Technical Paper for Texas Tech University

Receiver Front End for 28-30𝑀𝐻# Transceiver Project

Taslim Anupom

R11368261

Texas Tech University

July 27, 2016

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Abstract

The technical paper describes the process of how a receiver front end of a transceiver

operates and what are the components required to achieve the optimal performance for the

system. The system receives a signal of frequencies between 28𝑀𝐻# – 30𝑀𝐻#. The signal is then

passed through a series of RF filters, amplifier and a mixer to get an output of 9𝑀𝐻# with a

selectable impedances as discussed in the report. The paper elaborates the above mentioned

topologies of how the system is designed along with full detail about how the system operates.

The paper also includes images and schematics along with detailed explanations for each section

for reference.

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Table of content:

1. List of Figures………………………………………….…………………….…………………3

2. Introduction………………………………………………….………………………………….4

3. RF Bandpass Filter………….…………………………………….……………….……………5

4. RF 2 Stage Amplifier…………………………...……………….….………….……….….…...8

5. SA 612 Frequency Mixer………………………………………………………….….…….…10

6. Conclusion…………………………………………………………………….....……………12

7. References……………………………………..…………………………………..…….….…15

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List of Figures:

Figure 1: System Block Diagram………………………………………………………………….5

Figure 2: Bandpass Filter LTSpice Circuit Schematic………………...………………………….5

Figure 3: LTSpice Bandpass Filter Simulation…………………………..……………………….6

Figure 4: Bandpass Filter Spectrum Analyzer Image………………………….………………….7

Figure 5: Suppressed Signals Around Center Frequency……………………...………………….8

Figure 6: RF Amplifier LTSpice Circuit Schematic………………………………………………9

Figure 7: RF Amplifier Resultsat-50dBminputsensitivity………………….……………….10

Figure 8: SA612 Internal Block Diagram…………………………………………..……………11

Figure 9: SA612 LTSpice Circuit Schematic………………………………………...………….11

Figure 10: SA612 Output………………………………………………………………………...12

Figure 11: Receiver Front End Schematic………………………………………….……………13

Figure 12: Receiver Front End Prototype……………………………………….……………….13

Figure 13: Receiver Front End Green Board Design…………………………….………………14

Figure14:ReceiverFrontEndOutputfromGreenBoard………………………………………………..14

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Introduction:

The main objective for the receiver front end system is to receive a signal, of frequency between

28𝑀𝐻# – 30𝑀𝐻# with center frequency at 29𝑀𝐻# having a characteristic impedance of 50Ω. The

signal is received with a power sensitivity of -100dBm and after filtering the 29𝑀𝐻# signal with

the RF bandpass filter, the signal is later amplified with a two stage RF amplifier and passed to a

SA612 dual balanced mixer and oscillator IC. Finally, at the output of the SA612 mixer, the

system generates multiple frequency but only the 9𝑀𝐻# signal with a selectable impedance of

50Ω or 1500Ω is required from the output of the receiver front end. The Local Oscillator (LO) of

the circuit along with the RF filter to suppress the 9𝑀𝐻# signal was achieved from other systems

developed by separate groups. But for testing purpose, a signal generator with a frequency of

20𝑀𝐻# was used for the LO. Figure 1 below represents the block diagram for the entire system.

The system achieves a gain of 40dB at the end of the receiver which is shown in the following

sections.

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Figure11:SystemBlockDiagram[1]

RF Bandpass Filter:

The RF bandpass filter used for the system is a coupled resonator filter. To maintain the

center frequency of the filter, the equation 𝜔 = '()

with 𝜔 = 2𝜋𝑓 is used. The tank circuit is

resonant with the frequency(f) at 29𝑀𝐻#. Figure 2 below represents the schematic for the filter.

Figure12:BandpassFilterLTSpiceCircuitSchematic[2]

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The signal is first stepped up using a transformer. A turn ratio of 3:8 was used for

simulation purpose, but to achieve the actual performance a turn ratio of 2:6 was used. At the

output of the filter another steps down transformer was used with a turn ratio of 6:2 in the actual

system. The 50Ω resistor in the schematic acts as a load for simulation purpose only. The actual

circuit does not have the 50Ω resistor as it is compensated by the input impedance used for the

input source.

Figure 3 below shows the result from the simulation done in LTSpice. From the simulation, the

signal out of the filter has a loss of 6.41dBm. But the actual circuit shows a much better

performance. The actual loss out of the filter is 2.47dB which meets the system requirement as

shown in figure 4 below, the image from the spectrum analyzer (The input 29𝑀𝐻# signal was at

a 0dBm for testing purpose).

