Welcome to this presentation which helps to highlight the capability of Genesys in

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the design and optimization of an LTE receiver front end.

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We will use several of many tools available in the Genesys to design our front end

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receiver including Mixer, Passive and Mfilter and post processing math to evaluate

the envelope information from the modulated data carrier.

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The receiver front end dictates the frequency, sensitivity, and bandwidth of the

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signals that we wish to detect and extract information.

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Modern front end topologies consist of an antenna(s), a diplexer for signal

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separation, front end amplifier, mixer, filter etc.

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To improve selectivity downconversion to a lower frequency band is performed with

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additonal IF amplification.

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System level design and evaluation provide a “tops down” approach to receiver

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design allowing the designer to specify component parameters and analyzing the

results

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Where there is no filter the noise figure is 3.5 dB even though the amplifier has a

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.5dB NF. This is because the amplifier also amplifies the image noise which is

folded over in the mixer, adding 3dB. When placing a filter in front of the mixer

adjacent signals are attenuated improving selectivity but adding to the noise figure

by 2dB. Finally placing the filter after the amplifier attenuates the amplified image

noise and provides the best NF but potentially desensitizes the receiver due to large

adjacent signals.

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First our focus we be designing and optimizing a front end amplifier stage(s) which

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will a balance between gain, noise figure, and dynamic range. The receiver will

cover the 1930-1990 MHz of the LTE band.

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We start with a workspace that has the bias network and 50 ohm load. Using this

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saved workspace reduces the time needed to find the best active device. Shown

above are four different devices selected from a library. Each is substitued in the

network and subsequently analyzed for gain, noise figure, stability, and intermod

etc.

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We use built-in linear measurements to evaluate noise figure, noise measure,

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minimum noise figure as well as plotting the stability circles. The stability circles

indicate regions where oscillation may occur. This particular device cannot be

simultaneously matched at inpujt and output.

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Surprisingly, we have improved the stability of our network so that simultaneous

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conjugate matching is possible. Also the spread in parameters over frequency is

smaller which makes matching simpler.

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We start the synthesis tool from the workspace tree. From the Match settings tab

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we select the frequency band and number of points. The sections tab provides us

with the topology for the matching structure. Note that we use the sub-network

SCH1 representing the two stage amplifier schematic for our device to match to.

This produces a network which may be further optimize to meet our amplifier goals.

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Using Match synthesis tool for the cascaded pair we arrive at compromise between

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noise figure and match. The final network uses fewer components and results in a

compromise between gain, noise figure and match.

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We now set up a harmonic balance simulation and sweep the input power to

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determine the 1dB compression point and gain. Note that at the 1dB compression

point (2dBm output) that the harmonics are below -30dB. Linear Gain is

approximately +33dB.

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When we apply two RF signals simutaneously to the input we can measure the

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intermodulation performance. Based upon the intermod values our predicted

intercept point is ~+16 dB and predicted 1dB compression point is +6dBm. This is

close to the simulated compression point of +2dBm. The rule of thumb more

accurately applies to single stage amplifiers.

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Having completed our front end amplifier we next move to the post RF filter. We will

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investigate both a distributed and lumped filter approach using the Genesys

synthesis tools for comparision. Launch Mfilter from the workspace tree. We next

choose the type(bandpass) and shape for our filter.

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Under the settings tab the ripple, termination, order as well as bandpass

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frequencies are selected. As a comparision point we evalute the filter based upon

the characteristics of Green tape or LTCC technology. Even with the higher Er, the

size of a distributed filter is large and may be more costly to implement depending

on volume etc.

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Using the same procedure to launch the Mfilter synthesis tool we use the Passive

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filter synthesis tool to produce and evaluate a lumped element equivalent filter.

Allowing for loss or finite Qs for the lumped elements generates loss in the pass

band as would be expected.The size of our passive filter is comparable to the

distributed filter so for this example we will use the lumped element bandpass filter.

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The next step in the receiver design is the mixer for downconverting our signal to

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the IF band. The Genesys Mixer synthesis tool aides us in selecting one of many

pre-designed mixer types including single ended, balanced and active mixers. The

procedure should look familiar, first select the syntheis tool from the workspace

menu, select a topology and then the mixer parameters as shown in the slide.

Shown are the frequencies for the RF and LO as well as input power and LO power.

