A Novel C-Band Single Diode Mixer with Ultra High LO/RF and LO/IF Isolation Abdul Maalik, Zohaib Mahmood SUPARCO SRDC Lahore, Pakistan E-mails: [email protected], , [email protected]

Abstract- A novel single diode mixer with ultra high LO/RF, LO/IF isolation and having a reasonably good conversion loss has been designed. RF and LO signals are applied through bandpass and lowpass filters respectively. The mixer operates in C-Band and downconverts a block of frequencies (5.925GHz – 6.425GHz) to a block of frequencies (3.7GHz – 4.2GHz) using a fixed frequency local oscillator at 2.225GHz. The proposed mixer structure achieves double balanced functionality with a single diode and obviates the need for RF and LO baluns. The measured conversion loss ranges from 8 – 10dB and port isolations are in excess of 60dB.

Index Terms—Mixers, Single diode mixers, Passive mixers, Mixer operation, port-to-port isolation

I. INTRODUCTION

M

ICROWAVE mixers refer to a nonlinear element embedded in associated circuitry to provide frequency changing for up or down conversion. They are used in a wide variety of applications including heterodyne receivers, radars of all types, electronic warfare, guided weapons, communication, instrumentation, transportation, radio astronomy etc [1]. Mixer design is a quite mature field and has seen quite interesting circuit topologies ranging from unbalanced design to single balanced, double balanced and triple balanced designs and various mixing elements ranging from point contact diode to shottkey diode to FET to more recently HEMT and HBT [2]. These various designs cover different performance parameters of microwave mixer and an exact topology and mixing device selection is dictated by particular application requirements. For example in applications where high isolation and higher third order intercept point are required, double balanced mixers are preferred, whereas in those applications where high dynamic range is required, triple balanced mixers are used. Similarly applications requiring low conversion loss and high frequency of operation, single diode mixers employing schottky diode are the design of choice [2]. However if we require the mixer to be integrated in an MMIC, we will make use of FET or HEMT mixers since they are easier to integrate as compared to diode mixers. Every RF/microwave mixer falls into one of the following two categories.

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A. Passive mixers These are the mixers that use the devices that exhibit positive nonlinear resistance in their working. The most common device used in passive mixers is schottky diode; FETs can also be used for this purpose where we make use of resistive channel of a MESFET. These are also called diode mixers and are characterized by conversion loss. They are very broadband, inexpensive and easier to design. However they require more LO drive as compared to active mixers. [2], [3]. At millimeter wave frequencies diode mixers are by far the widely used mixer type. B. Active mixers These are the mixers that make use of active devices like FET, BJT,HEMT,HBT or a passive device like RTD (which exhibits negative resistance) to achieve gain. In active mixers, the emphasis is not solely put on achieving maximum possible gain. Rather, the gain is set slightly greater than unity and low noise figure is made sure, since high gain degrades the noise performance of the receiver. They require low LO drive and provide superior noise figure performance and some conversion gain as well. [4], [5]. However their bandwidth is limited by high-Q gate-input impedance. Also active balance mixers require baluns or IF hybrid couplers that have very large size when IF is low and difficult to design at very high frequencies. A very common problem in microwave mixer design has been that of port to port isolation and various designs have been proposed to address this problem [6], [7], [8]. The significance of having very large isolation between mixer ports arises from the requirement of better overall receiver performance. If the isolation between RF and LO port is not large enough, the LO signal, being very strong in amplitude, would leak through RF port and would reduce the dynamic range of LNA or RF preamplifier. If LNA is not present in the receiver, the leaked LO signal will be radiated by the antenna and thus would interfere with other receiver systems. Similarly, LO/IF isolation is of great significance as well. If this is poor, the LO signal escaped through the IF port would saturate the IF amplifier which is already operating in high gain mode.

One of the techniques to improve port-to-port isolation is to design RF and LO baluns in such a way that electric fields of RF and LO signals are orthogonal to each other [6]. In the present paper we are presenting a new design approach for a double balanced passive mixer with emphasis on achieving very high port to port isolation. Instead of using a quad ring topology and baluns for injecting LO & RF signals to the diode ring, as is traditionally done, we have made use of a single diode to achieve a double balanced design. RF signal is injected to the anode of diode through a bandpass filter and LO signal is injected through a lowpass filter to the cathode of diode. The downconverted IF signal is separated from undesired mixing frequencies by a bandpass filter at the cathode side. Inductors are used to provide DC return path for the diode current. The RF signal fundamental has been resistively terminated on the cathode side and this termination has been embedded in the LO filter. The LO signal fundamental has been terminated at anode side and the termination has been made part of RF filter. The current paper proceeds with the principle of diode mixer operation in section II. Design procedure has been described in section III. Simulated results have been shown in section IV. Various measured performance parameters along with simulated results have been given in section V. Conclusions are drawn in section VI. Acknowledgements and references have been presented in sections VII & VIII respectively.

