Radio Frequency - Receiver

In a receiver processes modulated signals are induced into antenna and the receiver electronics delivers a reproduction of the original modulating tone, audio or video. The signal can then be amplified to drive a reproducing device such as a loudspeaker, earphone, tape recorder or video monitorGenrally reception is the induction of EM waves into the antenna to produce a voltage in that antenna, and amplifying it. Selection is tuning of one particular frequency from all the signals induced into the antenna. This is called selectivity. The better the receiver is at differentiating between the desired and undesired frequencies, the better the selectivity rating. Sensitivity of a receiver is based on its noise figure, the minimum required S/N ratio for detection of the modulation, and the thermal noise of the system. The equation for the minimum input signal is noted below:

S = NF + n0 + S/N

where S is the minimum input signal required (dBm), NF is the noise figure of the receiver, S/N is the required output signal to noise ratio (for adequate detection, usually based on the acceptable bit error rate), and n0 is the thermal noise power of the receiver (dBm).For sake of simplicity, we will estimate the required output S/N ratio (Manchester data) to be 5dB. To calculate S, we still need n0. n0 is defined as:

n0 = 10log10 (k T B / 1E-3) in dBm

where k is the Boltzmann's constant (1.38 E-23), T is temperature in Kelvin, and B is the noise bandwidth of the system. At room temperature (T = 290�K) in a 1Hz bandwidth, n0 = -174dBm (commonly expressed as = -174dBm/Hz).For a 300kHz IF bandwidth, n0 is calculated to be .119dBm.Detection is the action of separating the low frequency audio or video signals from the higher frequency carrier. This is also called a demodulator. Perhaps the simplest way to study receiver technology is to keep this in mind: whatever is done to change the modulating signal at the transmitter, it must be undone at the receiver. Propably the simplest form of receiver is rystal radio set with one tuned circuit and one rectifier as the demodulator for AM signal.The first radio receivers were Tuned Radio Frequency (TRF) receivers. In TRF these receivers, all the RF amplification is carried out at the incoming received frequency. This kind of receivers ver bulky to use, because in order to be able to tune to different signals, all of these RF stages must be tunable in step with each other.A dramatic improvement was made in receiving efficiency with the discovery and introduction of the superhetrodyne receiver. Basically, the output from a variable "local" oscillator in the receiver is mixed or hetrodyned with the signals from incoming radio transmissions. In mixing an incoming radio signal with the local oscillator signal, there will be present at the output the original two signals plus the sum and the difference signals of the two, plus harmonics of these sum and difference signals. For instance, in receiving an FM station (Station "A") on 99.7 MHz, the local oscillator could be tuned to 89 MHz. The only modulation product we are interested in is the difference frequency, 10.7 MHz, which is called the I.F. or intermediate frequency. 10.7 MHz is the normal FM receiver intermedia frequency (some other radio types use different frequencies). The advantege of fixed I.F. is that this tuned amplifier stage can be made to operate at fixed frequency, so it is much easier tomake high perfomance than variably tuned stages. Some receivers are "double-superhetrodyne", and this means that the output from the first intermediate frequency is hetrodyned with a second (fixed) local oscillator to produce a second I.F. for further amplification. It is possible to produce substantially greater selectivity by this means. For example 455 KHz is a common second I.F. for VHF radiotelephones operating on NBFM, and requiring the extra selectivity required for the narrower channel spacings. 455 KHz is also the normal (single) I.F. for AM receivers.In most superhet receivers, especially those in the commercial broadcast range, the I.F. is constant. Commercial AM I.F. is 455 KHz and FM I.F. is 10.7 MHz. For brodcast television signals an IF frequency of around 39 MHz is quite common.The detector, or demodulator, allows the extraction of the original modulating signal (audio). It essentially pulls the intelligence from the I.F., leaving a usable audio signal by filtering out the I.F. carrier. Different types of demodulators are needed for different modulations (for example AM and FM need a different kind of modulator).Sensitivity is the ability to receive weak signals and amplify them to a usable level. Most quality receivers will be able to amplify signals (lower) than 5 microvolts. That is, the smallest discernible signal is 5 uv in amplitude. Increasing the sensitivity in a receiver can be accomplished by adding more stages of amplification prior to demodulation (but there are physical limitations how much amplification can be done before component noise levels kick in). The signal-to-noise ratio is a comparison of the signal power to the noise power. This result should be high as possible. Noise should be kept to a minimum as it tends to cover up the weaker signals.Selectivity is the ability of a receiver to tune to a particular station without any other signal interfering with the reception. Selection of a proper I.F. frequency is important to image rejection. In most cases, the larger the I.F., the better the rejection. In some receivers, the oscillator frequency is higher than the received signal; in some cases, lower. Image frequencies are always taken into account in the design of all receivers.One of the most important aspect of a superhet receiver is the constant intermediate frequency. Signal demodulation methods:
