By: Supervisors:

PO-HSUAN HUANG Andreas Axholt

ATEEQ UR R. SHAIK Mattias Andersson

AMIR SHADEMANI

LUND UNIVESITY Electrical and Information Technology

Radio project ETI041

By:

BABAK MOHAMMADI

FARHAD SHOKRANEH

Supervisor:

GÖRAN JÖNSSON

Submission date: 2011-05-12

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Table of contents

1. Abstract………………………………………………………………………………………………..3

2. Introduction………………………………………………………………………………………….3

3. Design…………………………………………………………………………………………………..4

3.1. Biasing……………………………………………………………………………………………4

3.2. Input and Output…………………………………………………………………………. 5

3.3. Layout…………………………………………………………………………………………….7

4. Verification………………………………………………………………………………………………8

4.1. Setup……………………………………………………………………………………………..8

5.2. Gain………………………………………………………………………………………………..9

4.3. LO-IF isolation……………………………………………………………….…………………10

4.4. IF-RF isolation………………………………………………………………………………….11

4.5. LO-RF isolation…………………………………………………………………………………12

4.6. RF BW………………………………………………………………………………………………13

4.7. 1 dB compression point……………………………………………………………………14

4.8. S11 and S22…………………………………………………………………………………..…16

5. Conclusion……………………………………………………………………………………………………....17

6. Acknowledgement ………………………………………………………………………………………....18

6. References……………………………………………………………………………………………………....18

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1. Abstract Mixers are widely used in radio systems and having a mixer with good

characteristics can help designers to save design time and complexity in other

design stages. Having not much loss (or even having some gain), high BW which

covers working band and low noise are key points for designing a good mixer.

The goal of this project was to design, implement and verify a FM mixer using

discrete components. During design we found out that the most challenging part

was matching ports while keeping the gain of the mixer in the desired value.

Unfortunately we couldn’t fulfill this requirement, but on the other hand we

achieved higher gain than requirements.

2. Introduction Mixers are frequency translators. Radio frequencies are relatively high in

frequency and it is difficult, ineffective and costly to deal with signals in high

frequencies. Thus this high frequency should be lowered in some way.

Fig.2. FM Mixer converts high frequency at the input to lower one in the Output

(IF)

The concept of frequency mixers lies on their nonlinear transfer function.

Nonlinear transfer function could be expanded using Taylor series, thus if

) = ( )+ + … (1)

(2)

The first term in eq.(1) will occur as DC voltage, the second term will have the

same frequencies as input, but amplitude will depend on a1 and the third

…

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expression will produce our desired multiplication (using eq.2) and the higher

terms will cause intermodulation and harmonics which in most cases are not of

interest and are removed using filters.

3. Design During design different structures where studied. Our project was about designing

an active mixer. There are lots of different active-mixer structures with both

Bipolar and FET transistors. One of the most well-known FET topologies is Dual

gate FET transistor. This topology is known to have very high isolation between

ports, but it is difficult to match its ports, since the input port of FETs are

capacitive. Bipolar transistors are easier to match, but their isolation is relatively

lower.

In this project we chose an available bipolar transistor from lab which was

fulfilling the projects requirements like frequency and gain. Our first choice was

BFR93A from NXP.

3.1.DC

The first parameter in the design to think about was determining DC point, since

it affects almost all device parameters. To do so we should refer to the datasheet.

Plots in the datasheet show that for IC=3-5 mA we will have minimum noise

figure which is good to have in mixer. To achieve this current we used a single

resistor from collector to base to provide required base current. The supply

voltage was decided to be 5 V. The resistor is 100 KΩ which gives us 3 mA

current at the collector. Also a big inductor (fsrf=2.52 MHz) is used to block AC

signal to pass from collector to base and vice versa. At the output a 10 uF

capacitor is used to block DC current. 330 Ω resistor at the emitter is to load the

LO. Simulations were done and result was 2.96 mA.

Fig.3. Block diagram of mixer with matched input and output

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Fig.4. DC setup schematic

3.2. INPUT AND OUTPUT

The next step was designing input and output matching networks. Input and

output should be matched to 50 Ω and it should have filters at input and output to

fulfill these requirements:

At IF, RF should be filtered allowing just IF (10 MHz signals to pass)

At RF, IF signals should be passed, allowing just RF signals to pass.

At the output port we used a tank to have a resonance frequency at 10 MHz

which was our IF frequency.

Simulations were done and output is shown in fig.6. It should be noticed that this

is not the output of mixer and just shows the filter’s response.

To have filtering at the input port we used a parallel inductor at input to filter the

lower frequencies and block higher frequencies. Then we needed to have another

inductor to shift the input impedance to the center of smith chart at 100 MHz.

Designing input and output ports in mixer was not easy and is different from

amplifier, since we should consider and work in 2 different frequencies.

