Tuning RF amps

Discussion in 'General Electronics Chat' started by DexterMccoy, Mar 8, 2014.

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  1. DexterMccoy

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    Feb 19, 2014
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    How do you tune RF power amplifiers? do you use an RF signal generator? and you use the RF power meter to measure what? you use the VNA vector network analyzer to measure what for RF power amplifiers?

    When the transmitter is not clean? what are the other signals of?

    Noises from what? EMF? oscillation? crossover?

    Does the Transmitter need more filtering to clean it up?

    What does it mean to Tune an RF amplifier? tuning what? and how do you tune it? trim pot?
     
    Last edited by a moderator: Mar 8, 2014
  2. bertus

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  3. DexterMccoy

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    Why do you need to know the input impedance and output impedance of the amplifier?

    When is the input and output impedance measurement bad and when is the measurement good? in what ranges
     
  4. DexterMccoy

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    What does it mean to Tune an RF amplifier? tuning what? and how do you tune it? trim pot?

    When is the tuning bad and when is the tuning good?
     
  5. antonv

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    How do you tune RF power amplifiers? It depends on the design
    do you use an RF signal generator?
    You could
    and you use the RF power meter to measure what?
    Power
    you use the VNA vector network analyzer to measure what for RF power amplifiers?

    Quote:
    you would also use a spectum analyser on the output to make sure the transmitter has a clean signal
    When the transmitter is not clean? what are the other signals of?
    Byproducts of non-linearities in the circuitry

    Does the Transmitter need more filtering to clean it up?
    Typically yes

    What does it mean to Tune an RF amplifier? tuning what? and how do you tune it? trim pot?
     
  6. antonv

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    What does it mean to Tune an RF amplifier? tuning what? and how do you tune it? trim pot? and variable capacitors and inductors.
     
  7. Papabravo

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    Q:What does it mean to Tune an RF amplifier?
    A:It means that at the frequency of operation you need to match the source impedance of the amplifier with the impedance of the load.
    Q:tuning what?
    A:Typically an LC resonant circuit.
    Q:and how do you tune it?
    A:With a mechanical knob.
    Q:trim pot?
    A:No a trim pot won't help you since it's impedance is not frequency dependent.
    Q:When is the tuning bad and when is the tuning good?
    A:The tuning is bad when the VSWR is greater than about 2.5:1, and the tuning is good when the VSWR approaches 1:1.
     
  8. DexterMccoy

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    What are you tuning it to? to the receiver frequency or transmitter frequency

    What can be the byproducts? what are the byproducts? oscillations, crossovers?
     
  9. DexterMccoy

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    What is the VSWR?

    Do you use an VNA vector network analyzer to set the VSWR? and tune the RF amplifier to match its source and load impedance?
     
  10. Papabravo

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    RF Power amplifiers are used in transmitters, not receivers and the tuning is typically for an entire band although you could re-tune each time you changed frequency.

    Mixing two signals of different frequencies by a non-linear circuit produces signal energies at frequencies besides the frequencies of the two inputs. You can derive this result for yourself with basic trigonometric identities. Just multiply two cosine functions of different frequencies and look at the result.
     
  11. Papabravo

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    VSWR - Voltage Standing Wave Ratio
    It measures the relative magnitudes of forward power and reflected power.

    Most amplifiers will measure this for them selves, tell you what the value is, and shut themselves down if it is too high.

    A VNA is typically used to measure passive circuits and individual active devices like RF transistors.
     
  12. DexterMccoy

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    Thanks Papabravo for your time and info

    So what do you use to Tune the RF amplifier to the right RF frequency? do you use a spectrum analyzer on the output of the RF amplifier and tune the LC network to the receiver's frequency or the transmitters frequency?

    When tuning the VSWR ratio, do you use a RF power meter on the output of the RF amplifier or a spectrum analyzer on the output of the RF amplifier to tune the VSWR ratio?
     
  13. Papabravo

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    The RF Amplifier I am most familiar with is the KPA500 from Elecraft.

    http://www.elecraft.com/KPA500/KPA500.htm

    It has band selector switches on the front panel, and the transceiver, a K3, communicates with the KPA500 and tells it what band the transmitter has selected. For an antenna I use resonant inverted V, one for each band.

    In this case no tuning is required, and the amplifier has green, yellow, and red LEds to monitor the power level and the SWR. I can also measure and monitor the heatsink temperature of the RF power transistors.
     
  14. DexterMccoy

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    So tuning the RF amplifier, is tuning the frequency band? for that RF power amplifier?

    What is the Band selector switches are choosing different frequency of what inside the RF power amplifier?

    Does using a Spectrum analyzer help to tune the transmitter/reciever bands? or what sets the bands?
     
  15. DexterMccoy

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    I thought tuning a RF amplifier was MATCHING the Transmitters frequency with the Reciever's Frequency

    Is this right or wrong?
     
  16. Papabravo

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    The concern for a transmitter is that if the output stage is not matched to the antenna (the load), the antenna will reflect a significant amount of power back to the source (transmitter output stage) instead of radiating it. This reflected power must be dissipated as heat. If the mismatch is serious, the reflected power can destroy the amplifier. The KPA500 is extraordinarily aware of the situation and will shutdown before the reflected power can destroy it.

    A spectrum analyzer would be used to assess the quality of the transmitter's signal.
     
  17. DexterMccoy

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    So tuning means: adjust the LC circuit to a frequency ( band ) of the RF amplifier that will match the Antenna ( load ) frequency, using a Spectrum Analyzer?

    Tuning Mean: to adjust the LC circuit to match the output impedance of the RF amplifier to the Antenna ( load ) Impedance using a VNA Analyzer?
     
  18. Papabravo

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    Neither instrument is required. If you know the antenna is a match for the frequency band, and the amplifier can monitor the SWR then no instruments or manual tuning procedures are required.

    You could use the VNA to measure the characteristics of the antenna, but you would normally do this with the amplifier disconnected. Don't know about you but I sure couldn't afford a VNA at prices north of $80,000.00
     
  19. DexterMccoy

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    What kind of manual tuning procedures would you do? if it is required that you can think of please?

    What would you use a VNA for when testing or test procedures for RF transmitter circuits and RF reciever circuits?

    What would you use a Spectrum Analyzer for when testing or test procedures for RF transmitter circuits and RF reciever circuits?
     
  20. BReeves

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    On the bench you need a spectrum analyzer to be sure you are not tuning it to a harmonic, dummy load to simulate the antenna and a watt or power meter to measure the output. You are either adjusting a variable cap or a coil and tuning each stage for max output at the desired frequency.

    Transmitters are not always on the receive frequency.
     
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