II. DIODE MIXER OPERATIONAL THEORY

(1)

Where I represent the o/p current of a mixer diode and V represent the total voltage applied across the diode (the sum of RF and LO signal voltages). If we put V =VLO +VRF =A (t) cos (wpt) +A (t) cos (wst)

III. DESIGN PROCEDURE Our design procedure is clearly depicted in Figure 1.

Fig. 1. Block diagram of the designed mixer.

The design of individual modules/circuits is described below.

In a diode mixer, the nonlinear mixing element is a semiconductor diode. Today’s high frequency, high performance mixers almost invariably make use of shottkey diodes because of the absence of minority carriers in these diodes, which result in very high switching speeds [5]. Current-Voltage (IV) characteristics of a semiconductor diode is given by

I = ao + a1V + a2V 2 + a3V 3 + ...

wave and large number of its harmonics. This results in numerous o/p products of which the desired product is filtered out. In essence, LO voltage changes the diode’s small signal junction conductance (or resistance) and capacitance and frequency conversion occurs via these linear time varying small signal elements. This is called the small-signal approach for describing the operation of diode mixer [5].

(2)

in Eq. (1) and solve, we get output current at different mixing frequencies n*wp+/-m*ws where m = n = 0, 1, 2, 3, … . Further, if it is assumed that amplitude of the applied RF signal is much smaller than the LO signal, which is usually the case, the o/p current contains small signal components at wn= (ws-wp)+n*wp [5]. Of all these components, the frequency of interest can be filtered out by IF filter (or RF filter in case of an up converter). Alternatively, a mixer can be viewed as a switch where the RF signal is interrupted periodically at the LO frequency. This corresponds to RF signal being multiplied by a square wave. Thus, according to Fourier series, the RF signal is being multiplied in the diode by a DC signal, a fundamental LO sine

A. Diode selection Proper diode selection is at the heart of microwave passive mixer design. The selected diode should have strong nonlinearity, low noise, low distortion and good frequency response in the regions of interest [7]. A diode mixer’s performance depends upon diode parameters (junction capacitance, series resistance and barrier voltage) and circuit parameters (DC bias, load resistance, harmonic termination & matching networks). Whereas a design can optimize the circuit parameters to achieve good performance, the device parameters solely rely on proper device selection. As far as the device selection is concerned, it depends upon two different aspects. (i) Device Structure (ii) Device Material. Device structure includes dot-matrix diodes, beam lead diodes and packaged diodes. We selected packaged diode because of its superior performance and simplicity of handling. From the aspect of device material, GaAS devices are the clear winners because of their high cut-off frequency, high breakdown voltage and high resistance to voltage transients. [10] We have selected Agilent Technologies’ HSMS2850 single shottkey diode. This device is optimized for C-Band operation. B. RF Filter Design A band pass filter for injecting the RF signal to the diode has been designed using M/s Agilent’s Advanced Design System (ADS) momentum. The filter has the following specifications.

= = = > >

6.175GHz 500 MHz 1.5 – 2.5 dB 20dB 15dB

S21 m1 0 -2 Mag. [dB]

Center Frequency 1 dB B.W Insertion loss Out of band rejection Return loss

The filter has been designed as a microstrip half-wave edge coupled filter. The simulated wideband performance of RF Filter is shown in Figure 2. S21

-4 -6 -8 -10

0

1.0

1.5

2.0

Mag. [dB]

-10

2.5

3.0

Frequency

-20

Fig. 4. Simulated S-parameter (S-21) of Low Pass Filter (ADS Momentum)

-30

Layout of this filter is shown in Figure 5.

-40 -50 -60 4

6

8

10

12

14

16

18

20 Fig. 5. Physical layout of Low Pass Filter (ADS Momentum)

Frequency Fig. 2. Simulated S-parameter (S-21) of RF Band Pass Filter in broadband (ADS Momentum).

