Patch antenna design tutorial with CST microwave

This is a tutorial series on microstrip patch antenna design with inset feed in CST microwave.  The antenna will be designed to work in 2.4GHz ISM band used in WiFi devices.

The overall design steps for designing the patch antenna in CST microwave studio involves-

1. Calculation of the dimension of the patch antenna, substrate, ground plane, effective dielectric constant, impedance etc.
2. Inputting the parameter obtained into the CST microwave studio, draw the substrate,, ground plane, patch antenna, gap between the patch and microstrip feed and microstrip feed.
3. Simulation and Optimization

The patch antenna picture that will be designed is shown below-

 Because it is a lengthy process, each step is described in different blog post. The first step which involves the calculation is described in this post. Readers can skip to-

2nd Part- Design of Microstrip Patch Antenna in CST Microwave Studio
3rd Part- Inset Fed Patch Antenna Simulation and Result

Design Specification:

Suppose we want to design the patch antenna that resonates at 2.45GHz on a RT/Duroid 5870 substrate. The RT/Duroid substrate has a dielectric constant of 2.33 and a thickness (or height) of 0.787mm.

The first step is determine the length and width of the patch antenna. These can be determined using the following equations-

For Width,

\(\begin{equation}W=\frac{c}{2f_r}\sqrt{\frac{ 2}{\epsilon_r+1}} \end{equation}\)              ........(1)

where,
   c is the speed of light, \(\begin{equation}c=3\times10^{8}m/sec \end{equation}\)
   fr is the resonant frequency, \(\begin{equation}f_r=2.4835GHz \end{equation}\)
   \(\begin{equation}\epsilon_r\end{equation}\) is the dielectric constant of RT/Duroid substrate, \(\begin{equation}\epsilon_r=2.33\end{equation}\)

substituting the values gives us, W = 47.44mm

For Length,

first determine the effective dielectric constant,

 \(\begin{equation}\epsilon_{eff}=\frac{\epsilon_r+1}{2}+\frac{\epsilon_r-1}{2}\frac{1}{\sqrt{1+\frac{12h}{W}}} \end{equation}\)                                       ............(2)

 \(\begin{equation}\epsilon_{eff}=2.272 \end{equation}\)

then find the extended incremental length,

 \(\begin{equation}\triangle{L}=0.412*h\frac{(\epsilon_{eff}+0.3)(\frac{W}{h}+0.264)}{(\epsilon_{eff}+0.258)(\frac{W}{h}+0.8)} \end{equation}\)                                       ............(3)

using the values we find,

\(\begin{equation}\triangle{L}=1mm \end{equation}\)

Now the length of the patch antenna can be obtained using the equation below,

\(\begin{equation} L=\frac{c}{2f_r\sqrt{\epsilon_r}}- 2*\triangle{L}\end{equation}\)                              ........(4)

Substituting the known values we obtain the length of the patch antenna as, L = 39.618mm

The next step is to determine the Inset feed point.

What is the inset feed point? It is a point located inside the patch where the impedance of the feed microstrip and the patch antenna is matched or becomes equal and thus least amount of energy is reflected back to the microstrip and most of the energy is radiated from the antenna.

Let the distance of the inset feed point be d from one of the edge of the patch antenna. Let us use a 50ohm microstrip feed and find the inset point where this 50ohm will match the input impedance of the patch antenna.

The equation to find out the value of d is,

\(\begin{equation} R_{f,in}(d)=R_{p,in}(0)*cos^2(\frac{\pi*d}{L})\end{equation}\)                              ........(6)

where,
     \(\begin{equation} R_{p,in}(0)=\frac{1}{2(G_1\pm G_{12})}\end{equation}\)                              ........(7)

   here \(\begin{equation}G_1\end{equation}\) is the conductance at the entry slot and \(\begin{equation}G_{12}\end{equation}\) is the mutual conductance between the entry and second slot.

