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RF Amplifier devresi

Konusu 'Fm Verici Devreleri' forumundadır ve guclusat tarafından 27 Mayıs 2014 başlatılmıştır.

  1. guclusat
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    guclusat Tanınmış Üye Süper Moderatör

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    [​IMG]
    The circuit above is the exact one used
    in the Progressive Communications Receiver.





    One of the best RF amplifiers ever published was Wes Hayward's post mixer amplifier in the Progressive Communications Receiver. It quickly became known as a great high level RF amplifier. With a standing current of 50ma, it takes a very strong signal to upset this amplifier. Check out page 15.24, figure 15.33 of the 2000 ARRL Handbook for Radio Amateurs for more information.




    [​IMG]


    Instructions for bypassing the RF amplifier are near the bottom of this page.

    In between the Hamstick and dream antenna farm, the RF amplifier is left in and a 50 ohm pad is adjusted for the right amount of gain.

    The right amount of gain is a compromise between sensitivity and dynamic range. When using a Hamstick, signal levels are so low that going all the way for sensitivity gives a very good performing setup.



    Low Noise Antennas


    An excellent article that shows the effectiveness of using Hamsticks in a dipole configuration is "Receiving Antennas", by Robert L. Nelson, K6ZGQ, Ham Radio, May 1970, pp 56-63 (Subtitled: "A discussion of special purpose antennas for receiving - including some novel ideas for improved performance on the lower bands").

    Low noise receiving antennas, combined with a high level RF amplifier gives the ideal receiving setup.

    Mr. Nelson states, from the article above (page 62), "I have obtained good results with both a helical dipole and an untuned five-foot-long dipole with a capacitive hat when worked through an antenna coupler. However, the helical arrangement is somewhat better."

    Again, on the helical dipole (page 62), "This antenna, if mounted horizontally, will be horizontally polarized, electrical-field sensitive and balanced with respect to ground, and will discriminate rather well against local noise."

    Even though this receiver has plenty of gain to work with small antennas, they must be tuned and matched to the 50 ohm input of the receiver. An antenna tuner must be used with random wires for best results.

    A Hamstick used at its design frequency gives the best reception. However, a 17 meter Hamstick gives even reception throughout all the bands of this receiver. My ground lead was a #12 wire to the pipes in the bathroom. A counterpoise wire run along the baseboard of a room will also work as a ground.

    Other Hamsticks give different results. A 30 meter Hamstick will work on 17 meters, but on none of the others. A 20 meter Hamstick will work ok on 40 meters, but hardly at all on 30. A 40 meter Hamstick will not work very well on 20 meters, etc. For general listening, use a 17 meter hamstick.

    In many mobile installations, several Hamsticks are mounted on a common mount. A multiple Hamstick installation would work well with the receiver.



    Adjusting Gain


    To gage proper gain at the RF stage, listen to the signal in the speaker and check for 455 kHz IF strip noise. This receiver was designed so IF noise is masked by the signal levels entering the IF strip.

    If you do not get a definite rise in background noise when tuning a band with the bandpass filter pot, more amplification is needed. If the band noise blows you off the table, add more loss in the 50 ohm pad.

    A 3dB pad is installed at the output of the RF amplifier. Higher loss pads may be used. Tests on loss/gain through the front end and bandpass filters indicate the receiver has the same gain with or without the 3 dB pad.

    The reason surmised is that the gain is improved by better matching with the 3dB pad versus a little mismatch that loses some signal strength without the pad. The 3dB pad is recommended in every situation. Real differences begin with a 6db pad.

    The 50 ohm pad used in the receiver is a Pi-Network Resistive Attenuator. The following are some values that would be useful in this receiver. Reference for this information came from the 2000 ARRL Handbook, page 30.24. Only the most recent editions of the Handbook carry this information.



    [​IMG]


    dB Attenuation R1 (ohms) R2 (ohms)
    3 292 17.6
    6 150.5 37.3
    9 105.0 61.6
    12 83.5 93.2
    15 71.6 136.1


    Fixed Attenuator Calculator Online:
    Will calculate any impedance and any desired attenuation.
     
    Son düzenleyen: Moderatör: 10 Ocak 2015
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