Rf Transmitter Using Behavioural Models Computer Science Essay

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Advanced Design System (ADS) is the industry leader in high-frequency design. It supports system and RF design engineers developing all types of RF designs, from simple to the most complex, from RF/microwave modules to integrated MMICs for communications and aerospace/defence applications. With a complete set of simulation technologies ranging from frequency- and time-domain circuit simulation to electromagnetic field simulation, ADS lets designers fully characterize and optimize designs. The single integrated design environment provides system and circuit simulators, along with schematic capture, layout, and verification capability - eliminating the stops and starts associated with changing design tools in mid-cycle.

Objectives:

Create a system project for an RF transmitter using behavioural models.

Use an RF source, LO with the phase noise and a Noise controller.

Perform a harmonic balance simulation.

Test the system: spectrum, gain, compression, available power, noise figure and other specification.

Perform system budget analysis.

Write an equation in the data display.

1_ Theory

Modern communication systems require RF and microwave signals for wireless transmition of information. These systems employ oscillators, mixers, filters and amplifiers to generate and process various types of signals which are often affected by noise.

Noise

It's one of the most complex unwanted factors of the communication systems. It is defined in nearly every aspect of attenuation, distortion of the signal and has many forms:

external noise noise that is generated outside the device or circuit. The three primary sources of noise are : atmospheric, extraterrestrial and man-made

Internal noise - electrical interference generated within a device or circuit. There are three primary kinds of internally generated noise: shot, transit time, and thermal

Correlated noise - noise that is correlated (mutually related) to the signal and cannot be present in a circuit unless there is an input signal. Correlated noise is produced to be nonlinear amplification and includes harmonic and intermodulation distortion which are both forms of nonlinear distortion. Correlated noise is also a form of internal noise.

Amplifiers

Amplifiers are the basic components of any communication circuit's .The main use of the amplifiers in RF or Microwave circuitry is to improve signal to noise ratio that was generated by unwanted effects of the filtering and mixing of signals in the circuit. We also want our signal to be amplified to the power that was decreased due to attenuation of the filter and the transmition line.

Filters

Filters are used in most of the nonlinear systems for either noise filtering purposes or frequency filtering purposes. If we want to minimise the noise in the system, it is important to filter the signal as soon as any device generate the noise, i.e. place a filter after each device. Different types of the filter approximations are available to the engineers. Depending on the purpose of the design, we can estimate the approximation that is suitable for our specifications. For this experiment, we will use the Butterworth approximation. Here are the different approximations available:

Mixers

Mixers consist of any devices capable of exhibiting non linear performance. It's essentially a multiplier or a chopper. If we input two signals in a mixer, their product will be available at the output. The following picture illustrates the concept:

Oscillators

Oscillators are used for two main purposes:

To deliver power within narrow bandwidth

To deliver power over frequency range.

Each purpose has many subcategories and large range of specifications to define the oscillator

2_ Construction of the circuit

We manage to construct the RF transmitter circuit using the ADS tools with respect to the given specifications:

Oscillator

Amplifier

Schematic in ADS:

Filter

Mixer

Components:

These are the components being used in the schematic with their given values

3_ Calculations

The 40MHz baseband signal needs to b translated to a frequency of 1.95GHz using the super heterodyne transmitter system. Determine the frequency of the tow oscillators ( LOfreq1 & LOfreq2) and the centre frequency of the four bandpass filters ( IFfreq & RFfreq )

40 MHz baseband signal is being translated to a frequency 1.95 GHz

IFreq=?

RFfreq=?

LOfreq1=?

LOfreq2=?

Assuming

LOfreq1= 150 MHz

Given

RFfreq=1.95 GHz

IFreq= LOfreq1 + Baseband single

IFreq= 150 MHz + 40 MHz = 190 MHz

LOfreq2=RFfreq-IFfreq

LOfreq2=1.95GHz-190MHz = 1.760 GHz

4_ Simulation Tools

Simulation Components:

This measurement is used to determine the transducer power gain (in dB) and is defined as the ratio of the power delivered to the load, to the power available from the source. (where power is in dBm)

Harmonic Balance (HB) Analysis produces complex voltages and currents as a function of frequency or harmonic number.

Large-signal S-parameters are based on a harmonic balance simulation of the full nonlinear circuit. Because harmonic balance is a large-signal simulation technique, its solution includes nonlinear effects such as compression. This means that the large-signal S-parameters can change as power levels are varied. For this reason, large-signal S-parameters are also called power-dependent S-parameters.

This function measures the deviation of x from the average of x

Defines multiple variables or equations.

In ADS, by selecting the ParamSweep and SweepPlan controllers from any of the simulator palettes, you can sweep a variety of parameters and construct a series of sweep plans for special purposes.