Using approximation,
     \(\begin{equation} R_{p,in}(0)=\frac{1}{2G_1}\end{equation}\)                                  ........(8)

The value of  \(\begin{equation} G_1\end{equation}\) is given by,

\(\begin{equation}  G_1=\frac{cos(k*W)+k*W*S_i(k*W)+\frac{Sin(k*W)}{k*W}-2}{120*\pi^2}\end{equation}\)                    ...............(9)

where, k is the wave vector, \(\begin{equation}k=\frac{2\pi}{\lambda}=51.98m^{-1}\end{equation}\)

Substituting the value of W=46.8mm(0.0468m) and k=51.98 per m in equation (9), we obtain \(\begin{equation} G_1=2.855*10^{-3}S \end{equation}\)

Now, using equation (8) we obtain,

\(\begin{equation} R_{p,in}(0)=175ohm\end{equation}\)

Substituting the values of  \(\begin{equation} R_{p,in}(0),R_{f,in}(d)=50ohm\end{equation}\) obtained above in equation (6) and solving for d we get,

\(\begin{equation}d=\frac{L}{\pi}\cos^{-1}(\sqrt{\frac{R_{f,in}(d)}{R_{p,in}(0)}})\end{equation}\)

d=12.697 mm

Now we have to calculate the width of the microstrip feed line with impedance of 50ohm. The width of the microstrip line can be found out using microstrip line calculator such as ADS LineCalc or using the following formula-

\(\begin{equation}Z_f=\frac{120\pi}{\sqrt{\epsilon_{eff}}[\frac{W_f}{h}+1.393+0.667*ln(\frac{W_f}{h}+1.444)]}\end{equation}\)                                     ............(5)

Using the ADS LineCalc the width of microstrip feed \(\begin{equation}W_f\end{equation}\) and length were obtained as below-


\(\begin{equation}W_f=2.291mm\end{equation}\) and \(\begin{equation}L_f=31.949mm\end{equation}\)        

We have calculated all the required dimension for the microstrip patch antenna design.

Now continue reading Design of Microstrip antenna in CST Microwave Studio

AMI Bipolar Encoding Circuit design example

This tutorial shows how to design a AMI Bipolar encoding circuit and simulate it using Orcad Capture and simulate the circuit to obtain the waveform. AMI(Alternative Mark Inversion) is a bipolar encoding technique that was and is still used in older PCM modems. The reason why it is useful is because if there is any bipolar violation then it is detected at the receiver and the error can be detected. The another reason is- because AMI is a bipolar RZ(Return to Zero) scheme it does not contain any DC component and therefore can used in telephony circuit where coupling transformers are used.

Circuit Schematic for AMI Bipolar Encoder
The circuit schematic of the AMI bipolar encoding circuit is shown below:

Fig 1: AMI Bipolar Encoding Circuit Schematic

In the circuit, NRZ unipolar signal is generated using the SIM1 part from the source library. With SIM1 part we can generate a series of binary data with some defind bit period. In this example, a 7 binary bits "1011001" each of 100us(micro second) is generated and looped for 3 times.

The 7486 XOR gate, 7408 AND gate, 7401 AND gate, the transistors Q2N3906 and Q2N3904, the J-K flip flop 74107 are found in eval library. The diode D1N750 is found in the diode library. The rest are resistors, capacitors and voltage sources that are easily found.

After connecting the components as shown in the above figure, the AMI labelled wire is where the AMI bipolar signal appears.

Simulation
Create a new simulation profiel and set the simulation time for 2ms and run it. The resulting waveform is shown below-

Fig 2: AMI Bipolar Encoding Pulse Waveform

Each bit duration is 100microsecond(100us) or 0.1 millisecond(0.1ms). From the waveform we can see that the 1 alternatives between +3V and -3V while 0 remains at 0V.

When clicking the FFT buttom on the probe we get the frequency spectrum of this RZ bipolar waveform as shown below-

Fig 3: AMI Bipolar Encoding with No DC component

From the spectrum figure 3, we can see that the AMI bipolar encoding has no DC component and thus is suitable for transmission over telephone lines and is useful for repeaters where ac coupling is required.

Also see baseband line code analysis blog post and NRZ Unipolar and NRZ Bipolar signal generation

NRZ Unipolar and NRZ Bipolar signal generation in Orcad capture

This cadence orcad tutorial shows how to generate NRZ unipolar signal and convert the NRZ unipolar to NRZ bipolar format. Next it is verified that the NRZ unipolar signal contents DC component whereas the NRZ bipolar signal does not contain any DC component.

