Amplitude shift keying (ASK) in detail
In this article, I will explain Amplitude shift keying (ASK) along with the waveform and frequency spectrum in detail. This article is easy to understand, clear and with all the information. Suitable for engineering and diploma students.
A digital signal is a sequence of high and low pulses of defined amplitudes, which act on the carrier amplitude. The simplest method is a so-called ASK, on off keying. In digital high bit (1) the maximum carrier amplitude is sent. A low bit (0) switches the carrier. This method has the disadvantage that a temporary failure or a reception station trouble can not be distinguished from the low bit. An improved modulation technique in this regard to the carrier amplitude samples of 100% for the high-bit and 25% for the low bit. As modulators of the known circuits can be used in analog AM. The following figure shows a generated by both methods ASK signal.
The ASK can be performed with a plurality of gradations of the carrier amplitude. Four amplitude levels may be the same 2-bit (00, 01, 10, 11) transmitted. The transmission speed is doubled. A multivalent ASK, in the following image, the quaternary signal requires a good quality connection. As almost always show disturbances at an amplitude modulation and amplitude noise, the polyvalent ASK is susceptible to interference and is less often used. The transmitter needs to transmit a synchronous or start signal, so that in the receiver the correct bit sequence can be detected and demodulated.
The oldest use of the ASK is probably the Wireless Telegraphy Morse signals. If only the carrier amplitude is switched, this is called toneless keying. The carrier may additionally be amplitude modulated with an audio frequency permanently of 1 kHz sine wave. The method is called sounding keying. The evaluation at the receiver is then acoustically possible. Modem connections often work with ASK. The location of the air traffic beacon use, which are amplitude modulated with an audible tone and their station identification is based on ASK. In digital fiber optic systems ASK is used frequently.
A significant ASK signal is sent from the long wave transmitter DCF77. It is intended for the Germany radio signal which switches the 77.5 kHz sinusoidal carrier in seconds between 100% and 25% rated power. The duration of a reduction, the bit can be 100 ms as a logical low signal or 200 ms as a logical high signal. Within a minute, let it transfer all the time and date information. The coding in the BCD code of 1, 2, 4, 8, 10, 20, 40, 80 ...Frequency spectrum of the ASK
The frequency spectrum of an ASK signal clearly indicates that it is a modulation and not a simple switching on and off of the carrier oscillation. Appear on both sides of the carrier frequency line side lines and the switching frequency is no longer included in the spectrum. Compared with the analog AM signal bandwidth of the ASK is substantially greater.
The picture shows a simulation program created in the ASK spectrum, in which a 1 kHz carrier signal is keyed with a 100 Hz binary clock signal. The binary information corresponds to a symmetrical square wave. In the sequence of the side lines and their amplitude attenuation can be seen, the range of key function.
In comparison to the bandwidth of the analog AM ASK is much greater. This is a disadvantage because fewer stations can work undisturbed next to each other within a frequency range. If the bandwidth is reduced, so that the step frequency f p is still transmitted without attenuation, it can be uniquely decoded in the receiver the signal. In the foregoing examples, the frequency is 100 Hz, a step of low-pass with a bandwidth of 1.6 x B = f p satisfies this condition. Because of the symmetrical sidebands the ASK signal requires twice the bandwidth.Soft keying using low-pass filters
Going through the digital signal before modulation, a low pass filter, then the bandwidth requirement of the ASK signal decreases significantly. A steep LC low-pass filter is an RC-pass preferable. The cutoff frequency corresponds to the value presented above 1.6 · f p, the phase shift occurring to the original bit clock has no negative impact on the recovery of information.
Terms of data transfer
Step frequency
The step frequency or dot frequency is the highest fundamental frequency of a pulse train 1-1 in the pulse duration and pulse interval are identical in length. It is also referred to as the Nyquist frequency. The step duration T s, the duration of a bit, then half as long as the period of time calculated from the step frequency.
Step time
The smallest time interval between two successive state changes at discrete-time signals is referred to as a step. The step signal from high to low, with a defined duration, the step duration. A digital signal having a constant step duration T S is referred to as step clock.Isochronous data signal
A data signal is called the isochronous if the change of state take place within a fixed time frame the same.Step speed
The walking speed or symbol rate is calculated from the reciprocal of the shortest step time. The number of state changes per second signal parameters is given in honor of the French telegraph engineer as baud Baudot. The same terms are significant for walking speed bit rate or bit rate.
Transmission speed or baud rate
For bivalent, binary signals, where each bit encodes a state change, walking speed in baud is equal to the transfer rate in bit / s It is referred to in this case as a bit rate or bit rate.
For multi-level signals with nmöglichen value levels the relationship: v u = v s · lb (s) in [bit / s].
In this equation, lb () the logarithm to the base 2 It applies lb (n) = lg (n) / log (2).Data Rate
The term is used synonymously to transmission rate or transfer rate. The data rate in bits / sec, the number of information units is again transferred in a certain time.Drawing speed
The data bits in a character frame of start, stop and parity bits are included. The number of unit steps z is the sum of all bits in the character frame (z = start bit + data bits + parity + stop bit (s)). For the transmission of the full frame, it requires the transmission duration T Z whose inverse gives the drawing speed v Z.
v Z = 1 / T Z Nyquist bandwidth
Thus, the signal from the receiver can be uniquely reconstructed, at least the first harmonic of the highest frequency occurring in the signal to be transmitted. It is the frequency of a point-symmetrical high-low-high change, which is referred to as a Nyquist frequency. The bandwidth is then calculated as:
B N = f p = 1 / (2 · T S)Source of images
Images are personally created using CAD software