are connected in series with the mixer. If a filter is connected to a 50 Ω resistor at one end, the impedance seen from the other port in the passband and stopband is significantly different. This will affect the performance of the mixer. Therefore, this effect should be considered in the design of the mixer.

1.3 Design of harmonic suppression tunable bandpass filter

Figure 3 is a schematic diagram of the key part of the upconverter to suppress image frequency, LO leakage and parasitic response, namely the tunable bandpass filter with harmonic suppression. Unlike non-tunable filters, its harmonic frequency changes with the change of the passband frequency. The passband frequency range and the second harmonic may be aliased. In this case, it is impossible for an ordinary tunable bandpass filter to suppress harmonics. This design adopts a new harmonic suppression tunable bandpass filter based on the center-loaded resonator [5]. It consists of two identical microstrip resonators. Two varactor diodes are connected to the open microstrip port of each resonator. The silicon varactor diode is produced by Toshiba, model 1SV277, and the adjustable capacitance range is 1.8 pF~4.5 pF. By changing the bias voltage to adjust the capacitance value of the varactor diode, the overall electrical length also changes. In this way, the passband is adjusted to track the frequency change of the mixer output. In the middle of each resonator, a resistor and a capacitor are loaded to suppress even harmonic response. At the fundamental resonant frequency and the passband frequency, the voltage value is zero. At the second harmonic, the voltage value is no longer zero [5]. Therefore, loading a resistor at this point will not affect the passband performance, while at the second harmonic, the resistor will consume some power. Therefore, at the second harmonic, the Q value of the resonator will be greatly reduced, thereby suppressing the second harmonic. The combination of resistors and capacitors presents a high-pass response, reducing the impact on the performance near the passband frequency. The filter passband frequency can be tuned in the range of 1.65 GHz to 2.1 GHz. In the low stop band, the attenuation is greater than 40 dB. Due to the use of resistors, the filter has a wide high stop band with a frequency range of 2.5 GHz to 4.5 GHz and an attenuation of about 35 dB, thereby suppressing the mixer parasitic response.

1.4 VCO voltage-controlled oscillator design

The structure of the voltage-controlled oscillator is shown in Figure 4. It consists of three parts: a transistor, a resonator, and a feedback network. The transistor used is a bipolar silicon germanium transistor BFP640 produced by Infineon. A 3 V DC power supply is added to the collector. Resistors R1 and R2 are used to adjust the voltage and current. The total current is 25 mA. Two varactor diodes are used to adjust the oscillation frequency. In order to simplify the design, they share a bias voltage Vt. The measured varactor diode oscillation frequency-when the bias voltage changes from 0 to 6.5 V, the oscillation frequency changes from 1.27 GHz to 1.87 GHz, and the relative tuning range is 38.2%, which meets the requirements of the up-converter.

The phase noise of the VCO voltage-controlled oscillator is about -90 dBc/Hz and -108 dBc/Hz at 100 kHz and 500 kHz offset from the carrier frequency.

2 Experimental results

When all the components are realized, they are integrated together for performance testing. The output frequency of the voltage-controlled oscillator is adjustable within 1.3 GHz~1.7 GHz, and the intermediate frequency is fixed at 400 MHz. The overall performance of the upconverter is evaluated using a signal generator and a network analyzer. The conversion gain

of the upconverter at different RF frequencies is measured as shown in Figure 5. In the experiment, the LO signal power fed into the mixer is about 2 dBm, and the intermediate frequency input power is -15 dBm. In this case, the conversion gain is between 6.3 dB and 7.6 dB. In terms of nonlinearity, the 1 dB compression point is measured to be -4 dBm, and the output third-order intercept point (OIP3) is -1.5 dBm.

This paper proposes a reconfigurable upconverter. The image frequency, local oscillator leakage and parasitic response are suppressed by using a new harmonic suppression bandpass filter. The upconverter has a simple structure and is compact, which is suitable for discrete circuits in multi-band applications.

References
[1] MASS SA, MIXERS M, 2nd ed., Boston: Artech house, 1993.
[2] BEHBAHANI F, KISHIGAMI Y, LEETE J, et al. CMOS mixers and ployphase filters for large image rejection[J]. IEEE Journal of Solid-State Circuits, 2001,36(6):873-887.
[3] LERSTAVEESIN S, SONG B S. A complex image rejection circuit with sign detection only[J]. IEEE Journal of Solid-State Circuits, 2006,41(12):2693-2702.
[4] VIODJKOVIC, TANDG V D. LEEUWENBURGH A, et al. Adaptive multi-standard RF front-ends. Dordrecht, Netherlands: Springer, 2008.
[5] ZHANG X Y. Novel RF resonators and their applications bandpass filters : theory[D]. design and application,2009.