Radio Frequency Theory

For Surveillance Systems




Radio systems exist in form of transmitters and receivers or combined transmitters and receivers also called transceivers. The radio frequency (RF) receiver must have the ability to receive the intended signals only. Intended signals include data and audio and other forms of meaningful signals as well. The Transmitter and receiver must both be able to tune to one another and secure the signal being communicated. There are various ways of accomplishing this ultimate goal of radio communication. In the old days radio communication security was not evolved and security was not as urgently needed as it is today. There are probably tens and hundreds of radio signals around you now - not utilizing security measures and radio tuning electronic circuits will make radio communication very inconvenient if not impossible. The radio history started with very simple forms of communication circuitries such as manually driven continuous wave (CW) data transmitters, tuned radio frequency circuits, and the similar. Among the latest advents and greatest accomplishments of wireless data communications is the spread spectrum technology.


Fact: Data or audio signals can’t travel long distance over air or cables at their own; they need the help of higher energy and higher frequency signal that can travel un-exhausted long distance at its own. The higher energy higher frequency signal will give data or audio signals a safe ride to its destination. The data or audio signals may vanish prior to reaching their destination if they travel at their own. This is the reason the higher energy higher frequency signal is called the carrier – it carries the audio or data un-exhausted to their destination. The act of riding the data or audio signal on the carrier signal is called modulation. The data or Audio is called the modulating, and the carrier is called the modulated.


Continuous Wave CW transmitters and receivers usually use the sinusoidal waves to transmit and receive signal. The following wave resembles the cosine wave which is the sine wave shifted by 90 degrees:


This wave has a frequency of 868 MHz. The above CW signal can be modulated with data or voice. The CW wave is called in this case the carrier wave. The signal to be transmitted or communicated is not the carrier but is the data or voice signal and usually has a much lower frequency than the CW carrier signal. For example a 40 MHz data signal is to be communicated between the main office and the security system inside the merchandise store:



The radio transmitter will combine the data and carrier signals together and send the resulted combined signal as following:



Notice that the above modulated carrier has two frequencies, unlike the original carrier signal and the data signal. An ON-OFF Keying (OOK) modulation is another method used to send data over the air or cables. This is also called Amplitude Modulation (AM). The same 868 MHz carrier above could be used for example to send the letter A from the transmitter to the receiver. The letter A data is represented by the ASCII value of 41 hex of binary digital code 01000001:


The binary 41 data above could be used to modulate the following carrier wave signal of 915 MHz:



The RF transmitter will send to the air the following modulated signal via the antenna:

The above method is not pretty secured against hackers and thieves so data scrambling, encoding and decoding schemes along with random number generators could be used to establish secured communication. For example the word “HELP” could be wirelessly sent as “z4&0d$” instead. One issue with that is the speed may then be reduced. To overcome this issue a modulation scheme called IQ could be used to speed up the communication and processing capabilities for a dramatically reduced latency time. The IQ stands for In Phase / Quad Phase, where Quad Phase refers to the 90 degrees of phase shift in the carrier signal. 


The above example transmitted the zeros and ones bits of the letter “A”. The presence of the carrier represented the bit 1 and the absence of the carrier represented bit 0.

Fact: Communication devices are required to perform fast and secure transactions. Battery operated wireless devices need not spend too much energy and time transmitting data for long battery life. It is also required that wireless devices need to operate in very noisy environment that is congested with many other wireless devices. Among the commonly used communication modulation schemes are Direct Sequence Spread Spectrum (DSSS) and the Frequency Hopping Spread Spectrum.


Depending on the system used the rules may vary; for example the presence of the carrier could alternately represent bit 0 and absence of carrier represent bit 1. Whenever the presence of carrier is used to present bit one or zero the number of the carrier cycles per bit may also vary depending on the system rules. The larger the number of carrier cycles per bit used for this presentation the longer the processing time is required and therefore the system latency may be affected. Also more power will be required to transmit data for larger number of cycles. There are many advantages and disadvantages to using larger number of cycles per bit transmitted. Wireless features that differentiate systems from one another include power consumption, modulation scheme, baud rate or communication speed, communication errors and error correction, and communication immunity against adverse environment.   




Radio Frequency Heterodyning – Mixing:


Radio and wireless systems are used to send data and audio signal to remote places. Different methods are used to enable this transaction. These methods have advantages and disadvantages. The application usually dictates the type of system to use. Applications range from opening your car by the key fob, operating the mobile phone, listening to the music, watching the highway traffic remotely, withdrawing money from your account, reading the temperature of the backyard, and so on. The application carries with it requirements that demand certain wireless system characteristics. These characteristics include cost and affordability, safety and security, immunity against electrical noise such as high power machines, operating in hostile environment or congested radio traffic, sending guidance data command to a guided weapon, and so on. It is thus obvious that not all wireless systems use same electronics and technology. 