• Probably the simplest of all methods of demodulation is AM. It usually consists of a single diode and filter.
• For DSB and SSB demodulation, the carrier frequency from an internal oscillator must first be introduced to the signal to replace the suppressed carrier in the received signal. The signal will then appear as a standard AM signal which can be easily detected by a diode-filter arrangement. This oscillator is sometimes known as a beat-frequency oscillator (BFO), and has to be within a few Hertz of the original carrier, or carrier as hetrodyned down to I.F.
• FM demodulation is considerably more difficult than the demodulations described earlier. A FM demodulator produces an output voltage that is proportional to the instantaneous frequency of the input. There are three general categories of FM demodulator circuit: Phase-locked loop (PLL) demodulator, Slope detection/FM discriminator, Quadrature detector.
Practically all radio tuners are actually analog. There are some radios with digital control of tunign, but their actual tuning and radio processing is anolog electronics. Most radio receivers use a local oscillator for the tuning. There is an RF input amplifier and then a mixer stage. The idea is that when the mixer mixes the local oscillator frequency and the incomign signal, the result is that at ixer output the rest of the receiver electronics gets the wanted signal at suitable intermediate frequency. The frequency of what the RF amp and thelocal oscillator are tuned to is controlled by varactor devices. This is aspecial type of diode that changes it capacitive reactance with theinput voltage. There are some complex circuits around this device tomake it work. The net result is to imitate a variable capacitor foreach of RF stages, and for the local oscillator. This device is whatactually makes the tuned frequency change.Older radios used to have analogue tuning control (by adjusting a variable capacitor or varactor tunign voltage). Many modern radios with digital frequency control are controlled by a circuit called a PLL, that isalso working with a pre-scalar. When you tune to a frequency, you areseeing a pre-scaled display of the frequency that is to be tuned. ThePLL ( Phase Lock Loop ) is commanded to send a command to the varactorcontrol circuits to tune the station to the selected station. Thefrequency tuned is governed by a referenced, pre-scaled reference. Therate, linearity, and span of the tuning is controlled by a combinationof software, and adjustments of tuned circuits and bias references.When the frequency is close, there is an AFC ( Automatic FrequencyControl ) circuit that samples the output of the IF stages via an AFCdetector to make a feedback control to the PLL. This is to force it tosearch for the peak or centre of the carrier of the station being tuned.Peak signal is used for AM, and centre of span or bandwidth is used forthe FM reference.
• 45-860MHz Radio receiver based on UV916-tuner - This receiver use a TV-tuner, a simple radio-circuit and a interface to a computer. The computer control (set) the receiving frequency from 45-860MHz. The purpose of this project is to learn about tuners.  
• 50 MHz Receiver based on MC3372 - The purpose of this project is to build a simple receiver for 50MHz. The Receiver is built around the circuit MC3372, wich is a narrow band FM receiver. The receiving frequency can be set with a LC tank or with a crystal. 
• A Cameo - This is an FM receiver circuit which can receive signals between 88 and 108 Mhz. With the TDA 7000 you can build an FM radio with a minimum of components; most of them so easy to manage. Nevertheless, this IC in itself, is a complete superheterodyne receiver with all the problems of alignment previously solved by the constructing engineers.  
• A high-performance FM receiver for audio and digital applicatons - This receiver design offers high sensitivity and low distortion for today?s demanding high-signal environments.  
• Aircraft Receiver - A passive receiver for listening to the pilot, pdf file  
• AM radio circuit (by Fran Golden) - This AM radio circuit will demonstrate how a radio wave is received and detected, using a very simple tuned radio frequency design. 
• AM-Receiver for Aircraft communication (118.250MHz) - This is a tunable AM-receiver for 118.250 MHz frequency. This receiver is manually tunable with some 100kHz around the 118MHz. 
• A Short Wave Regenerative Receiver Project - A regenerative radio receiver is unsurpassed in comparable simplicity, weak signal reception, inherent noise-limiting and agc action and, freedom from overloading and spurious responses. In the comprehensive electronic project presented here, Charles Kitchin, N1TEV has provided us with a three stage receiver project which overcomes some of the limitations of this type of receiver, principally the provision of an rf amplifier ahead of the detector. 
• Building a Three-Penny Radio - A crystal radio is nice because it needs no power, and the materials can all be home-made or at least found around the house. This is a little bit more advanced device. The heart of the radio is a special 10 transistor integrated circuit in a tiny three-legged bit of plastic. This circuit comes ready-made with several amplifiers, the detector, and an Automatic Gain Control circuit that boosts the level of faint stations to match the strong ones, so no volume control is needed. This is called a "Three Penny" radio because it uses three shiny pennies as anchors for the various parts the radio needs. This makes the construction very easy.  