The LO fundamental has been resistively terminated by a λ/4 transmission line at 2.225 GHz. The filter was again optimized to mitigate the effect of this T.L on RF filter performance. The λ/4 transmission line also presents high impedance to IF signal. The layout of this filter is shown in Figure 3.

D. IF Filter Design A band pass filter to get the desired O/P frequencies has been designed with following specifications: Center Frequency 1dB Band Width Return loss Out of band rejection

= = > >

3.95GHz 500 MHz 15dB 20dB @ 100 MHz offset

Wideband simulated response of this filter is shown in Figure 6 and generated layout is shown in Figure 7. 20

Fig. 3. Physical Layout of RF Band Pass Filter (ADS Momentum).

Center Frequency Insertion loss Return loss Out of band rejection

= = > >

2.225GHz 0.2 – 1dB 20dB 20dB at 2.3GHz

Moreover, a λ/4 transmission line for resistively terminating the RF signal fundamental at 6.175GHz has been added to this filter structure. A notch filter to suppress the LO 2nd harmonics has also been designed. The filter was re-optimized in ADS momentum. Since these filters are embedded into the mixer structure individual testing could not be performed and only simulated results are provided (Figure 4).

0 Mag. [dB]

C. LO filter design A low pass filter for injecting the LO signal to the diode has been designed using the same software tool. It has following specifications:

m2

m1

-20 -40 -60 -80 -100 0

5

10

15

20

Frequency Fig. 6. Simulated S-parameter (S-21) of IF Band Pass Filter in broadband (ADS Momentum)

Fig. 7. Physical layout of IF Band Pass Filter (ADS Momentum)

E. DC Biasing No external dc bias was used in our design. Although external dc bias lowers the conversion loss but it may turn a balanced mixer into an unbalanced one and hence degrades some of the performance parameters of the mixer like rejection of LO noise and suppression of even order IM distortion. Rather, internal biasing has been used where the LO drive was increased a little to bias the diode. The LO level and hence bias was set carefully with the help of HB simulator. The drive levels lower than 5dBm and greater than 15dBm resulted in a greater conversion loss. The optimum biasing of the schottkey diode happened to be at 10dBm LO level. To provide a return path for dc-bias current to flow through the diode, we have used two chip inductors at anode and cathode of the diode. Their value has been chosen in a way to present high enough impedance to RF and LO signals so that they might not leak through these inductive paths and are applied fully across the diode. The variation of conversion loss versus different L1 and L2 values has been simulated using ADS Harmonic Balance and is plotted in Figure 8. The optimum values of L1 and L2 that resulted in least conversion loss are chosen. 0

fundamentals. The diode’s junction capacitance has been estimated as Cjo. The diode’s reactance at RF frequency is XRF = 1/ 2π*fRF*Cjo

(3)

It has been absorbed into the RF matching circuit. Similarly diode’s reactance at LO frequency is XLO = 1/ 2π*fLO*Cjo

(4)

It has been absorbed into LO matching circuit. The diode’s reactance at IF frequency is approximated twice as that at RF frequency [2]. A matching circuit was designed to resonate out this reactance and thus match the diode’s IF impedance to that of load.

IV. MIXER SIMULATION The complete mixer was simulated by using ADS, HB simulator. The circuit with simulation setup is shown in Figure 9. Matching circuits, reactance terminations and LO level were changed interactively to obtain the best possible conversion loss and port-to-port isolation.

Conversion Gain (dB)

-5

-10

-15

-20

-25 0

2

4 6 Inductor Value-L1 (nH)

8

10

(a)

Conversion Gain (dB)

0

-5

Fig. 9. Mixer Simulation setup in ADS

V. FABRICATION AND TESTING -10

-15 0

20

40

60

80

100

Inductor value-L2 (nH) (b) Fig. 8. Conversion gain as a function of inductor values (a) L1 (inductor placed at anode of the diode) (b) L2 (inductor placed after cathode of the diode)

E. Matching Circuit Design: Small signal junction resistance of shottkey diode varies from 40-100 ohm [2]. It has been matched to RF & LO

The mixer was fabricated on Duroid sheet with Er=2.2 and substrate thickness of 0.787mm. The circuit was etched using LPKF 100H PCB Prototyping mini CNC machine and tested on HP vector Network Analyzer. The measured port-to-port isolation and conversion loss have been plotted in Figures 10, 11, 12, and 13. The measured and simulated data show close compliance. Possible deviation could be the lack of accurate manufacturing and lack of all RF harmonic terminations. From the port-to-port isolation measurement results it is clear that we have been quite successful in achieving very high isolation as compared with the conventional mixers reported so far.