5_ Simulation of spectra

As we can see from the following graphs, the signal from OSC1 was 10 dBm strong with no side bands when mixed with the baseband the signal was added and left the mixer with around -15 dBm attenuation. As the signal travelled along the transmitter it was slowly filtered of unwanted harmonics and when mixed again to up-convert to 1.95 GHz the attenuation was only around -6.8dBm and we could see the difference between even and odd harmonics. Resultant signal has 1.95 GHz frequency and 31.399 dBm power gain with very low power sidebands between -430 dBm and -1250 dBm. This just stated that the most of the power went to our information signal and the sidebands did not take much of the bandwidth

RFout

baseband

OSC1

mixer1

filter1

amp1

filter2

mixer2

filter3

amp2

filter4

RFout in dB

6_ Power performance simulation

This part illustrates the plots of power performance. The plots describe the relationship between the input power and the power gain of the system. Due to the presence of nonlinear components which have nonlinear effects, as we increase the input power, the power gain decreases.

Sweep1.HB3.PWRgain1 Versus BasePower:

Sweet1.HB3.Gaincomp1 Versus BasePowThe plot above illustrates another unwanted effect of nonlinear circuits. By increasing the base power gain over -15Bm, the gain compression increases too and reaches almost 34dBm which is more than the output gain of the transmitter.

Sweetp1.HB3.HB.PortPower (2) versus Base Power:

The plot above shows the relation between the output port power and the input base power. As we can see, the system generates enough signal power because the output power is already at its maximum while the input power is still negative. The maximum value of 35 dBm is also considerable because the spectrum plot had a maximum of 31.4 dBm.

Sweep2.HB3.PwrGain1 versus IFAGC

1 db compression point at -10dBm base power

1dB compression point at 0dBm basepower

The plots above illustrate the 1dB compression points with -10 dBm base power and 0dB base power. The IFAGC at -10dBm basepower is around 7.5 dBm higher then at 0 dBm base power. We can therefore say that by increasing the input power of the system, the amplifier power saturates earlier.The maximum power output gain that can be achieved is nearly 35dBm. By increasing the basepower over 0dB the value of the output power remains the same but will saturates earlier.

Plot 1-amp2 gain changed from 21dB to 10dB

The plot 1 above shows the affect of gain changed from 21dB to 10dB of amp2. The 1dB compression point is at higher value and the power gain has increased up to 45dBm.The circuit shows almost linear response which is ideal for all RF transmitter

Plot2-amp3 gain changed from 19dB to 25dB

The plot 2 above shows the affect of gain changed from 19dB to 25dB of amp3. The 1dB compression point is at lower value and the power gain is still 45dBm. So by increasing the power gain, the saturation or maximum gain power is achieved earlier but keep the same value of 45dBm.The peak response is already achieved at 5dBm of IFAGC gain. This is nearly at one fifth of what is shown on plot 1

7_ RF System Budget Analysis

The Budget controller enables us to perform an RF system budget analysis to determine the linear and nonlinear characteristics of an RF system comprising a cascade of two-port, two-pin linear or nonlinear components. The RF system may also include automatic gain control (AGC) loops to control gain and set power levels at specific points in the RF system. The characteristics are derived at the system input, system output and at the nodes between the components.

RF budget analysis is based on using frequency domain characteristics for each top-level two-port, two-pin component in the RF system design. The components may include mixers and nonlinear amplifiers. The analysis is performed at the single RF tone with specified power from the system input signal source. Each component is characterized for its S-parameter (small-signal and large-signal) and noise parameters. The collection of these parameters for each component in the cascade of two-port, two-pin components composing the RF system design are then used by the Budget controller to calculate the system performance at each node of the system design for the RF budget measurements that you select.

AC Small Signal Simulation

Using Simulation AC palette

Budget incident Power

Budget Noise Figure Degradation

Budget Power Gain

Budget Noise Figure

The table and graph for budget power gain are shown below:

From the graph and table above, we can see that the gain decreases after the signal is mixed and stays constant at low gainof 9dB.It is then amplified and reaches 2dB,passes through the second mixer ,gets attenuated by 1.3dBand keep constant during the filtering process.. Second amplifier amplifies the signal up to 22dB and filter keeps it in the same power level. However from the budget table, we can read that the oscilators have the highest attenuation (lowest gain) due to the introduction of phase noise.

The table and graph for the budget incident power are shown below:

The table and graph above describe the incident power budget in the circuit. We can see that there is a very low incident power at the input of the mixer from the oscillator to the amplifier. This means that these components are having nonlinear effects on the signal. But the filters have good response over the incident power because of their linearity.

The table and graph for budget noise figure degradation are shown below:

We couldn't get the graph for this sweep

The table and graph for budget noise figure :

The table and graph above describe the budget noise figure of the circuit. Here we can also read the high noise figure at the first amplifier. We have a very low noise figure of 0.729 from the mixer to the amplifier 1. However, the noise gets amplified through the first amplifier, reaches a maximum of 7.714 and keeps constant.

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