10 Terms You Must Know About RF Technology Development
This article will cover some of the terminology associated with the antenna products sold by Digi-Key (this also applies to receivers, transmitters and transceivers, and anything RF related). For customers who are just starting to explore this type of technology, some of the terminology can be quite confusing.
decibel
The decibel is a unit of measurement that expresses the ratio of one value to another on a logarithmic scale. In the audio world, the decibel (dB) is a unit used to measure the intensity of sound. RF applications also use different forms of the decibel, such as dBA, dBm (sometimes just dB), dBi, and dBV. Because dB is "unitless," certain suffixes are added when discussing specific values. The following is a list of the uses of each unit:
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dBA : Stands for decibel amplifier and is used when measuring current amplitude in RF applications.
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dBm or dB: These two units are often used when describing power (watts) in RF applications. The "m" usually indicates the prefix "milli". Typically, RF power measurements will not be very high (depending on the application), so dBm tends to be more common.
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dBi : This measurement is specific to the directional gain of an antenna.
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dBV : Stands for decibel-volts and is used when measuring voltage amplitude in RF applications.
Frequency Range
The frequency range is the effective frequency at which the antenna operates. A minimum and maximum frequency are usually specified. The component is able to receive or transmit within this range with varying "efficiencies," depending on the center frequency. Antennas may also list several frequency ranges if they have broadband capabilities.
Center frequency
The center frequency is the position where the antenna produces or delivers the greatest signal strength (better gain). Some antennas have multiple center frequencies, which may allow for wideband communication. When developing your application, you do not need to match the center frequency, and sometimes you may not even be able to get the exact frequency. It is best to get closer to the middle because it will provide better performance.
bandwidth
Bandpass and Bandstop
The terms bandpass and bandstop are related. They usually apply to special filters that "pass" or "reject" a range of frequencies. Bode plots are often used when studying RF filters, where the X-axis shows increasing frequency (usually on a logarithmic scale) and the Y-axis usually shows the signal amplitude in dBV (decibel Volts). Take the following Bode plot I made for a bandpass signal (I forgot to note that the Y-axis of both plots is in dBV):
Note
that
this is not a typical frequency range used by most devices
,
and that
the X
is not on a logarithmic scale
,
I am just using a smaller range for illustration.
I have marked which frequencies will remain around 1V (0dBV) based on a filter that cuts off at around 810Hz on the low end and 777kHz on the high end. The bandwidth of this filtered signal is about 776,190Hz (or 776.19kHz), and all other frequencies will be dramatically reduced (attenuated) in amplitude. The opposite filter is called a bandstop:
Certain applications sometimes require that certain frequencies be suppressed. For RF components you will find plots very similar to these (Bode plots).
So why use these graphs? If it were a continuous sine wave with frequency and amplitude increasing at different points it would look pretty messy.
The bandpass graph is shown above. If it is a sine wave with increasing frequency, it will become a mess of vertical lines as the frequency increases.
broadband
The term is often used to describe Internet connections, but it is also a general term. Antennas that have a wide range of frequencies and several center frequencies are called broadband antennas.
Gain
Gain can describe several properties without context, but it usually describes an increase in some signal property. If it's an antenna, then gain is not the added power (antennas can't increase power), but rather a "directional gain". Antennas produce signals that are directional due to their design. Having more gain isn't always good, and if you don't want the signal to be stuck in a particular direction, you need to lower the gain. Directional gain depends on the application, which is why some antennas have negative gain (loss). If it's a filter or boosted signal, then gain can apply to other units of measurement. You can also increase power, current, and voltage, but that requires some external power source.
Return loss
Return loss is the ratio of the frequencies received by an antenna to those rejected by it.
VSWR
VSWR stands for Voltage Standing Wave Ratio. Standing waves represent the power that is not received by the receiver and is reflected back on the transmission line. VSWR is the ratio of the maximum voltage to the minimum voltage on a lossless line. The standing wave is highly dependent on the impedance of the transmission line, the receiver, and the transmitter.
impedance
Impedance is a combination of reactance and resistance. Reactance is also measured in ohms, but is completely dependent on the frequency of the signal. Here is an article on impedance: Impedance in a nutshell . However, I did not mention the impedance of transmission lines or other components.
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