0

0

Simulated Measured

RF/IF ISOLATION (dB)

CONVERSION GAIN (dB)

Simulated Measured -5

-10

-15

-20 5.9

6

6.1 6.2 6.3 RF FREQUENCY (GHZ)

6.4

6.5

-20

-40

-60

-80

-100 5.9

6

6.1

6.2

6.3

6.4

6.5

RF FREQUENCY (GHz)

Fig. 10. Simulated and measured conversion gain of mixer (LO Power = +11dBm)

Fig. 13. Simulated and measured RF/ IF Isolation

0 Simulated Measured

LO/IF ISOLATION (dB)

-10 -20 -30 -40 -50 -60 -70 -80 -90 5.9

Fig. 14. Fabricated Mixer 6

6.1

6.2

6.3

6.4

6.5

RF FREQUENCY (GHz)

Fig. 11. Simulated and measured LO/ IF Isolation 0 Simulated Measured

LO/RF ISOLATION (dB)

-20 -40 -60 -80 -100 -120 -140 5.9

6

6.1

6.2

6.3

6.4

RF FREQUENCY (GHz)

Fig. 12. Simulated and measured LO/ RF Isolation

6.5

VI. CONCLUSIONS A broadband ultra high port-to-port isolation double balanced mixer was designed for C-band downconverter using single shottkey diode. The RF/LO isolation measured in excess of 65dB.The LO/IF isolation measured in the range of 55dB.The authors believe that this is the highest port-to port isolation for C-band mixer which has been reported The conversion loss was bit on the higher side (8 – 10dB). This was due to the fact that HSMS2850 is optimized at the lower end of RF frequency band (i.e. upto 6GHz). One possible remedy to this could be to use another diode in parallel to this diode (e.g. HSMS8101). It will result in a flat conversion loss over the whole frequency band. The second thing that could be done to this mixer is with the size of the mixer. This could be reduced by designing miniaturized filters at RF, LO & IF frequencies. ACKNOWLEDGEMENTS

This work couldn’t have been accomplished without the auspicious guidance of Dr. Inam Elahi Rana, the Director Bismillah Electronics Lahore, Pakistan. We are also indebted to Dr. M. Riaz Suddle (SI), Director SUPARCO Lahore and Mr. M. Yousuf Khan, GM SUPARCO Lahore for their unprecedented encouragement and support.

REFERENCES [1] [2] [3] [4] [5]

T. H. Oxley, “50 years development of the microwave mixer for heterodynereception,” IEEE Trans. Microwave Theory and Tech., Vol. MTT-50, No. 3, pp. 867-876, Mar. 2002. S. A. Maas, Microwave Mixers, 2nd Edition. Norwood, MA, Artech House, 1993. G. D Vendelin, A. M. Pavio, and U. L. Rhode, Microwave circuit design using linear and nonlinear techniques. New York: Wily, 1990. S. Weiner, D. Neuf and S. Sophrer, “2 to 8GHz double balanced MESFET mixer with 30dbm input 3rd order intercept,” in IEEE MTT-S Int. Microwave Symp. Dig. 1988, Vol. 11, pp. 1097-1100. S. A. Maas, “A low distortion GaAs MESFET resistive mixer,” Microwave J., vol. 31, no. 3, pp. 213–220, Mar. 1988.

[6]

J. J. krantz, “An S-Band Double Balanced Mixer with very high LO/RFport and LO/IF-port isolation,” in IEEE MTT-S Int. Microwave Symp. Dig. , vol. 2, 1990, pp. 765-766. [7] A. M. Pavio, R. H. Halladay, S. D. Bingham, C. A. Sapashe, ”Double Balanced Mixers using active and passive techniques,” IEEE Trans. Microwave Theory and Tech., Vol. 36, pp. 1948-1957, Dec. 1988. [8] A. C. Azevedo Dias, D. Consonni, M. A. Luqueze, “High isolation subharmonic mixer,” in Proc. Microwave and Optoelectronices Conf., vol. 2, Aug. 1999. [9] K. Chang, I. Babl, V. Nair, RF and Microwave Circuit and Component Design for Wireless Systems, Wiley March 2002. [10] Y. Anand, W. J. Moroney,” Microwave mixer and detector diodes,” Proc. IEEE Vol. 59, pp. 1182 – 1190,Aug 1971