A Low Conversion Loss Eighth Harmonic Mixer with Wide Band-Stop Filters for Low Cost 94 GHz Receiver Front-Ends

1 Introduction

Millimeter-wave mixers are widely used in the field of radar, communications, satellite remote sensing and so on. There are two kinds of mixers, such as fundamental mixers and harmonic mixers, Comparing with fundamental mixers, the biggest advantage of harmonic mixers is low frequency of LO which is particularly difficult to achieve in millimeter-wave. The defect of harmonic mixer is high CL. Therefore, broadband, high-order and low CL harmonic mixers remain to be a technical problem, there are still many research papers on it [1]–[14]. Due to the high CL of the high-order harmonic mixers, their main application is restricted in field of millimeter-wave test instruments such as vector network analyzer and spectrum analyzers.

To solve high CL of the eighth harmonic mixer, wide band-stop filters with radial stubs and phase shift microstrip lines are designed and combined for RF and idle frequency signals recycling. A novel compact micrstrip resonant cell (CMRC) IF low pass filter (LPF) is adopted for extraction of IF signals and rejection LO and unneeded signals. LO band pass filter (BPF) with parallel coupled lines is designed for suppression of IF signals. The designed harmonic mixers achieved low CL of 16.5 dB in 90–100 GHz. By combing with high gain and low noise amplifier (LNA), we can develop miniature, low power consumption and low cost 94 GHz receivers.

2 Circuit design

2.1 The eighth harmonic mixer

The mixer circuit layout is given in Figure 1. It consists of following parts, anti-parallel mixing diode, RF waveguide-finline-microstrip transition, RF matching networks, RF and idle frequency recycling reflector networks, IF LPF and LO BPF. First, the different parts of the circuit are optimized, such as LO BPF, IF CMRC LPF, and RF and idle frequency recycling network, respectively. Then, the different parts are combined with physical structure of the diode for whole circuit simulation, and its multi-port S-parameter file is extracted and combined with the diode SPICE mode for CL analysis. We optimized the matching circuit until the performance of the mixer meets the requirements.

Figure 1:

Circuit layout of the eighth harmonic mixer.

2.2 The mixing diode

The anti-parallel mixing diode presented in Figure 2 is realized by GaAs foundry of NEDI. The main diode SPICE model parameters are listed as follows [15]: R_s=6.4 Ω, C_jo=7.7 fF, n=1.4, I_s=1.03e^–12A, V_f=0.525 V (1 µA), V_br=4.625 V (10 µA). The diode main physical parameters are presented as follows, anode and cathode pad area 0.06 × 0.06 mm², air bridge length 0.03 mm, Schottky contact area 5 µm². The diode uses Ti/Mo/Au multilayer metal system, wherein the Au layer is used as the bonding metal, and Ti as a barrier metal. Due to the high operating frequency, parasitic parameters of the diodes would have a great impact on circuit performance. Electromagnetic simulation software is applied for extracting the diode S-parameters, and the diode material settings are same as those in Table 1. By combing with its SPICE model, harmonic balance analysis (HBA) is used for RF, LO, IF port impedance discussing.

Figure 2:

Diode picture.

Table 1:

Diode material setting.

Material	Doping	Thickness (µm)	Setting
N+ GaAs epitaxial	2×1017	0.2	Perfect conductor
N GaAs buffer layer (εr=12.9)	5×1018	4	Lossless dielectrics
GaAs substrate (εr=12.9)	0	50
SiO2 layer (εr= 4)	0	0.1

2.3 The IF LPF circuit

The novel CMRC IF LPF circuit is given in Figure 3, and its equivalent circuit is presented in Figure 4. Circuit substrate is RT/duroid5880 with dielectric constant of 2.22, and thickness of 0.127 mm. It can be found in Figure 5 that the simulated 3 dB cutoff frequency is around 1.7 GHz, and the suppression of LO signals is higher than 35 dB in 9–14 GHz, guaranteed a high LO-IF port isolation.

Figure 3:

Circuit layout of IF LPF.

Figure 4:

Equivalent circuit of IF LPF.

Figure 5:

Simulated S-parameters of IF LPF.

2.4 The LO BPF circuit

The LO BPF is parallel coupled lines structure, its circuit test setup is shown in Figure 6, which consists of five resonators, and each resonator is about half of one wavelength of the operating frequency. Circuit board is ceramic dielectric with thick of 0.254 mm and dielectric constant is 9.6. The tested results are presented in Figure 7, IL is less than 2 dB, return loss is greater than 15 dB, and suppression of 0–1.5 GHz IF signals is greater than 50 dB.

Figure 6:

Test setup of LO BPF.

Figure 7:

Measured S-parameters of the LO BPF.

