There were only a couple of very minor wording changes in this section…Dan
Filters are very important circuits in amateur radio equipment. As the name implies, these circuits are used to clarify or process radio signals. For example, one type of filter—a low-pass filter—passes all signals whose frequencies are below a certain frequency, called the “cutoff frequency.”
One application of a low-pass filter is to block the VHF and UHF harmonics produced by an amateur transceiver from reaching the antenna. To do this, you would connect the input of the filter to the output of your transceiver and the output of the filter to your antenna system. When used in this way, the impedance of a low-pass filter should be about the same as the impedance of the transmission line into which it is inserted. (G7C06)
Filters are also used in amateur radio transmitters. A filter is used to process signals from the balanced modulator and send them to the mixer in a single-sideband phone transmitter. (G7C01) A balanced modulator is the circuit used to combine signals from the carrier oscillator and speech amplifier and send the result to the filter in a typical single-sideband phone transmitter. (G7C02)
These days, many transceivers use digital circuits, instead of analog circuits, to filter RF signals. This technique is called Digital Signal Processing (DSP). Digital Signal Processor filtering is accomplished by converting the signal from analog to digital and using digital processing. (G7C10) What’s happening is that a specialized computer chip, called a Digital Signal Processor, is running software that performs many millions of calculations on the digital representation of the signal.
All of the these choices are correct when talking about what is needed for a Digital Signal Processor IF filter (G7C09):
- An analog to digital converter
- A digital to analog converter
- A digital processor chip
The superheterodyne receiver is the most popular type of amateur radio receiver. Superheterodyne receivers convert the received signal to an intermediate frequency (IF) and then process that IF signal. A mixer is the circuit used to process signals from the RF amplifier and local oscillator and send the result to the IF filter in a superheterodyne receiver. (G7C03) A product detector is the circuit used to combine signals from the IF amplifier and BFO and send the result to the AF amplifier in some single-sideband receivers. (G7C04) The simplest combination of stages that implement a superheterodyne receiver is HF oscillator, mixer, detector. (G7C07)
FM receivers have different types of circuits than the superheterodyne receivers designed for AM, CW, and SSB reception. A discriminator circuit is used in many FM receivers to convert signals coming from the IF amplifier to audio. (G7C08)
Most modern transceivers use digital circuits called synthesizers to control the receive and transmit frequencies. An advantage of a transceiver controlled by a direct digital synthesizer (DDS) is that it provides variable frequency with the stability of a crystal oscillator. (G7C05)
Digital techniques have proven to be so effective at generating and receiving radio signals, that some transceivers now implement most functions using digital signal processors. We even have a special term for these types of radios. The term “software defined radio” (SDR) means a radio in which most major signal processing functions are performed by software. (G7C11)
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