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Fig.5. Output with matching filter

Fig.6.Forward transmission

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3.3. LAYOUT

Since we were worried about the models that we used in simulation and also the

difficulty in adjusting the ports, we decided to adjust the ports practically. We

knew that how circuit should look like and which components will be placed. So

we designed a very general layout using Protel DXP.

Fig.7. Layout designed by Protel DXP

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4. VERIFICATION

After mounting the components we verified DC operation. It was correct and

circuit was consuming about 3 mA for 5 V supply voltage.

Then we used spectrum analyzer to verify the output. We used two different

signal generators as RF and LO sources. RF was adjusted to 100MHz and -

20dBm and LO was adjusted to 90MHz and 8dBm. The output was visible at 10

MHz, but unfortunately the gain wasn’t fulfilling the requirements. After looking

for the reason, we found it out that the series inductor at the input which was used

to match RF port was big enough to decrease our gain considerably. So after

trying to fix this problem we had to remove those series inductors to keep our

gain.

4.1. SETUP

The test setup for mixer is shown in figure below. Since we need two different

sources for mixer, we have to connect an external signal generator. We will

sweep the frequency from 88 MHz to 190 MHz and because of this these two

should be synchronized. ZVR has the ability to control signal generator using a

reference signal and an IEC cable.

Fig.8. Mixer Measurement setup.[2].

The ZVR´s port 1 and signal generator’s output are fed to the mixer. RF signal

which was connected to ZVR’s port one was configured to provide -20 dBm from

80 MHz up to 180 MHz where external source was configured to provide 8 dBm

from 69 MHz to 170 MHz. Filter’s exact resonance frequency was 11 MHz and

we used that frequency as our IF frequency.

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4.2. CONVERSION GAIN

Next figure shows conversion gain over FM band. As figure shows we have a

maximum of 4 dB gain. It decreases to 1.8 dB at 120 MHz but then it continues

increasing for higher frequencies. So we improve the required specification by

having 4 dB more gain. Figure 9 shows the result (with and without auto-scale).

Fig.9.Gain over the band

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4.3. LO-IF ISOLATION

Next figure shows LO-IF isolation. As it can be seen we have high isolation from

120-180 MHz. The max isolation happens at 160 MHz. There is gain at IF

frequencies, but we shouldn’t care about that, since high-pass filter will omit

those frequencies in previous stages.

Fig.11. LO-IF isolation

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4.4. IF-RF ISOLATION

The next figure shows IF-RF isolation over the band. We have a max of -38.14

dB isolation over the band.

Fig.12. IF-RF isolation over the band

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4.5. LO-RF ISOLATION

Fig.13. LO-RF isolation

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4.6. RF BW

We swept the RF-LO over a wider range to see the effect. We recalibrated the

ZVR OVER 1 GHz. Result comes next.

Fig.14. Sweeping the RF-LO over a wider range to find RF BW

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4.7. 1 DB COMPRESSION POINT

1 dB compression point for RF=111MHz is -24.12 dBm . Figure below shows that.

Fig.15: 1 dB CP

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The setting used with ZVR comes next.

Fig. 16: settings used for finding CP.

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4.8. S11 AND S22

As mentioned before we faced some problems for matching networks. But it

should be noticed that we matched both ports, the only problem was reduced

gain.

Fig. 17: S11(RF)

Fig. 18: S22(IF)

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Fig. 19: The implemented PCB

4. Conclusion In this project we designed an active mixer for FM band (88-180 MHz) using a

bipolar transistor (BFR 93A with ft=500 MHz). All the requirements except for

matching ports are fulfilled. Working over a wide band and not a single frequency

made matching very difficult, since the impedance of transistor and other passive

elements are changing with frequency.

At the other hand because of lack of practical experience in RF design we didn’t

paid much attention to component selection and used DC/AC blocking

components were too big. After receiving feedback from supervisor we found it

out that selected components may have self-resonance frequencies lower than our

frequencies, but because of time problem we couldn’t verify and replace

components. Maybe the most important experiences that we achieved from this

project are:

Don’t trust simulations which are done with ideal components. In low

frequency it’s never a problem and produces a few percent error. But in

RF, this error will be huge.

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Be very careful with component selection. The characteristics in the

working frequency is very important and it’s not always the same value

that you read.

We have learnt lots of new and practical things which were really important and if we

do the project again we will have better results. We learned a few new tools and

became more familiar and maybe more professional in using tools like N.A. .

5. Acknowledgment At the end we want to thank our supervisor Göran Jönsson which always had a

complete support in all steps of this project and shared his valuable knowledge and

experience with us. I think we achieved what we were looking for when selecting

this course. Now we have a higher self-confidence for doing real work as well as a

wider overview over the RF design.

Also we should have a special thanks to Lars Hedenstjerna which helped us with

PCB board.

6. References [1]. ETI031 - Radio slides, GÖRAN JÖNSSON - Lund University, Department

Of EIT

[2]. Radio Electronics, L.Sundström, G. Jönsson, H. Börjeson

[3]. RF Microelectronics, Behzad Razavi

[4]. www-users.cs.york.ac.uk