2.5 The mixer circuit analysis

Commonly, millimeter wave eighth harmonic mixer CL is high. Therefore, wide band-stop filters with radial stubs and phase shift microstrip lines are designed and combined for recycling of RF and idle frequency signals. When the RF signals are transmitted to the diode, their leakage to LO and IF common port must be taken into consideration for recycling, then are 2ω_P±ω_s, 4ω_P±ω_s frequency signals. As presented in Figure 8, at the right end of the diode, a butterfly radial stub with radius of r₁ is applied to present short circuit to RF signals (75–110 GHz) at center frequency of 90 GHz. The other radial lines r₂, r₃, r₄, r₅, combined with phase shift lines L₁ and L₂, are designed to present short circuit at 147.5 GHz and 139 GHz (about center frequency of 2ω_P+ω_s and 4ω_P+ω_s signals), 50 GHz and 42.5 GHz (about center frequency of ω_s – 2ω_P and ω_s – 4ω_P signals), respectively. Simulated reflection phase angle of the circuit is given in Figure 9. The reflection angle shows that the deviation from 180° is less than 15 degrees, and the short circuit effect is perfect to idle frequency signals in 40–150 GHz. The simulated results in Figure 10 show that the attenuation in 40–150 GHz is greater than 15 dB. For harmonic mixing products such as 6ω_P ± ω_s (131–192 GHz and 19–27 GHz), and 8ω_P + ω_s (150–220 GHz) frequency signals, recycling are omitted because that the energy can be reused is very limited.

Figure 8:

Recycling circuit layout.

Figure 9:

Reflection phase angle of recycling circuit.

Figure 10:

Simulated S-parameters of recycling circuit.

3 Implementation and measurements

3.1 Implementation

The RG5880 circuit substrate and the mixing diode are mounted to block with 220°C and 150°C solder, respectively. The LO BPF is mounted to block with silver epoxy H20E. IF and LO interface connector are SMA-KFD45, and RF interface is standard WR-10 waveguides. The mixer block is aluminium and gold-plated, and its photo is presented in Figure 11.

Figure 11:

Photo of the mixer.

3.2 Measurements

The RF signals are generated by Agilent U85104A and attenuated to –10 dBm by Mi-Wave 510W. The LO signals are provided by Agilent 8257D, and down-converted IF signals are monitored by Agilent E4440A spectrum analyzer. The output IF is fixed at 1 GHz and LO signals’ pumping power is set at 18 dBm. It can be found in Figure 12 that the tested CL is less than 28 dB, 18 dB, 16.5 dB in 75–110 GHz, 87–104 GHz, 90–100 GHz, respectively. The minimum CL is 14.7 dB at 94 GHz. The plot of the two mixers’ CL deviation is lower than 3 dB, which shows good conformity. Figure 13 shows that the tested typical suppression of IF harmonics such as second, third, and fourth is 45 dBc, 40 dBc, 60 dBc respectively. As described in Figure 14, the measured LO-IF port isolation is better than 35 dB. Table 2 demonstrates that the CL of the designed mixer is much lower than Millitech, HXI, Farran’s commercial products, and reached RPG’s performance level.

Figure 12:

Measured CL of mixer.

Figure 13:

IF Harmonics suppression.

Figure 14:

Measured LO-IF port isolation.

Table 2:

Performance of eighth harmonic mixers.

Corporation	Module	RF (GHz)	CL (dB)
This paper	–	75–110	Typ. 20, Max. 28
		87–104	<18
		90–100	<16.5
Millitech [1]	MHP-10	75–110	Typ. 35
HXI [2]	HHM10	75–110	Typ. 35
Farran [3]	WHMB-10	75–110	Max. 38
RPG [4]	SAM-110	75–110	Max. 30, Typ. 23

4 94 GHz receiver front-ends

Two 94 GHz receiver front-end with different mixer types are designed as in Figure 15. For conventional receiver with fundamental mixer, LO signal is realized by a complex multiply chain (Ka-band active tripler, Ka-band BPF, Ka-band drive amplifier, W-band diode tripler, W-band BPF, W-band drive amplifier). It can be found in Table 3 that their noise figure is same. The gain of the receiver with eighth harmonic mixer is low about 6.5 dB because of its high CL, and its gain can be easily compensated by IF amplifier. The receiver volume, power consumption of this work is low and it is simple and easy to realize. The receiver is composed by LNA and mixer block, it can be further minimized if the LNA MMIC chip is integrated with harmonic mixer circuit. The selling price of the conventional receiver is one hundred and fifty thousand Yuan, and the receiver in this work is fifty thousand Yuan, it’s very attractive for low cost 94 GHz receivers.

Figure 15:

The 94 GHz receiver front-ends. (a) Receiver with eighth harmonic mixer, (b) conventional receiver with fundamental mixer.

5 Conclusion

Hybrid integrated 75–110 GHz low CL eighth harmonic mixer is achieved. RF and idle frequency signals are recycled by wide band-stop filters for low CL. A novel CMRC IF LPF is adopted for extraction of IF signals and its suppressing to LO signal is high. The measured CL is less than 28 dB in 75–110 GHz, less than 18 dB in 87–104 GHz, and less than 16.5 dB in 90–100 GHz. Because of its low CL, it can be widely applied in miniature, low cost, and low power consumption 94 GHz receiver systems.