Figure13:LTSpiceBandpassFilterSimulation

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Figure14:BandpassFilterSpectrumAnalyzerImage

For optimal performance of the system, the filter needed to suppress the harmonic

frequencies by a certain power level. Frequencies greater than 2𝑀𝐻#from center was suppressed

by >3dBm, frequencies greater than 5𝑀𝐻# from center was suppressed by >6dBm and signal

greater than 10𝑀𝐻# from center was suppressed by >10dBm. Figure 5 below shows the result

achieved from the filter design. Frequency at 27𝑀𝐻# yields a loss of 9.4dB, 25𝑀𝐻#yields a loss

of 13.34dB, 31𝑀𝐻# yields a loss of 6dB and 33𝑀𝐻# a loss of 12.95 dB, that is all from the

center frequency of 29𝑀𝐻# that only loses 2.47dB.

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Figure15:SuppressedSignalsAroundCenterFrequency

RF 2 Stage Amplifier:

Following the filter is a 2 stage RF amplifier. The purpose of the two stage amplifier is to

improve the power level of the signal, since the signal coming out of the filter is significantly

weak. The amplifier designed provides a gain of 20 dB which is enough for this part of the

system as more power can be achieved from the latter section of the procedure. Figure 6 below

shows the schematic of the RF amplifier.

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Figure16:RFAmplifierLTSpiceCircuitSchematic[3]

Signal coming out of the filter goes past a step-up transformer for impedance matching

the filter with the amplifier. To achieve maximum gain from the amplifier along with a 10mA

current stability performance, a 220Ω resistor in parallel with a 1000pF capacitor is used. The

signal output from the amplifier has an impedance of 50Ω on the low side. But for the next part

of the system design, the signal needs to be fed to a 1500Ω mixer. To match the impedance of the

amplifier with the mixer, another step-up transformer was used with a turn ratio of 5:18.

Figure 7 below shows the performance of the amplifier output. The output from the amplifier

gives a gain of 19.5dB from the output of the filter. This time, the 29𝑀𝐻# signal was at a -

50dBm sensitivity. To calculate the turn ratio for the transformers the following equation was

used:

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𝑧01𝑧234

=𝑁'𝑁6

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Figure17:RFAmplifierResultsat-50dBminputsensitivity

SA612 Frequency Mixer:

The general description of the mixer “is a low power VHF monolithic double-balanced

mixer with on-board oscillator and voltage regulator.” SA612 is used due to its low cost and low

power communication systems. The frequency mixer is used to produce an intermediate

frequency (IF) of 9𝑀𝐻#. The mixer receives the 29𝑀𝐻# signal from the amplifier and also LO

signal of 20𝑀𝐻#. The output results from the mixer is based on the addition and the subtraction

of the two signals. The signal that is required out of the mixer is the difference between the two,

which is (29-20)𝑀𝐻# = 9𝑀𝐻#. The internal block diagram of the SA612 is shown in figure 8.

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Figure18:SA612InternalBlockDiagram[4]

The SA612 has two inputs for local oscillator but the project only requires one LO for the

20𝑀𝐻# signal. From the datasheet of the SA612, the single ended untuned configuration was

used due to its simplicity, as well as best fit for the system. The IC is powered using an 8V

regulator which is connected to pin 8. The system uses the same setup configuration provided by

the datasheet for the power. Figure 9 below shows the schematic for the mixer.

Figure19:SA612LTSpiceCircuitSchematic[5]

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The result from the mixed output is show below in figure 10. This time the input 29𝑀𝐻#signal

was set at -100dBm and from the figure below, at 9𝑀𝐻#a gain of approximate 40dB is achieved.

The output also shows other signals from the mixer as explained earlier but those will be

suppressed from the system added to the output from the mixer from the other group.

Figure20:SA612Output

Conclusion:

The entire system was powered using a 12V source. Figure 11 below shows the entire

circuit schematic combining all the sections that is explained above. The RF filter, amplifier and

mixer schematic contains all the corresponding component values actually used in the system

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prototype.

Figure11:ReceiverFrontEndCircuitSchematic[6]

Figure 12 below shows the prototype from which all the results have been achieved. A green

board has also been designed as per project requirement which is shown in figure 13 below.

Figure 14 shows the output result from the greenboard system.

Figure12:ReceiverFrontEndPrototype

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Figure13:ReceiverFrontEndGreenBoardDesign

Figure14:ReceiverFrontEndOutputfromGreenBoard

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Reference:

1. Dennis F., “System Block Diagram,” (July 02, 2016)

2. Dennis F. & Anupom T., “Bandpass Filter LTSpice Circuit Schematic,” (July 02, 2016)

3. Dennis F. & Anupom T., “RF Amplifier LTSpice Circuit Schematic,” (July 02, 2016)

4. NXP Semiconductors, “SA612A Doubled Balanced Mixer and Oscillator,” 2014,

http://www.nxp.com/documents/data_sheet/SA612A.pdf (July 25, 2016)

5. Dennis F. & Anupom T., “SA612 LTSpice Circuit Schematic,” (July 02, 2016)

6. Dennis F. & Anupom T., “Receiver Front End Circuit Schematic,” (July 02, 2016)