barcode in vb.net 2005 Detection of CW signals in Software

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Detection of CW signals
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If you tune in an unmodulated carrier with an envelope detector, you won t hear anything. A keyed carrier might produce barely audible thumps or clicks, but it will be impossible to read the code. For detection of CW, it s necessary to inject a signal into the receiver a few hundred hertz from the carrier. This injected signal will beat against the carrier, producing a tone whose frequency is the difference between the carrier and injected-signal frequencies. The injected signal is produced by a beat-frequency oscillator (BFO). The beating occurs in a signal combiner or mixer. A block diagram of a simple CW receiver is shown in Fig. 27-5. The BFO is tunable. Suppose there is a CW signal at 3.550 MHz. As the BFO approaches 3.550 MHz from below, a high-pitched tone will appear at the output. When the BFO reaches 3.549 MHz, the tone will be 3.550 3.549 MHz, or 1 kHz. This is a comfortable listening pitch for most people. The BFO setting isn t too critical; in fact, it can be changed to get a different tone pitch if you get tired of listening to one pitch. As the BFO frequency passes 3.550 MHz, the pitch will descend to a rumble, then to a swish-swish sound. As the BFO frequency continues to rise, the tone pitch will increase again, eventually rising beyond the range of human hearing.
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27-5 Block diagram of a simple direct-conversion receiver for CW, FSK, and SSB.
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506 Data reception This demodulation scheme is called heterodyne detection. A receiver that makes use of a heterodyne detector is called a direct-conversion receiver.
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Detection of FSK signals
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Frequency-shift keying can be detected by using the same methods as CW detection. The carrier beats against the BFO in the mixer, producing an audio tone that alternates between two different pitches. The block diagram of Fig. 27-5 can therefore apply to FSK reception as well as to CW reception. With FSK, the BFO frequency is always set well above, or well below, both the mark and the space carrier frequencies. The offset affects the audio tone frequencies and is set so that certain standard pitches result. There are several sets of standard tone frequencies, depending on whether the communications is amateur, commercial, or military. To get the proper pitches, the BFO must be set precisely at the right frequency. There is little tolerance for error.
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Detection of SSB signals
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A single-sideband signal is really just an AM signal, minus the carrier and one of the sidebands. If the BFO frequency of a CW receiver is set exactly where the carrier should be, the sideband, either upper or lower, will beat against the BFO and produce audio output. In this way, the receiver of Fig. 27-5 can be used to detect SSB. The BFO frequency is critical for good SSB reception. If it is not set exactly at the frequency of the suppressed carrier, the voice will sound bizarre, like monkey chatter. A more advanced method of receiving CW, FSK, and SSB makes use of a product detector. This is a specialized form of mixer and is discussed a little later in this chapter.
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Detection of FM signals
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Frequency-modulated or phase-modulated signals can be detected in several ways. The best FM receivers respond to frequency/phase changes, but not to amplitude changes.
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Slope detection
An AM receiver can be used to detect FM. This is done by setting the receiver near, but not exactly on, the FM signal. An AM receiver has a narrowband filter with a passband of about 6 kHz. This gives a selectivity curve such as that shown in Fig. 27-6. If the FM carrier frequency is near the skirt, or slope, of the filter response, modulation will cause the signal to move in and out of the passband. This will make the receiver output vary with the modulating data. Because this scheme takes advantage of the filter slope, it is called slope detection. It has two disadvantages. First, the receiver will respond to amplitude variations (because that s what it s designed for). Second, there will be nonlinearity in the received signal, producing distortion, because the slope is not a straight line.
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