In AM transmission, the carrier signal is modulated so that its amplitude varies with the changing amplitudes of the modulating signal.
The frequency and phase of the carrier remain the same. Only the amplitude changes to follow variations in the information. The following figure shows how this concept works. The modulating signal is the envelope of the carrier. AM is normally implemented by using a simple multiplier because the amplitude of the carrier signal needs to be changed according to the amplitude of the modulating signal. AM Bandwidth:. The modulation creates a bandwidth that is twice the bandwidth of the modulating signal and covers a range centered on the carrier frequency.
However, the signal components above and below the carrier frequency carry exactly the same information. For this reason, some implementations discard one-half of the signals and cut the bandwidth in half. The total bandwidth required for AM can be determined from the bandwidth of the audio signal:. Standard Bandwidth allocation for AM Radio:. The bandwidth of an audio signal speech and music is usually 5 kHz. Therefore, an AM radio station needs a bandwidth of 10kHz.
AM stations are allowed carrier frequencies anywhere between and kHz 1. However, each station's carrier frequency must be separated from those on either side of it by at least 10 kHz one AM bandwidth to avoid interference. If one station uses a carrier frequency of kHz, the next station's carrier frequency cannot be lower than kHz.
In FM transmission, the frequency of the carrier signal is modulated to follow the changing voltage level amplitude of the modulating signal. An integrated ADC creates a saw-tooth signal that ramps up, then falls to zero.
When the ramp starts, a timer starts counting. When the ramp voltage and the input match, a comparator fires. At this point, the timer's value is recorded. The ramp time is typically sensitive to temperature because the circuit generating the ramp is often a simple oscillator. Calibrating the timed ramp, or using a clocked counter driving a DAC and then use the comparator to preserve the counter's value are two solutions for this.
By continuing to browse this website without changing your web-browser cookie settings, you are agreeing to our use of cookies. Please use latest browser to ensure the best performance on ROHM website. Rohm Breadcrumb. Below we cover a few examples. Successive Approximation Conversion A successive-approximation ADC settles on a final voltage range, by using a comparator to reject ranges of voltages. Previous Next.
Electronics Basics What is a Transistor? What is a Diode? What are SiC Power Devices? What are SiC Semiconductors? What is IGBT? What are LEDs? What is a Photointerrupter? What is a laser diode? What is a Resistor? So, an input voltage of mV should give an output of , or 0xFFF.
What happened is that over the entire input range, value of 1-bit changed ever so slightly to say, 1. The accumulated error resulted in a full-scale error of one or two bits of accuracy.
As will be seen later, there are many external factors that will degrade the ADC output accuracy even more. Figure 1- Schematic representation of an ADC. There are many ways of implementing an ADC.
The next few sections present several of the more common ones. To keep this article relatively short, only simple and somewhat incomplete descriptions of each such implementation are given.
The block diagram of a single-slope ADC is shown in figure 2. Basic operation is very simple. A capacitor is charged from the input source until its voltage reaches V Ref , at which point the comparator trips. While charging, a digital counter fed by a clock was also counting. It stops counting when the comparator trips, at which point the count reached is a representation of the analog input. Figure 2 — Block diagram of a single-slope integration ADC.
One of the most common variations of this method is the dual-slope integration ADC. In it, the capacitor is then discharged, and the counter value is averaged.
This technique mitigates the effect of dielectric absorption, an effect that can cause errors in the ADC reading, in the integrating capacitor. This type of ADC is accurate, but very slow; it is mostly used in multimeters, for example, where accuracy is more important than speed. Starting with the input side, the difference amplifier produces an output that is the difference between V in and the DAC output.
The integrator in this case can best be thought of as taking the moving average of the previous value and the current input value.
So, to start, assume V in is fixed at just a very tiny bit above 0V so that the comparator will trip. Its value will be high, or 1.
On the next round this value will be subtracted from the current value of V in. The integrator output will now be at — V ref since the previous value was 0V.
On the next sample, the integrator output will be 0 since the previous value was at — V Ref , and the difference amplifier actually subtracted -V Ref , and thus added, V Ref to V in. The comparator output will thus be 1. This process continues, and the comparator output will thus be a steady stream of … for a V in of 0V.
Remembering that logic 1 means V Ref , and 0 means -V Ref , then if N number of samples are taken and averaged, it is easy to see that the average will be 0V. The processing block after the comparator will simply output this as a single value of … assuming a reference of V Ref — -V Ref , or 2 x V Ref. Working out the same steps as before, the output will be: … This works out to be 1. However, if more samples are taken the precision becomes greater, and the value gets closer to 1.
Thus, the Sigma-Delta requires many samples in order to generate one output. In other words, the input signal needs to be over-sampled to reduce the ADC conversion errors. The operation of a flash ADC is perhaps the easiest to understand.
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