Amplitude modulation was the first modulation type to be considered in analog communication systems. Amplitude modulation has the obvious advantage of being simple and relatively bandwidth efficient.
The disadvantages of amplitude modulation are :. In the first experiment, we analyzed the effect of varying the amplitude of a sinusoidal carrier in compliance with the baseband information signal. A major improvement in performance in the transmission is achieved with angle modulation.
In this type of modulation, the amplitude of the carrier is kept constant. Angle modulation provides the improved noise performance. Phase Modulationand Frequency Modulation are both the modulation techniques analyzed in angle modulation. In this second experiment, we will examine the most common modulation scheme in daily life, namely, the Frequency Modulation, or FM.
Please see the Fundamentals of Analog Communications section as discussed in the first experiment as a reference. The complex received FM signal have both real and imaginary components. This signal has the form:. The complex received signal is the input to two blocks. You can easily recognize that this result is very similar to the signal by the demodulation using differentiator method.
This method is often referred to FM to AM. It basically converts frequency changes to amplitude variations.
The message is recovered as the control input of the VCO . In the simulation experiment section-2we used the VCO to demodulate the information signal to make life easier. The frequency modulator and demodulator structures are as explained below. In the first model, you are provided a FM structure that is very similar to the theoretical background of this experiment.
In this case, you will use the modulator and demodulator blocks provided by Simulink. In model-1, you have already learnt the theoretical foundations for FM. In the second model, instead of using complex modulator and demodulator structure, we will implement an FM system using direct modulator and demodulator blocks defined in Simulink.
The input, in this case, has three different forms: sine wave, rectangular pulse train and triangular waves. Therefore, we will be able to observe the frequency variations using variety of inputs. The model-2 is expressed as:. The signal generator block is simply an analog input. In order to use this block as an input of the FM Modulator, we need to digitalize it.
The rate transition block zero-order-hold, or ZOH will sample the analog information by the sampling period please see the Review Manual for the sampling process. Here, we will implement the FM modulator and demodulator using a music file as a source.
In this case, since the source is a multimedia file rather than a pure sine wave, we need DSP processing, which is resampling and filtering. You will not be kept responsible for DSP processes. However, you can find them very useful when comprehending sampling rate, rate conversion, Finite Impulse Response FIRdecimation and interpolation etc. Now, we will go a further step to transmit a music file, and then receive it via USRP hardware. In this case the transmission is real time, therefore unlike the simulations, you will observe the transmission through the air as well as the noise.
Resampling and Filtering blocks are the same as the music simulation as well as the baseband modulator. Stewart, Kenneth W.Learn about two techniques for recovering the baseband signal from a frequency-modulated carrier. Frequency modulation offers improved performance over amplitude modulation, but it is somewhat more difficult to extract the original information from an FM waveform. One of these is quite straightforward, and the other is more complex. If you look back at the page on analog frequency modulationyou will see that the mathematical relationship is less straightforward than that of amplitude modulation.
With AM, we simply added an offset and then performed ordinary multiplication. With FM, we need to add continuously varying values to the quantity inside a sine or cosine function, and furthermore, these continuously varying values are not the baseband signal but rather the integral of the baseband signal.
It turns out, though, that it is actually easier to generate an FM signal. We simply use the SFFM option for a normal voltage source:. Note that the modulation index is five; a higher modulation index makes it easier to see the frequency variations.
The following plot shows the waveform created by the SFFM voltage source. We need to design the high-pass filter such that the attenuation will vary significantly within a frequency band whose width is twice the bandwidth of the baseband signal. The received FM signal will have a spectrum that is centered around the carrier frequency. Thus, the lowest frequency in the modulated signal is approximately equal to the carrier frequency minus the highest frequency in the baseband signal, and the highest frequency in the modulated signal is approximately equal to the carrier frequency plus the highest frequency in the baseband signal.
Our high-pass filter needs to have a frequency response that causes the lowest frequency in the modulated signal to be attenuated significantly more than the highest frequency in the modulated signal.
If we apply this filter to an FM waveform, what will be the result? It will be something like this:. This plot shows both the original FM waveform and the high-pass-filtered waveform, for purposes of comparison.
The next plot shows just the filtered waveform, so that you can see it more clearly. By applying the filter, we have turned frequency modulation into amplitude modulation. This is a convenient approach to FM demodulation, because it allows us to benefit from envelope-detector circuitry that has been developed for use with amplitude modulation. The filter used to produce this waveform was nothing more than an RC high-pass with a cutoff frequency approximately equal to the carrier frequency.
The simplicity of this demodulation scheme naturally makes us think that it is not the highest-performance option, and in fact this approach does have a major weakness: it is sensitive to amplitude variations.Frequency modulation FM is a technique in which the frequency of a transmitted waveform is varied according to the variations in the message wave.
