Module 8 Theory dbs AM Detector Ring Modulator Receiver Chain Functional Blocks Parameters
Decibel (db) The term db or decibel is a relative unit of measurement used frequently in electronic communications to describe power gain or loss. In general: Positive value indicates Gain Negative value indicates Loss In terms of power In terms of Voltage
dbw Power Values (memorise) dbw Watts -6 = 0.25 W -3 = 0.5 W 0 = 1 W 3 = 2 W 10 = 10 W 20 = 100W 23 = 200 W 30 = 1000 W Powers of 10 10 0 = 1 10 0.3 = 2 10 0.6 = 4 10 1 = 10 10 2 = 100 10 3 = 1000 10 6 = 1000000
Db Gain and Loss The following can be expressed in dbs: Antenna Gain Amplifier Gain Attenuator Path Loss
Stage Gain and Loss 10 db (x 10) -3 db (1/2) -3 db (1/2) 23 Db (x 200) 10 db (x 10) -3 db (1/2) -3 db (1/2) 23 db (x 200) --------------------------------------------- 27 db (x 500)
Decibel Reference Quantities Decibels are sometimes quoted relative to a reference quantity dbw = Decibels against a reference of 1 Watt dbm = Decibels relative to reference power of 1mW dbi = Antenna gain relative to a perfect ISOTROPIC antenna. dbd = Antenna gain performance expressed relative to a DIPOLE
Ring Modulator
Voltage Diode Detector Circuit Diode D1 rectifies AC into DC C1/R1 is a lowpass filter - filters out the RF D1 conducts on positive half-cycle only Output D1 RF Input C3 100pF R1 100k Demodulated AF Output Time Input
Simple Diode Detector AM
AM Envelope Detection Demodulation of a modulated audio signal Detector output follows envelope of RF Otherwise known as an envelope detector Envelope Modulated RF Demodulated Audio
Receiver Chain & Parameters
Purpose The purpose of a radio receiver is to acquire a radio (RF) signal containing information in the form of modulation and to process it into an audible (AF) sound A receiver should have: Sensitivity - to resolve weak signals satisfactorily without introducing noise Selectivity - to separate the required signal from unwanted or interfering ones
Essentials Amplify weak signal from antenna Select required signal Filter out unwanted signals Demodulate (detect) signal Amplify audio output
Types of Receiver TRF Single conversion superheterodyne Double conversion superheterodyne Direct conversion
TRF Receiver Earliest receiver very basic not used much today Simplicity Limited Selectivity Poor Sensitivity
Superheterodyne Receiver Most common form of receiver The incoming signal is converted to a fixed Intermediate Frequency (IF) Receiver selectivity and gain are determined at this fixed frequency
Double Conversion Superheterodyne The incoming signal is first converted to an I.F. of say 10.7 Mhz or 1.6 Mhz where it is filtered Then converted to final I.F. (455 Khz) for further filtering and amplification
Direct Conversion Can be thought of as a TRF with product detector Superhet with an I.F. of 0 khz Easily to build receives SSB/CW
RF (Radio Frequency) Amplifier Amplifies weak input signal Provides some selectivity Should be low noise Has a manual or automatic gain control (AGC) or switched attenuator to prevent overload by strong signals
Local Oscillator Produces a local signal offset from incoming (RF) signal) by Intermediate Frequency (IF) Combined with incoming signal in Mixer to produce the IF. Has variable tuned circuit or may be digitally synthesised
Mixer Has two inputs RF signal and Local Oscillator (LO) One output Intermediate Frequency (IF) A Mixer produces an output equal to the difference of the input signals
Mixer f out = f o - f s Difference of two frequencies 455 khz = 7455 7000 khz
IF Amplifier IF Intermediate Frequency Provides gain and Selectivity at IF Fixed selective tuned circuits at IF Usually preceeded by a crystal or ceramic filter
Detector (Demodulator) Recovers modulation from the signal Amplitude modulation (AM) detector is often a simple diode rectifier
Product Detector For CW and SSB a product detector (mixer) is used SSB Oscilllator on IF frequency is mixed with IF signal to give audio as difference (Carrier Insertion Oscillator, CIO) CW Oscillator offset from IF frequency by say, 800Hz, is mixed giving an audible beat note (Beat Frequency Oscillator, BFO)
FM Detector In the case of FM (frequency modulation) the modulation causes the carrier frequency to vary Amplitude variation due to propagation or noise is first removed by a LIMITER if a Foster-Seeley Discriminator is used as a demodulator Ratio detector does not require a Limiter
Audio Amplifier Amplifies the audio signal Has manual gain control Provision for loudspeaker, headphone or line output
Automatic Gain Control (AGC) Automatically controls receiver gain to maintain constant output level Gain of RF and IF amplifiers reduced proportionate to signal strength Prevents overload of amplifier stages May drive S-meter
S meter Indicates the strength of the received signal usually as measured at the output of the I.F. stage or detector (audio-derived) Calibrated in S-points (0 9) and 10 db points above S9 No real standard. S9 about 50μV. One S point about 6dB. Varies with receiver and band
S Meter Readings (Guide only) S Reading V (50 ) dbm db above 1 V S9 + 10dB 160 63 44 S9 50.2-73 34 S8 25.1-79 28 S7 12.6-85 22 S6 6.3-91 16 S5 3.2-97 10 S4 1.6-103 4 S3 0.8-109 -2 S2 0.4-115 -8 S1 0.2-121 -14
Squelch Used primarily in FM receivers Suppresses audio output in the absence of sufficiently strong input signal and thus excludes lower-power signals or noise present on or near the receiver frequency Has a control knob which sets the threshold level at which signals will open the audio output
Receiver Parameters Important performance measures for receivers Frequency stability Selectivity Bandwidth Sensitivity Dynamic range Signal-to-noise ratio (SNR) Effect of RF amplifiers & pre-amps
Frequency Stability Frequency stability is the same as for transmitters Accuracy of tuning to an entered or displayed frequency Ability to remain on frequency without drifting off Often given in ppm parts-per-million 1ppm error at 28MHz is 28Hz.