• Build the 'Moorabbin': A regenerative receiver for the AM broadcast band - This circuit first appeared in Amateur Radio, November 1999. It doesn't need an antenna, gives speaker reception of local AM broadcast stations and also receives amateurs talking on the 160 metre band. 
• Build your own superheterodyne receiver - simple three-IC superheterodyne radio to receive stations in the 4.5- to 10-MHz range 
• Classic detector enables low-cost receiver - super-regenerative receiver's sensitivity is better than 1 mV, and it operates over a wide range of VHF  
• Compact 20: A 14 MHz Direct Conversion Receiver - This is a no-frills homebrew direct conversion receiver that is capable of receiving local and overseas stations on the popular 14 MHz band. The well-known NE-602 is used as the product detector. A MPF102 RF amplifier assures adequate sensitivity. The audio stage uses the common 741/386 combination. Component values provide high and low frequency roll-off suitable for SSB (can be mofied for CW also). This set is ideal for portable use because of its low power consumption.  
• Crystal Radio - very simple circuit  
• Deluxe Direct Conversion Transceiver (Super Sprint Rig) - This direct conversion receiver uses 74HC4053 as a mixer and is virtually immune to AM SWBCI. Unique 74HC86 XOR gate based heterodyned VFO, crystal oscillator and mixer. 
• Doppler D/F Instruments - a doppler type VHF radio direction finder for HAMs and experimenters  
• Electronic Eaverhopping Device Detector - detects transmitters in 1-1000 Mhz range 
• FM Band Preamplifier - This low noise amplifier will bring in those low-power stations 
• FM crystal receiver with slope demodulation construction project - This is a passive FM receiver for 87-108 MHz band. It works only near transmitter sites. Reception range is approx. 4-7 miles using a 3 Ele. Yagi antenna. 
• FM Radio Spectrum Analyzer - a device which will display the strength of all FM Radio stations simultaneously  
• FM-ULA-vastaanotin - This circuit is a receiver for FM radio band 88-108 MHz. It operates from 8-12V DC. This circuit is based on TDA7000 IC. The documentation for this circuit is in Finnish. 
• Improving Receiver Sensitivity with External LNA - This application note presents the system level trade-offs involved in adding a low-noise amplifier (LNA) to a remote keyless entry (RKE) receiver. The system's sensitivity improves 3.77dB, but the third order intercept degrades by 15dB.  
• Improving the performance of an a.m. radio receiver - Here are some suggestions for improving the performance of the simple radio receiver circuit. Both simple AM receiver and improvements are shown in this article. 
• Old Time Crystal Radio - plans of one electronics kit  
• One Transistor FM Radio Project - description of low-cost FM-receiver kit 
• Op Amp Radio - very simple AM radio 
• Poor Man's Spectrum Analyzer - information about an electronics kit  
• A Simple-To-Build Superhet Receiver - a simple circuit for a superhet radio receiver that can be built up in sections, with each section tested before assembly, can be tuned to different bands 
• Simple AM Radio Receiver & Amplifier - This is an extremely simple AM radio receiver and amplifier circuit which is capable of driving a small, 8 ohm speaker. Construction is simple and could be performed on a breadboard with decent results. 
• Simple Op-Amp Radio - This is basically a crystal radio with an audio amplifier which is fairly sensitive and receives several strong stations in the Los Angeles area with a minimal 15 foot antenna.  
• Single chip builds tiny aircraft receiver - This is a simple AM receiver for 108 to 135 MHz using single-chip NE605. 
• Spectrum analyser kid - simple spectrum analyzer for frequencies between 0 and 33 MHz  
• The R2 Single Sideband Direct Conversion Receiver, Revisited: The R2a  
• $10 receiver has microvolt sensitivity - three-transistor circuit receives signals in the 5- to 15-MHz short-wave band and operated from 9V battery   
• Super Scanner 45-860MHz with 0.01Hz stepsize. - This receiver is based on a TV tuner, a DDS circuit and a radio circuit. This receiver will work from 45 to 860 MHz and the step size can be down to 0.01Hz Why not use this receiver as a Spectrum Analyzer or a NOAA satelit receiver? How about that!  
• 45-860MHz Radio receiver based on UV916-tuner - This receiver use a TV-tuner, a simple radio-circuit and a interface to a computer. The computer control (set) the receiving frequency from 45-860MHz. The purpose of this project is to learn about tuners. The UV916 or UV918 tuner is easy to find in broke TV or VCR:s because it is a common tuner. This circuit can receive FM transmissions. 
• Crystal Radio Set Systems: Design, Measurement and Improvement - here are some early Blonder-Tongue Products to look at Electronic design by co-founder (now retired) Ben H. Tongue  

Receiver accessories

 

• Add a signal-strength display to an FM-receiver IC - The Philips TDA7000 integrates a monaural FM-radio receiver from the antenna connection to the audio output. External components include one tunable LC circuit for the local oscillator, a few capacitors, two resistors, and a potentiometer to control the variable-capacitance-diode tuning. The IC has an FLL (frequency-locked-loop) structure. You can obtain the information related to the intensity of the received signal at the output of the IF filter.