The FM is a very popular technique since they are widely used by the FM radio stations. The main reason behind using the FM modulations by the radio stations is the quality of the signal that can be recreated in the FM receiver.
The signal to noise ratio is comparatively very high at the output of FM demodulator circuits. However the FM modulator and demodulator circuits are complex compared to other modulation and demodulation techniques. In the demodulator circuit the VCO generates a frequency which matches the original carrier frequency and compares the phase of that with received FM wave using the Phase comparator.
The output of the Phase comparator is filtered out using the LPF and is current amplified using a Source follower. The output of the source follower matches the original message signal. To implement all the above mentioned circuitry is a difficult task, but there are ICs available which has all these circuit blocks embedded in it. This IC can recreate the FM signals with good quality using only a few external components. The functional diagram of the IC is given below:.
The value of the C1, R1 and R2 should be selected in such a way that the VCO produces pulses which matches the original un-modulated carrier frequency of the FM wave. The value of the R3 and C2 should be selected in such a way that the RC constant should match the range of the frequencies at which the message signals can be expected.
The pure sine wave is generated using a Wien Bridge Oscillator WBO which is then clamped to the positive voltage side using a positive clamper circuit. The block diagram of the entire set up for FM generation is shown in the following diagram:. The WBO circuit is designed to generate pure sine wave of 1 KHz with peak-to-peak amplitude around the supply voltage of 5V.
The WBO circuit and the image of the waveform generated is shown in the following figure:. A positive clamper circuit follows the sine wave generator and is made using a single capacitor and diode. The clamper circuit and the clamped sinusoidal waveform are shown in the following figure:. The clamping circuit is followed by a potential divider and current amplifier. The significance of the potential divider here is that by adjusting the entire amplitude of the signal input to the VCO, the range at which the frequency varies in the FM produced can be adjusted.
The clamper circuit with potential divider and current amplifier is shown below:. A timer IC is wired as a normal astable mutivibrator with constant on-off time period. The only difference in this circuit is that the pin number 5 is not connected to the positive via a capacitor, but it is used to receive the modulating signal. The voltage at the pin number 5 of the timer IC controls the frequency generated by the astable circuit and since the voltage applied here at the pin 5 is a sine wave of 1 KHz the output pulse frequency varies according to the sinusoidal variations in the amplitude.
The circuit diagram of the modulator is given in the following diagram:. The un-modulated frequency of the pulses generated by the above circuit can be calculated using the following equation:.In an FM signal, the instantaneous frequency varies in accordance with the modulating signal. For a sinusoidal modulating signal, the frequency deviation in an FM signal is sinusoidal, and it is proportional to the modulating, amplitude. Recall that the changes in the instantaneous frequency of the carrier signal occur with respect to the previously attained value of the carrier frequency.
Suppose the center frequency of the FM signal is fc, and it lies within the hold-in range of PLL the VCO is locked to fc, by applying an demodulated carrier at the input of the phase detector. When VCO is locked to fc, the error signal is zero, and therefore, the control signal that changes the VCO frequency is also equal to zero. The control signal is produced in proportion to the phase difference at an instance of time.
This control voltage will modify the VCO frequency, which is again compared with the incoming frequency. In this way, the current incoming frequency is compared with the previously attained value of the VCO frequency, which is the previously attained frequency of the FM Signal. The control signal is produced in proportion to the difference between the VCO frequency and the instantaneous frequency of FM signal.
In other words, the control signal so produced is proportional to the frequency deviation in the FM signal. Since the frequency deviation si proportional to the modulating signal, the control signal appearing at the output of LPF is the modulating signal.
The amplifier also functions as the low pass filter. This is a 14 pin dual in line package. The positive terminal of the Vcc is connected to pin number 10, and the negative ground terminal of Vcc is connected to pin number 1.
FM Modulator and Demodulator with PLL CD4046
The output signal to the phase detector is applied to pin numbers 2 and 3. The VCO output is applied to the phase detector through pin number 5.
The output of the phase detector is internally connected to the amplifier low-pass filter. The output or the phase detector is low-pass filtered and amplified by the amplifier stage.
The Output or the amplifier is the control voltage that is applied to VCO to force it to track the incoming frequency. The control voltage is also available at pin number 7. This is the output signal. In the ease of FM demodulator, the signal at pin number 7 is the modulating signal.FM demodulation Part-2
The amplifier also generates an output at pin number 6for reference purposes. The VCO gets its control voltage internally from the amplifier and its output at pin number 4. The VCO output should be given to the phase detector through pin number 5. It is customary to short pin numbers 4 and 5 so that the VCO output is applied directly to the phase detector. The external resistor and capacitor can set the free-running frequency of the VCO.Selection of components to set the lock field and the capture field.