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An extremely helpful portion of the synthesis tools is that pertinent measurements

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are made automatically. Here we witness the RF and IF spectrums for the double

balanced diode mixer selected.

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In addition to spectral information Mixer automatically sweeps the LO and RF power

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and plots the resulting conversion loss or gain verses power. Note for this mixer and

LO power of 7dBm is minimum for efficient conversion. Also the sweep RF power

graph shows a compression point of -4.5 dBm.

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When manufacturer’s diodes such as the Infineon BAT series are substitued,

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spectral purity is improved and LO drive levels have a wider range.

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We employ the Passive filter synthesis tool as before, to provide us with a low-pass

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IF filter following the downconversion. The filter eliminates undesired mixer products

prior to final amplification. The MMIC amplifier is an Avago(Agilent) broadband

general purpose amplifier. This completes the front-end receiver design.

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In an effort to reduce clutter and simplfy the visual desgin we use the hierarchical

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feature in Genesys to represent sub-networks or components. We may use any

library symbol or custom symbol to represent a component or subcircuit. In this

example we use the Spectrasys symbols to represent filters and mixer. Each

symbol points to a subcircuit which was previously designed and resides in the

current workspace.

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Adding the amplifer symbol to represent the front-end amplifier stage we complete

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the hierarchiacal representation. Each spectral graph represents the signal

spectrum as we pass through the receiver front end.

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Analyzing the completed chain we note that the input impedance has worstened

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because of the filter/mixer termination instead of 50 ohms. The power spectral

output shows spurious, image, and other harmonics are below the noise floor.

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The 1dB compression point, referred to the input is -45dBm, hard compression

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doesn’t begin until -20dBm input. Note that the noise figure bath tub plot is below

.9dB across the band.

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Another benefit of using heirarchical design is the ability to manuver our

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components and view the results. The graph shows the difference in noise figure as

a function of topology as was illustrated in the beginning.

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We use swept input power and non-linear noise analysis to view the effect of

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compression on noise figure.

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Since the information carried by the RF is digital in nature, transient simulation is

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necessary to view the composite response of our network.

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Text or ASCII PRBS code generated with MAXIM utility

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To use the PRBS code sequence we copy the text string shown into an equations

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block. Then use the name of the text block “PRBS127” in the VSTR field of the

MOD_VBIT source. Here we also input the power, frequency, time step, and rise

and fall time for the bit sequence.

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Substituing the power port with the modulated source and accompanied source

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resistance of 50 ohm we plot the modulated RF signal as it appears at the receiver

input. The second graph shows the downconverted and amplified signal. There is a

slight shift in phase and distortion that occured in the receiver process.

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The pulsed input appears to have additional modulation however this is a visual

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artifact of the sampling used in the time domain simulation. Slightly higher sampling

rate would result in longer simulation with no increase in accruacy however the

interveining graph would be filled. On the right is the down converted pulsed signal.

Note the ringing that occurs during the intersymbol period.

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To view the effects of non-linearity, filter parameters etc. viewing the baseband

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signal via eye-waveforms is a useful tool. Qualitative information can be gleaned

from eye shape and closure as it pertains to bit error rate. With this in mind we

introduce the “poor man’s circuit envleope”.

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The same procedure is performed in post processing of the transient data. An FFT

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is performed on the product of the signal with itself. All frequencies outside of the

baseband spectrum are attenuated or made zero. This is the mathematical filtering

referred to. An IFT is performed on the filtered data set resulting in the recovered

modulated signal.

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Here are three examples of modulated sources. 1) PRBS ,500 MHz RF. 2) AM

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modulated source. 3) Square wave modulated source.

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Note the PRBS product spectrum and resulting filtered spectrum which leaves only

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the baseband(folded)

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Overlayed on the modulated RF signal is the recovered modulation. Note how the

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modulation replicates the original signal.

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For our receiver front end our recovered modulation follows the original envelope.

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The down converted signal has slightly less fidelity due to distortion and the overlap

spectrum discussed in the previous slide.

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Using the ‘eye’ function in Genesys we can view the quality of the recovered

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modulation.

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Note the eye closing slightly and increase in apparent jitter.

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With a large input signal distortion, intersymbol interference reduces the eye to an

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apparent random looking bit stream.

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