The circuit schematic to produce NRZ unipolar and the NRZ bipolar signal is shown below:

Fig 1: Schematic to produce NRZ unipolar and NRZ bipolar

In the schematic the NRZ unipolar signal is generated using the STIM1 part in the source library. The signal is feed into the LF411 opamp which converts the NRZ unipolar to NRZ bipolar signal at its output. Proper biasing voltage is applied to the LF411 opamp as shown in figure.

New simulation profile with run time set to 1ms and leaving the max step size blank gives the following digital waveforms.

Fig 2: Unipolar NRZ and Bipolar NRZ waveform

The upper digital waveform is the bipolar NRZ signal and the lower digital waveform is the unipolar NRZ signal.

Now taking FFT of this signal waveform we get the following respective spectrum,

Fig 3: Spectrum of NRZ Unipolar and Bipolar

The lower NRZ unipolar signal spectrum contains dc component at 0 Hz whereas the NRZ polar signal spectrum contains no dc component at 0 Hz. This is a reason why NRZ encoding is unsuitable for transmission of signal over telephone copper line.

As mentioned above the schematic and simulation was done using orcad capture. Also see the analysis of polar, bipolar, NRZ and RZ blog post.

Analysis of Polar, Bipolar, NRZ and RZ baseband encoding

This is about the baseband signal encoding done in modern digital communication system. Analog signal is converted to Digital binary string such as "01011001010010..." using Analog to Digital Converter(ADC). The digital data from the ADC is then next encoded into the following formats:

Polar NRZ
Polar RZ
Bipolar NRZ(RS-232 serial communication)
Bipolar RZ
(and other formats such as manchester but only the above techniques are considered here)

The reason for encoding the binary data into these above mentioned format is to extract timing information at the receiver, eliminate DC components, produce better SNR, power utilization and combat channel noise.

Polar and Bipolar Signalling
When long strings of 1 or 0 appears in the binary data then polar signalling produces dc components that will not pass through transformers used in telephone circuit. Hence polar signalling is not used in PSTN circuit. Bipolar signalling will not produce DC components when there is long string of 1 or 0 and hence it is used in the PSTN circuit. The polar or bipolar encoding can be further be divided into NRZ and RZ scheme.

NRZ and RZ Format
Polar and Bipolar signals can have both either NRZ and RZ form. The distinguishing feature between the NRZ and RZ form is that NRZ is a full pulse width form of encoding whereas RZ is a half pulse width form of encoding. Whether it is logic 1 or 0 the pulse stays for full bit duration in NRZ encoding whereas the pulse returns to zero from the mid bit duration in RZ encoding.

Because the NRZ has full pulse width while RZ has half pulse width the NRZ encoding produces higher energy pulses compared to RZ encoding technique. This higher energy form is an advantage of NRZ encoding technique. The disadvantageous feature of NRZ is that it is not self-clocking, meaning that timing information cannot be extracted from a signal that is NRZ encoded and hence separate clocking signal must be sent.

As an example, NRZ Polar signal has a DC component unsuitable for transmission over telephone channel. NRZ Biploar signal does not contain any DC component thus is suitable for transmission over telephone channel.

download Microstrip Antenna Design Handbook eBook free

Download Microstrip Antenna Design Handbook eBook free written by P.Bhartia, Inder Bahl, R. Garg, A. Ittipiboo. This book provides a detailed theory and practical aspects of microstrip antenna that is used for short range hand held devices such as mobile handset.

The various antenna described in this book includes the rectangular patches, circular patches, triangular patches, dipole, circularly polarized antenna, antenna array etc. The advantage and disadvantage of microstrip antenna is provided. The substrates onto which these antenna are fabricated and its impact on radiation pattern is also discussed throughout the book. Each antenna type is explained with the radiation pattern calculation, the dimension of the antenna, feed into the antenna, antenna efficiency, gain, bandwidth and surface wave propagation that reduces the radiation efficiency in antenna. After explaining the theoretical aspect, the application and design of each antenna is provided followed by exercises.