Ok Read more.To know the basic principles of FM demodulation as well as the different circuits used to detect information from a received FM signal. For communication to work, both the sender and the receiver must agree on what communication channel to use.
After which, the sender encodes the message and transmits it to the receiver. Then, the receiver receives the message and decodes it. This holds true to FM: the transmitted FM signal is received and must be demodulated to take the information.
This is what FM detectors do. FM detectors are circuits that instantaneously convert the frequency changes from the carrier signal to its output voltage counterpart. They are also known as frequency demodulators or discriminators.
The input to the circuits is a frequency-varying signal with a constant amplitude.
FM Demodulator with CD4046
The circuits then transform these instantaneous frequency variations to amplitude variations, thus each voltage level in the output corresponds to its instantaneous frequency variation counterpart in the input.
Just like AM, FM also has a modulation index. It is equal to the ratio of the frequency deviation to the modulating frequency.
The frequency deviation is the amount of change or swing in carrier frequency produced by the modulating signal. This is a way to express the peak deviation frequency as a multiple of the maximum modulating frequency.
To illustrate this, refer to the figure below:. The carrier signal frequency is 1kHz, the modulating frequency Hz, and the modulation index is 3. Taking note of the modulation index, this makes the peak frequency deviation Hz. The frequency will swing between and Hz. On the other hand, the function of the modulating frequency is to know how fast the cycle is completed.
For the sake of simplicity, we will dive in to the Slope Detector to know the basic function of an FM demodulator.FM demodulation is a key process in the reception of a frequency modulated signal. Once the signal has been received, filtered and amplified, it is necessary to recover the original modulation from the carrier.
It is this process that is called demodulation or detection. FM demodulator circuits are found in any receiver that uses FM: broadcast receivers, two way radios like walkie talkies and handheld radios that use FM, and any receiver where frequency modulation is used.
In any radio that is designed to receive frequency modulated signals there is some form of FM demodulator or detector. This circuit takes in frequency modulated RF signals and takes the modulation from the signal to output only the modulation that had been applied at the transmitter. In order to be able to demodulate FM it is necessary for the radio receiver to convert the frequency variations into voltage variations - it is a frequency to voltage converter.
When the carrier frequency deviates to the lower end of the frequency range over which it deviates a lower voltage may be produced, then as it deviates higher in frequency, a higher voltage is produced.
Although it is easier to think of lower frequencies producing lower voltages, there is no need for this to be the case, it could be the other way around. One of the chief requirements for the FM demodulator is that it should have a response that is as linear as possible over the required bandwidth.
In other words a shift of a given frequency produces the same output change wherever it may be found on the curve. If the response is not linear, then distortion will be introduced. A further requirement for the FM demodulator is that it should not be sensitive to amplitude variations. As the modulation is carried by only the frequency deviation, no amplitude sensitivity is wanted. Any amplitude signal is likely to be noise, and by making the receiver insensitive to amplitude variations, the signal to noise ratio can be improved.
The resilience to noise is a major factor in providing low noise FM reception for applications like high fidelity audio broadcasts. It also means that for mobile radio, or handheld radio communications, the effects of signal level variations and fading due to movement is reduced. If an FM demodulator is sensitive to amplitude variations as well as frequency variations, then the demodulator can be preceded a limiting amplifier stage.
This stage runs into saturation when a signal of sufficient strength is present. By running in saturation, the amplitude variations are removed. There is a linear portion at the centre of the response curve and towards the edge the response becomes very distorted.
As can be anticipated, the detector response curve cannot remain linear over a huge range of frequencies. Instead it should be sufficiently wide to accommodate the width of the deviation of the signal and a bit more to provide additional margin.
To improve the noise performance of the FM receiver, typically the IF stage may operate such that the IF amplifier is driven into limiting. This removes the amplitude variations, that will result in noise, and only allows through the frequency variations.
These FM demodulators are used in different applications. The different types of FM demodulator provide designers with a choice of approaches dependent upon the application: broadcast, two way radio communications including walkie talkies and handheld radios, high specification communications receivers and the like.
Although the PLL FM detector and the quadrature detectors are most widely used, along with phase locked loop based circuits. The Foster Seeley and ratio FM detectors are still used on some occasions, but they are normally only found in older radios using discrete components. FM demodulation basics In any radio that is designed to receive frequency modulated signals there is some form of FM demodulator or detector.
FM demodulation principle In order to be able to demodulate FM it is necessary for the radio receiver to convert the frequency variations into voltage variations - it is a frequency to voltage converter. It is not particularly effective and is not used except when the receiver does not have an FM capability. This form of FM detection has very many limitations: the selectivity curve of the radio will not be at all linear and distortion will arise; the receiver will be sensitive to amplitude variations, etc.
Read more about. FM slope detector. FM ratio detector. Foster Seeley FM discriminator. PLL FM demodulator.