Receiver Characteristics Adjacent Channel Selectivity Ability of receiver to separate signals on closely adjacent frequencies (channels) is determined by its selectivity Switching to a narrow passband can reduce interference Usable passband width is determined by the bandwidth of the mode to be received
Amplitude Selectivity Selectivity is the ability to separate the wanted signal from nearby unwanted signals (other stations) Unwanted signal (strong) Filter response Wanted signal (weak) Frequency MHz
Amplitude Selectivity Measures Measures of selectivity 60dB bandwidth adjacent channel rejection ratio (VHF, UHF channelised) but how far away is the next channel (12.5kHz?) For SSB, may specify opposite sideband rejection 0dB - Filter response -60dB - 60dB bandwidth Frequency MHz
Amplitude Bandwidth Band of frequencies which the receiver should accept Eg. CW (A1A) morse typically 300Hz SSB uses 2.5 to 3kHz VHF FM typically 7.5 or 15kHz Usually 3dB BW specified, but not always! 0dB - -3dB - Filter response 3dB bandwidth Frequency MHz
Receiver Characteristics Sensitivity Capacity to amplify signals is determined by sensitivity Weakest signal discernible limited by noise generated externally and in early stages of the receiver where signal is still weak Minimum Discernable Signal (MDS) is the smallest input signal that will produce a detectable signal at the output
Sensitivity Sensitivity defines the limit of detection of weak signals. Receivers must have enough gain to bring weakest signals to comfortable level. the gain does not define sensitivity Sensitivity is determined by 2 factors: Bandwidth of the receiver the wider the bandwidth, the more noise power it lets in Noise figure of the receiver front-end a noisy receiver needs more signal to overcome the noise Receivers bandwidth should match the transmitted bandwidth. so as not to exclude any signal or accept unnecessary noise.
Receiver Characteristics S/N ratio Measure of sensitivity signal to noise ratio Defined by stating the minimum signal voltage at input to produce an output with a certain ratio of signal above noise level in a specified bandwidth at a particular frequency Typical 0.5μV for 10dB S/N in 3kHz for SSB at 28 MHz
Receiver Characteristics Noise Figure/Factor Noise figure is the degradation in signal to noise ratio as a signal passes through an amplifier or system (receiver) Vital amplifier and Rx characteristic at VHF/UHF, less so at LF
Receiver Characteristics Stability The ability of a Rx to remain tuned to a particular frequency is determined by its stability Depends on the electrical and mechanical stability of tuned circuits, particularily oscillators Effect of heat on tuned circuits and components Less of problem with modern semiconductor circuits than older valve equipment
Receiver Characteristics Image Frequency Sig Lo IF Sig Lo IF 7000 7455 = 455kHZ 7910 7455 = 455 KHZ In superhet, mixer can produce an output at IF for frequencies on either side of local oscillator Thus two signals separated by twice IF appear in passband Can only be removed by selectivity before mixer.
Receiver Characteristics Image Frequency Increasing IF separates image frequency further from required signal, easing requirements However with higher IF generally more difficult to remove adjacent channel Double superhet used with high first IF and low second IF
Amplitude Image frequency LO frequency RF frequency Image Frequencies Image is normally 2x the IF away from the RF frequency On the same side as the local oscillator Image has a band of frequencies that corresponds to tuning range IF IF Front-end RF filter may look like this Frequency MHz
Receiver Characteristics Densensitization Strong signals not far removed from the wanted signal may cause desensitization (SSB) or blocking (CW) of early receiver stages May come from local amateur stations or strong stations well outside the I.F. passband Amplifier overdriven so response to weak signal reduced
Receiver CharacteristicsIntermodulation Intermodulation distortion (IMD) is where two signals mix together due to non-linearity and produce spurious signals Raises receiver noise floor due to large number of products
Receiver Characteristics Cross-modulation Non-linearity may cause stage to act as a modulator AGC system may cause gain to vary with the modulation of the interfering signal Modulation of strong unwanted signal is impressed on the wanted signal
Receiver Characteristics Reciprocal Mixing Noise sidebands (phase noise) on receiver local oscillator mix with an incoming signal to produce a response in the IF passband This signal appears as noise Design of LO is important. A major problem with frequency-synthesised radios
Interference from Rx In superhet and direct conversion Rx, oscillators have potential to radiate and cause interference if not screened Reasonable standard of design/construction eliminates problem
SSB/CW Receiver
FM Receiver