Reception and Detection PDF

IT2311 RECEPTION AND DETECTION Communication Receivers (Frenzel, 2022) The transmitted signal in radio communication systems becomes weak when it reaches the receiver. This happens when the signal has traveled over a long distance and has instances of pickin...

IT2311 RECEPTION AND DETECTION Communication Receivers (Frenzel, 2022) The transmitted signal in radio communication systems becomes weak when it reaches the receiver. This happens when the signal has traveled over a long distance and has instances of picking up noise of various kinds. Thus, radio receivers must provide the selectivity and sensitivity that allow full recovery of the original signal. Selectivity is the communication receiver’s ability to identify and select a desired signal from others present in the frequency spectrum. At the same time, sensitivity lets communication receivers pick up weak signals and provide sufficient amplification to recover the modulating signal. Selectivity In a receiver, it is obtained using tuned circuits such as LC-tuned circuits (also called resonant circuits) made up of an inductor and a conductor. Selectivity is achieved in a signal by carefully controlling the Q or the quality factor of the LC-tuned circuit. The ideal bandwidth (BW) or the frequency range of the selectivity curve must be wide enough to pass the signal and its sidebands and narrow enough to eliminate or attenuate signals on adjacent frequencies. Figure 1. Retrieved from Frenzel, L. (2022). Principles of electronic communication systems: Fifth edition. McGraw Hill. Figure 1 is a selectivity curve of an LC-tuned circuit. It shows a gradual rate of attenuation of an LC-tuned circuit and the attenuation of the adjacent signals (𝑓1 and 𝑓2). If the Q is increased, the bandwidth will be further narrowed, thus improving the steepness of attenuation. But if the bandwidth becomes too narrow, it starts to attenuate the sidebands, causing information loss. The formula for the bandwidth of a tuned circuit is: 𝑓𝑟 𝐵𝑊 = 𝑄 06 Handout 1 *Property of STI  [email protected] Page 1 of 7  IT2311 Wherein: 𝑓𝑟 = resonant frequency 𝑄 = quality factor Figure 2. Retrieved from Frenzel, L. (2022). Principles of electronic communication systems: Fifth edition. McGraw Hill. Figure 2(a) shows the ideal receiver selectivity curve, which has perfectly vertical sides, but this cannot be easily obtained with just tuned circuits. Cascading circuits or crystal and ceramic filters should be used to achieve improved selectivity. The shape factor is used to express the steepness of the skirts. Skirts, or skirt selectivity, are the sides of a tuned circuit response curve. In Figure 2(b), the shape factor is the ratio of the 60-dB down bandwidth to the 6-dB down bandwidth or the bandwidths of the receivers at two levels of attenuation. The bandwidth at the 60-dB down points is 𝑓4 − 𝑓3 while the bandwidth of the 6-dB down points is 𝑓2 − 𝑓1. This results in the formula: 𝑓4 − 𝑓3 𝑆ℎ𝑎𝑝𝑒 𝑓𝑎𝑐𝑡𝑜𝑟 = 𝑓2 − 𝑓1 Such as if the 60-dB (𝑓4 − 𝑓3 ) bandwidth is 9 𝑘𝐻𝑧, and the 6-dB (𝑓2 − 𝑓1) bandwidth is 4 𝑘𝐻𝑧, the shape factor is: 𝑓4 − 𝑓3 𝑆ℎ𝑎𝑝𝑒 𝑓𝑎𝑐𝑡𝑜𝑟 = 𝑓2 − 𝑓1 9 𝑘𝐻𝑧 𝑆ℎ𝑎𝑝𝑒 𝑓𝑎𝑐𝑡𝑜𝑟 = 4 𝑘𝐻𝑧 𝑺𝒉𝒂𝒑𝒆 𝒇𝒂𝒄𝒕𝒐𝒓 = 𝟐. 𝟐𝟓 The shape factor of Figure 2(a) is 1, and the lower the shape factor is, the steeper the skirts get and the better the selectivity. Sensitivity This is mainly a function of overall gain, which is the factor wherein an input signal is multiplied to produce the output signal. Here, the higher the gain of the receiver, the better the sensitivity. SNR, or signal-to-noise ratio, also affects the sensitivity of a receiver. 06 Handout 1 *Property of STI  [email protected] Page 2 of 7  IT2311 A method of expressing the sensitivity of a receiver is by obtaining the minimum discernible signal (MDS). It is the input signal level that is almost equal to the average noise floor of the receiver. A noise floor is the internally generated noise value. MDS is expressed in dBm, which is the absolute power level in decibels with reference to one milliwatt (𝑚𝑊). 𝑃 𝑑𝐵𝑚 = 10 log 1 𝑚𝑊 Wherein: 𝑃 = Power in watts (𝑊) 1 𝑚𝑊= another used measure of receiver sensitivity For example, most receivers have an input impedance of 50 𝛺 in their antenna. A signal with 1 𝜇𝑉 produces a power P across the 50 𝛺 of: 𝑉2 𝑃= 𝑅 (1 𝜇𝑉)2 𝑃= 50 𝛺 1𝑥10−12 𝑃= 50 𝛺 𝑷 = 𝟐𝒙𝟏𝟎−𝟏𝟒 𝑾 To express in dB: 𝑃 𝑑𝐵𝑚 = 10 log 1 𝑚𝑊 2𝑥10−14 𝑊 𝑑𝐵𝑚 = 10 log 0.001 𝑊 𝑑𝐵𝑚 = 10 log(2𝑥10−11 ) 𝑑𝐵𝑚 = 10(−10.6990) 𝒅𝑩𝒎 = −𝟏𝟎𝟔. 𝟗𝟗 𝒅𝑩𝒎 Classification of Radio Emissions (Electronics Notes, 2023) Radio emission code designations are assigned to many types of signals that can be used to interpret a detected signal easily. The code is comprised of a capital letter, a number, and lowercase subscript letters. The capital letters are placed first and define the modulation type, the numbers are placed next and define the transmitted information type, and the lower subscript letters are used for more specific definitions and placed under the number. 