Why use IF amplifiers?

Discussion in 'Wireless & RF Design' started by firebreather, Jan 20, 2015.

  1. firebreather

    Thread Starter New Member

    May 27, 2013
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    When I can achieve a gain >1 for a narrowband signal using purely passive components like LC circuits, why use active devices like transistors for voltage gain ? Is it because of convention ? The overall circuitry becomes complex and one has to take care of biasing business,clipping and saturation ,nonlinear distortion,etc
    ..
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    the gains required to get a signal strong enough to dtive an output like an 8 ohm speaker require large amplifier stages and they are easier to design for the lower IF freq.
     
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
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    why dont you look up how much your reciever input is expected to handle? how many microvolts sensativity? how does that get to your speaier? there are riecivers without if amplifiers, such as direct conversion regenerative, and trf, the normal superhetrodyne with if amplifiers is the most practical.
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    There is voltage gain, current gain, and power gain. You cannot get power gain with passive components. Power gain is required to get an audio signal from a low level RF signal. It is true that a crystal set will drive high impedance headphones in the vicinity of a 50 kW clear channel AM station, but that is hardly the normal situation. Indeed why use an IF at all. The KX3 is by all accounts an extremely capable direct conversion receiver.
     
  5. profbuxton

    Member

    Feb 21, 2014
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    I don't know what sort of wonderful PASSIVE components you use to achieve >1 gain or what useful application you use it for but if you are talking radio you do need a number of ACTIVE components to generate a useful signal.
    In regard to why IF stages in radio(I guess thats what you mean) they provide fixed frequency gain stages to output a useful signal. With only RF stages and needing a number of them each one would need to be tuned each timed you changed frequency(different station). By using an IF stage/s you only need one Rf stage(generally) for tuning.
     
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  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    If you can get gain of more than 1 with passive devices, you have just invented perpetual motion.
     
  7. BR-549

    Well-Known Member

    Sep 22, 2013
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    It's mainly because of frequency.

    Imagine the number of coil and cap sets you would have to have....to RX from 25 kHz to 2.4 gHz.

    We can do the same with a dozen or so by using an IF amplifier.

    The reason we use IF amps is so when can have large bandwidth with few components.
     
  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    I worked on some very old HF receivers that were some of the first transistor PLL triple-conversion synthesizer models. The IF sections were easy to align but the RF stage took forever to setup from scratch because it needed complex impedance switching to cover the wide frequency range while still keeping the gain values consistent.


    http://www.schmitzhouse.com/Johns_Electronics_28.htm
     
  9. #12

    Expert

    Nov 30, 2010
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    That's how the VHF mechanical TV tuners worked in 1970.
    I cleaned a lot of contacts back then!
    Little guy, cross legged in the floor, with a bunch of tiny tools.
     
  10. Kermit2

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    radios were simpler but push button mechanisms could get me cussing. :mad:
     
  11. BR-549

    Well-Known Member

    Sep 22, 2013
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    That's why I'm so interested in the TAYLOE detector.

    This circuit should be able to RX DC to a couple of gig, without any tuned circuits.

    I bought the chips, so I could play around....but they are all surface mt. and I can not work with them.

    If my eyes and hands were young, I would be passing these detectors among all my friends.

    http://forum.allaboutcircuits.com/threads/dan-tayloe-circuit.100989/
     
  12. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    I/Q modulators/detectors have been around forever. (in FM/SSB equipment) It 's great now they are available as a commodity chip and when combined with software for cheap SDR receivers they can replace some conversion conventional models.

    http://forum.allaboutcircuits.com/threads/military-aircraft-radio-receiver.82143/#post-587944
    http://www.tpub.com/neets/book12/51e.htm
     
    Last edited: Feb 2, 2015
  13. BR-549

    Well-Known Member

    Sep 22, 2013
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    Has anyone here ever used diversity receiving?

    In the 70's diversity receiving had a different meaning than it does today.

    Just one antenna........but the Q signal went in the left ear and the I signal went to the right.

    When listening to the signal.........it gives the signal a unique location in your brain.

    It is very easy to listen to the signal you want....but at the same time.....ignore the others.

    It's like listening to one person talking in a noisy room full of other people talking.

    It's hard to explain....but one could easily listen to weak signals among all the strongest.....because of the location in the brain.

    Sort of like DSP between the ears.
     
  14. alfacliff

    Well-Known Member

    Dec 13, 2013
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    a detector by its self isnt a very good reciever. there are other conciderations, especially for a tunable reciever. such as bandwidth, not bit rates, but the actual bandwidth allowed through the reciever. in the days before the superhetrodyne, regens were used, even with a tuned rf amp on them, the bandwidth was wide, allowing many frequencies to come in at a time. the trf (tuned rf) reciever has several rf amplifier stages, that helps a bit, but the bdnwidth changes as the reciever is tuned. the super hetrodyne gets its bandwidth from its relatively low frequency tuned circuits, and stays the same at all frequencies the radio is tuned to.
     
  15. DickCappels

    Moderator

    Aug 21, 2008
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    The benefits of IF amplifiers:

    1) Thermal noise, such as that in RF circuits tends to be proportional to 1/ frequency (a gross simplification), so gain at the IF produces less noise than amplifying at baseband.

    2) In the case of an IF that is significantly lower than in incoming carrier frequency, IF amplifiers, by virtue of their controlled Q tuning, have better control of their bandwidth and therefore can have better selectivity and better signal-to-noise.
     
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