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====== Frequency display ======
Every receiver needs to have a useful frequency display, so you can determine the frequency of a received signal on the set, or you can search for a signal on a known frequency.

On domestic radios and older sets, there are frequency dials on which the operation frequency can be read, modern sets and specially in high-quality "world receivers" have a digital frequency display.

===== Frequency dials =====
{{ :images-small:kurz-skala-rund.jpg?150|Circular dial}}
  * **Circular dials**: Early receivers often had circular dials, which in the sets of the very early years only had a 0 - 100 graduation; the operation frequency had to be determined on the basis of a frequency calibration chart on graph paper. Later, the rotating or fixed round dials came with frequency dial marks with a rotating dial pointer or a fixed pointer and a rotating dial. The reading accuracy depends on the frequency range and is generally not sufficient to tune in a station on a known frequency directly.
{{ :images-small:kurz-skala-linear.jpg?150|horizontal dial}}
  * **Horizontal dials**: after the 1940s, horizontal or linear frequency dials from which the frequency can be read by means of a dial pointer moved by a cable, became industry standard. The usefulness of the frequency dial is still depending on the frequency range covered by the dial. If the medium wave range is covered on 30 cm dial length, a station can be located in the 9 kHz channel spacing, but if the entire short wave range from 6 - 16 MHz is covered on the same dial length, several hundred kilohertz are in a few millimetres and a frequency can no longer be determined accurately.
  * **Logging dials**: often a 0 - 100 division on the frequency dial can be found. The setting of a known frequency can be written down in the logbook and the frequency can be tuned again by setting the receiver to the corresponding number.
{{ :images-small:kurz-skala-bandspreiz.jpg?150|bandspread dial}}
  * **Bandspread dials**: in order to improve frequency readings in the shortwave ranges, on many receivers, the shortwave range has been subdivided. So one shortwave broadcast or amateur radio band covers the whole dial length. The reading accuracy here can reach 10 - 20 kHz, which is just about sufficient for a frequency determination.
  * **Projection dials**: in a few high-end receivers, the dial information has been printed or exposed on film strips or film discs; with a magnifying optic, this dial is projected on a groundglass screen and gives sufficient dial accuracy.

===== Digital frequency displays =====
A digital frequency display is very helpful in tuning a set to a known frequency, in determining the frequency of an unknown signal or identifying a station on a frequency according to a frequency covering plan.
  * **Frequency counter**: the frequency of a signal is determined by an electronic counter circuit. In the seventies, even external frequency counters were available as an accessory to be connected to certain amateur receivers due to their high prices. The reading accuracy is 1 - 5 kHz.
  * **Frequency display on a PLL synthesis receiver**: with the development of the PLL (Phase Locked Loop) synthesis circuit, it was made possible to display the frequency information from the frequency processing circuit of the receiver oscillator directly on a display. The accuracy depends on the quality of the PLL synthesis circuit, sometimes frequencies can be displayed with an accuracy of 1 Hz.

Different technologies are used for digital displays:
{{ :images-small:kurz-mechan-digital.jpg?direct&200|Mechanical digital display}}
  * **Mechanical digital display**: the tuning mechanism is directly coupled with a mechanical counter that works like an odometer - the frequency can be read from the mechanical counter.
{{ :images-small:kurz-frequenz-nixie.jpg?direct&200|nixie-tubes}}
  * **Nixie tubes**: the frequency is displayed on Nixie tubes, in which luminous filaments for all digits from 0 - 9 are controlled directly. Usually they are glowing in an orange-yellow colour, energy requirement is high and the Nixie tubes tend to wear out after some years of use.
{{ :images-small:kurz-freq-fluoreszenz.jpg?direct&190|fluorescence-display}}
  * **Fluorescent display**:\\ the frequency is displayed on a fluorescent tube display unit, which glows blue - greenish,\\ the power consumption is relatively high (often, the display can be switched off), good reading quality.
{{ :images-small:kurz-frequenz-led.jpg?direct&200|LED frequency display}}
  * **LED display**: \\ the frequencies are displayed with LED seven segment displays, sometimes there is also alphanumeric information on the operation mode or filter selection. The red or occasionally yellow or green numbers can be difficult to read when direct light falls on the display. The power consumption is medium.
{{ :images-small:kurz-frequenz-lcd.jpg?direct&200|LCD frequency display}}
  * **LCD display**: Liquid crystal displays, some as numerical seven-segment displays, some in the form of special symbols (narrow <> wide IF filter, keypad lock, etc.), some in the form of dot matrix displays, are most commonly used in the receivers of the latest generation. They require little energy (when a voltage is applied, the liquid crystals form a recognisable symbol without consuming current), but they usually require a backlight or a reflective background, which increases the energy requirement somewhat.
{{ :images-small:kurz-frequenz-dot-matrix.jpg?direct&200|dot-matrix-display}}
  * **Dot matrix display**, for example TFT displays: the display technology of modern smartphones gives multicolour display of frequency, operation mode, activities in a certain band (waterfall display) and even displays television pictures; it is used for example in semi-professional receivers of the latest generation.
  * **Blackbox** units: the entire receiver electronics are housed in a cabinet without operating or display elements, the set is controlled completely from a connected computer. There is virtually no limit to the display options, but the power requirement of the connected (notebook) computer is still a limiting factor, for example, in mobile operation.