06 Handout 1 *Property of STI  [email protected] Page 3 of 7  IT2311 Classification Symbol Definition Modulation Type A Amplitude Modulation F Frequency Modulation P Phase Modulation Information Type 0 Carrier ON only, no message (e.g., radio beacon) 1 Carrier ON/OFF, no message (e.g., Morse code, radar) 2 Carrier ON, keyed tone ON/OFF (code) 3 Telephony, message as voice or music 4 Fax, non-moving graphics (e.g., slow-scan TV) 5 Vestigial sideband (e.g., commercial TV) 6 Four-frequency duplex telegraphy 7 Multiple sidebands with different messages 9 General (all others) Subscript/Specifics None Double sideband, full carrier a Single sideband, reduced carrier b Double sideband, no carrier c Vestigial sideband d Carrier pulses only, pulse amplitude modulation (PAM) e Carrier pulses only, pulse width modulation (PWM) f Carrier pulses only, pulse position modulation (PPM) g Quantized pulses, digital video h Single sideband, full carrier j Single sideband, no carrier Table 1. Radio emission code designations For example, an AM voice signal heard on an AM broadcast band has the code A3. A = Amplitude modulation 3 = Telephony, message as voice All variations of AM using voice or video intelligence have the A3 code designation, such as: 1. DSB two sidebands suppressed carrier telephony: 𝑨𝟑𝒃 Modulation Type = DSB = A Information Type = Telephony = 3 Specifics = two sidebands, suppressed carrier = b 2. SSB vestigial single sideband, 30% pilot carrier: 𝑨𝟓𝒂 Modulation Type = SSB = A Information Type = Vestigial = 5 Specifics = single sideband, 30% pilot carrier = a 3. SSB single sideband suppressed carrier from voice: 𝑨𝟑𝒋 Modulation Type = SSB = A Information Type = voice = 3 Specifics = single sideband, suppressed carrier = j 06 Handout 1 *Property of STI  [email protected] Page 4 of 7  IT2311 ITU Emission Designation is another system used to describe a signal but includes more variations. It is used by the standard organization International Telecommunication Union (ITU), which agreed on a set of codes to describe a format and modulation for a radio transmission easily. Designators Format This format allows anyone using the ITU system to quickly identify the parameters of a specific transmission. BBBB 123 45 Wherein: BBBB = defines the bandwidth 1 = a capital letter indicating the modulation type 2 = a number indicating the modulating signal type 3 = another capital letter indicating the information type being transmitted 4 = a letter indicating practical details of the transmitted information, optional 5 = a letter indicating details about any multiplexing, optional Bandwidth Designator List This has the format of three (3) digits that express the significant figures and a capital letter to express the decimal point, such as: 2𝑀25 = 2.25 𝑀𝐻𝑧 6𝐾00 = 6 𝑘𝐻𝑧 Wherein: H = Hertz (𝐻𝑧) K = kilohertz (𝑘𝐻𝑧) M = Megahertz (𝑀𝐻𝑧) G = Gigahertz (𝐺𝐻𝑧) 06 Handout 1 *Property of STI  [email protected] Page 5 of 7  IT2311 Classification Symbol Definition Modulation Type N Unmodulated Carrier A Amplitude Modulation B Single Sideband C Vestigial Sideband D Combination of AM and FM or PM F Frequency Modulation G Phase Modulation H Single Sideband Full Carrier J Single Sideband Suppressed Carrier K Pulse Amplitude Modulation (PAM) L Pulse Width Modulation (PWM) M Pulse Position Modulation (PPM) N Unmodulated Carrier P Series of Pulses with no modulation Q Sequence of Pulses, Phase or Frequency Modulation R Single Sideband with reduced/variable level carrier V Combination of pulse modulating methods W Combination of any of the above X Cases not covered by the definitions above Modulating Signal 0 None Type 1 Digital, single channel, no modulation 2 Digital, single channel, with modulation 3 Analog, single channel 7 Digital, two or more channels 8 Analog, two or more channels 9 Analog plus digital Transmitted N None Information Type A Telegraphy, human B Telegraphy, machine C Fax D Data transmission, telemetry, control signals E Telephony (human voice) F Video, TV W Some combination of any of the above Table 2. ITU emissions designations. Examples: 1. 3k24 Amplitude-modulated Analog TV with 10 channels = 𝟑. 𝟐𝟒 𝒌𝑯𝒛 A8F BBBB = 3.24 𝑘𝐻𝑧 Modulation Type = Amplitude-modulated = A Modulating Signal Type = Analog with 10 channels = 8 Transmitted Information Type = TV = F 06 Handout 1 *Property of STI  [email protected] Page 6 of 7  IT2311 2. Phase modulation multi-channel with quantized or digital information = G7W Modulation Type = Phase Modulation = G Modulating Signal Type = multi-channel = 7 Transmitted Information Type = Quantized or digital information = W References: Electronics Notes (2023). ITU radio emission types/modulation designators. [Web Article]. Retrieved on August 16, 2023, from https://www.electronics-notes.com/articles/radio/modulation/itu-radio-emission-designators.php Frenzel, L. (2022). Principles of electronic communication systems: 5th ed. McGraw Hill. 06 Handout 1 *Property of STI  [email protected] Page 7 of 7