This book is for antenna designers and useful for circuit board designers. Also see the CST microwave software for designing antenna.

Download Link:

http://www.filefactory.com/file/cct1tqlodf9/n/bhartia.pdf

What is FM Preemphasis and deemphasis? Orcad Capture Tutorial

FM Pre-emphasis and De-empahsis is a technique in FM communication wherein a circuit called pre-emphasis is used at the FM transmitter to emphasis the high frequency components of the message signal before modulation and at the receiver after demodulation and discrimination a circuit called de-emphasis is used to suppress(or de-emphasis) the high frequency components of the message signal that was emphasized at the transmitter. That is the de-emphasis circuit restore the high frequency components. The reason for doing this is to lower the noise power and improve the SNR.

This blog post illustrates the concept and working principle of FM pre-emphasis and de-emphasis used in FM transmitter and receiver with simple circuits constructed in Orcad Capture. It is shown via simulation how high frequency component is emphasized in the transmitter and De-emphasized at the receiver.

Pre-emphasis:

Consider as an example the circuit schematic (Fig1) below that has a source of message signal with components at two different frequencies, one at 1kHz and the other at 15kHz. 

We want to emphasize the high frequency component magnitude at 15kHz using a pre-emphasize circuit.

Fig1: Signal Source with two frequencies(1kHz and 15kHz)

The output signal waveform which contains the two frequencies is shown below-

Fig2: Waveform of signal with two frequencies(1kHz and 15kHz)

 The corresponding waveform spectrum is shown below. Notice that both frequency components have equal magnitude.

Fig3: Spectrum of two frequency signal at 1kHz and 15kHz

Now, lets add a pre-emphasis circuit which is an RC HPF to the above circuit above as shown-

Fig4: Pre-emphasis circuit

The waveform of the signal at the output of the pre-emphasis is shown below-

Fig5: Waveform of output signal of pre-emphasis

Now the spectrum of this signal is shown below:

Fig6: Spectrum of the output of pre-emphasis

Now comparing the output signal spectrum of Fig 3 and Fig 6 we can see that the magnitude of the input signal at 15kHz is retained or equivalently emphasized while the magnitude of frequency component at 1kHz is decreased. This shows clearly that the RC HPF performs the pre-emphasis of the input signal.

De-emphasis:

Now at the receiver RC LPF is used to de-emphasis the high frequency component and thus restore the original signal. The circuit for the de-emphasis is shown below:

Fig7: Pre-emphasis and De-emphasis Circuit

The signal waveform of the transmitted and received signal is shown below-

Fig8: Signal waveform at the output of pre-emphasis and de-emphasis

The output spectrum of the pre-emphasis and the output spectrum of the de-emphasis circuit is shown below-

Fig9: Ouput spectrum of pre-emphasis and de-emphasis circuit

The Fig9 clearly shows that the pre-emphasis circuit does emphasis the high frequency component and that the de-emphasis circuit restore the high frequency component of the input message signal.

How to generate FM signal in Orcad capture

FM signal generation is required in many circuit design analysis. Instead of creating circuit level FM generator for example using BJT or FET transistor, one often requires just the FM signal to fed into the desired designed system. FM signal in orcad capture can be generated in many ways. One of the simple way is the use the VSFFM part which can be found under the source library.

 The circuit for FM wave using the VSFFM FM generator is shown below:

 In this VSFMM source, we can specify the modulating signal amplitude, the frequency of the modulating signal, the frequency of the carrier signal and the frequency modulation index. In this simple example the magnitude of modulating signal is VAMP=1V, modulating signal frequency, FM=1.5kHz, carrier signal frequency, FC=10kHz.

To check the Frequency modulated wave, we simulate and get the output as shown in the figure below:

To view the FM signal spectrum, we just click the FFT ion on the probe display in the Orcad capture. This gives us the following FM signal spectrum:

The spectrum clearly shows the absence of carrier part at the 10kHz.

Finally, the power developed on the resistor due to the FM signal is shown below:

See other Cadence Orcad Video Tutorials