Tip: We will assign to the data and audio signals the term “signal” only. This signal could also resembles any other source of meaningful signal including but not limited to video signal, temperature signal, pressure signal, light signal, and so on. All of these signals are to be communicated and transmitted between two points at a time.


The most commonly used modulation methods for radio and wireless communications are the Amplitude Modulation (AM) and Frequency Modulation (FM).  Both of these modulation methods share the same modulation block diagram:

















The above diagram is an overview of how the antenna signal (also called RF signal) is mixed (Heterodyned) with an internally generated signal by an RF circuit called the Local Oscillator. When both of the local oscillator signal (LO) and the Radio signal (RF) are mixed by the RF Mixer circuit the result is called Intermediate Frequency (IF) signal:


RF (MIXED with) LO > IF


The IF signal has new frequency equals to the Local Oscillator frequency less than the Radio RF frequency.


IF frequency = LO frequency – RF frequency


The LO signal frequency is generated by a special LO electronic circuit that allows the LO frequency to be varied either electronically or manually. This is similar to the tuning circuit in your radio which is connected to the knob you select the radio station from.  By rotating the knob or pressing on the channel buttons you are actually changing the LO local oscillator frequency. The LO frequency is usually higher than the incoming RF frequency so the IF frequency is positive in value. The radio circuit deals with one single IF frequency only. This unique IF frequency (signal) is used to extract the information needed such as security data or voice. In order to provide the fixed IF signal the LO frequency is brought up to a frequency higher than the desired channel frequency by a fixed value. For example assume that the system’s IF of 2 MHz have channels with assigned frequencies as follow:



























To select channel 1 you will need to bring the LO frequency to 12 MH since LO (12) – RF (10) = (IF) 2, and in order to tune to (select) channel 3 you will need to bring the LO frequency to 32 MHz.



Fact: The data or audio signal is usually of much lower frequency than the carrier frequency.  Human can only deal and interact with the common audio and data low frequency signals, and other signals such as visual, LCD, and similar. Radio science deals with the process of frequency conversion between the low to high and back to low frequency as the final beneficiary of this process is human.



Basic Modulation Techniques:


The origin of all radio modulation techniques stem back to the AM and FM basic modulation. AM modulation changes the carrier frequency according to the data signal while the FM changes the carrier frequency according to the data signal. The information signal varies with time such as it could increase, decrease, or vanish to zero. The following signal represents a temperature increase as function of time. The temperature starts from zero at the bottom left and increases as time goes by. This analog temperature signal is converted to a voltage signal voltage(t) where the voltage tracks temperature as function of time:    




The above temperature analog signal can modulate a fixed carrier signal below:  



In AM modulation the carrier frequency is kept the same but the amplitude of the carrier is changed according to the temperature signal as follow:


In FM modulation the amplitude of the carrier is kept the same but the carrier frequency is changed according to the temperature signal as follow:



The temperature reading is then transmitted in form of radio signal and picked by the radio receiver. The receiver then extracts the information from the radio signal (AM or FM) by analyzing the variation in amplitude or frequency. The same rules of amplitude and frequency variations are followed when transmitting or receiving the data signal.


From the above methods of AM and FM modulations different forms of radio modulations are derived. The ON-OFF Keying (OOK) technique described above is a derived form of AM modulation. Frequency Hopping Spread Spectrum (FHSS) is an advanced form of FM modulation.  Other modulation techniques such as Frequency Shift Keying (FSK), Phase Shift Keying (PSK), and In phase – Out of phase (IQ) modulation, Quadrate Phase Shift Keying (QPSK) modulation are also derived forms of FM modulation. The key is that the sine wave is divided into equal number of sections (4, 8, 16, 32) and the correct section is used to associate the data symbol to. Phase modulation and frequency modulation are related to one another and many rules equally apply to both of them. The sine wave can be divided into 4 quarters (Quad Modulation) each of the quad having 90 degrees. Here is how the sine wave is divided into 4 equal sections   



Details of the sine wave quads are used to carry useful information in the radio waves. These quads are transmitted wirelessly in series from the transmitter to the receiver. The IQ modulation could be use for very high speed communication. Each quad is assigned for example two digital bits as follow:




The digital 010110111100 code is radio transmitted by the following radio signal:



By doing so the power consumptions is minimized, communication speed is multiplied, but the possibility of transmit error is increased. Here is how the above radio signal may look like using the oscilloscope:



Different systems may have different rules and assign different bits for quad symbols.




End of Radio Frequency Theory for Surveillance Systems



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