NDR-oscillator
En NDR-oscillator (eng. Negative Differential Resistance oscillator) er en oscillator, som har en elektronisk NDR-komponent eller NDR-kredsløb, der på en del af sin overføringskarakteristik har negativ differentiel modstand. En NDR-oscillator er en to-terminal oscillator, da ingen tilbagekobling er nødvendig. Mellem de to terminaler kobles AC-mæssigt en elektrisk svingningskreds.[1]
NDR-komponent eller NDR-kredsløb
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Uddybende artikel: Negativ differentiel modstandSelvom NDR-komponenten er ulinear og har negativ differentiel modstand, er den som regel i mindre strøm- og spændings-intervaller linear, hvilket betyder at den differentielle modstand i intervallet er konstant.
I netop et nogenlunde lineart interval med negativ differentiel modstand, kan NDR-komponenten forstærke signaler nogenlunde lineart.
En af de simpleste eksempler er at sætte komponenten AC-mæssigt i serie med (lav NDR-impedans) eller parallelt over (høj NDR-impedans) svingningskreds. Når svingningskredsens samlede tab (inkl. evt. ekstern belastningsmodstand) mere end ophæves af den negative differentielle modstand, så vil kredsløbet fungere som en NDR-oscillator.
Eksempler
[redigér | rediger kildetekst]Eksempler på NDR-oscillatorer:
- Oscillator med lambda-diode.[2][3]
- Oscillator med tunneldiode: TDO; tunneldiode oscillator.[4]
- Dynatron oscillatorkredsløb med dynatronrør.[5] Se illustration.
- Dynatron oscillatorkredsløb med tetrode.[6] Se illustration.
- Transitron oscillator med pentode.[1][7][8][9] Se illustration.
- Visse IC-kredse kan have negativ differentiel modstand, negativ kapacitans eller negativ induktans i indgangen eller udgangen under visse kredsløbsforhold - se kilde.[10]
- En transistor emitterfølger kan lave utilsigtet oscillation, hvis belastet kapacitivt, da emitterfølgere udgang typisk har negativ induktans.[11]
Referencer
[redigér | rediger kildetekst]- ^ a b r-type.org: The Transitron. A P Blackburn, The Radio Constructor, October, 1955 Citat: "...The transitron makes a very useful sinusoidal oscillator. The frequency range obtainable can be from a few cycles per second up to nearly 100 MHz. The circuit is shown in below...One advantage immediately becomes clear. The oscillator circuit merely requires a tuned circuit. There are no taps on the coil, and no coupling winding. For this reason the circuit is sometimes called a 'two terminal oscillator'...It is worth mentioning a few suitable valves for transitron operation. Obviously, they must be pentodes and the suppressor grid must be available at a pin...", backup
- ^ A Dip Meter Using the Lambda Negative Resistance Circuit Lloyd Butler, Amateur Radio, January 1997, backup.
- ^ vk6fh.com: Negative resistance and the lambda diode, backup
- ^ clarku.edu: Tunnel Diode Oscillator Essentials. Chuck Agosta - Clark University Citat: "...The tunnel diode oscillator (TDO) is a highly stable self resonating oscillator system that we use to measure rf penetration depth in conducting, superconducting, and magnetic materials...", backup
- ^ "Radiomuseum: Dynatron elektronrør". Arkiveret fra originalen 4. marts 2016. Hentet 31. oktober 2020.
- ^ elprocus.com: What is Dynatron Oscillator: Working and Its Applications Citat: "...In that dynatron oscillator is one of the oscillators which shows a negative resistance characteristic...It is invented by Albert Hull in the year of 1918...The negative resistance effect of the tetrode tube cancels the positive resistance of the tuned circuit. Therefore the tuned circuit will have zero resistance. So, the oscillating voltage at the resonant frequency will be generated...Dynatron oscillator...wide operating frequency range. In the Second World War, these were used in many applications...", backup
- ^ Paywalled: The Transitron Oscillator. Proceedings of the IRE ( Volume: 27, Issue: 2, Feb. 1939)
- ^ THE TRANSITRON OSCILLATOR. Bulletin of the American Association of Jesuit Scientists, Eastern Section, Volume XVIII, Number 4, 1 May 1941
- ^ Paywalled: A Practical Negative Resistance Oscillator. Cledo Brunetti. Review of Scientific Instruments 10, 85 (1939) Citat: "...A simple, all‐purpose oscillator of the negative resistance type called the transitron oscillator is described..."
- ^ rf-microwave.com: SL610, SL611 & SL612 Citat: "...The input impedance is negative between 30MHz and 100MHz (SL610C, SL611C only) and is shown in Fig. 3. If the source is indictive [inductive] it should be shunted by a 1kohm resistor to prevent oscillation...The output stage is an emitter follower and has a negative output impedance at certain frequencies as shown in Fig.5. To prevent oscillation when the load is capacitive a 47 ohm resistor should be conncted in series with the output..."
- ^ sciencedirect.com: Emitter-Follower. Emitter followers are notorious sources of oscillation and should never be directly driven from low impedance sources (again, see References 43 and 44). From: Analog Circuit Design, 2011 Citat: "...In addition, emitter followers are known to oscillate very easily if capacitively loaded. This is due to the output impedance looking like a negative inductance as the ft rolls off. This inductance forms a tuned circuit with the capacitive load...", backup
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