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Receiver Design Help

Discussion in 'Homebrew and Kit Projects' started by NC4JB, Jul 16, 2010.

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  1. NC4JB

    NC4JB Ham Member QRZ Page

    I'd eventually like to start building my own rigs, and to start with I'd like to build a good quality general coverage receiver. Gotta crawl before you walk tho, so I figured I'd start out with a simple "crystal" receiver like the one in the Forest Mimms book.

    I know this is a very basic design for a receiver that won't really cut it. I'm familiar with the basic elements of a receiver: local oscillator, mixer, detector, and so on, but I'm not really sure what makes a receiver better. There don't really seem to be a whole lot of resources online that explain this either. Is there a book that starts with building a basic receiver and then goes through progressively more complicated receivers with better performance, explaining WHY and HOW they perform better? Or maybe someone can give me an explanation here about what makes a good receiver and what makes a better one?

    After I've gotten my feet wet and built a decent tube receiver, then I'll worry about transmitting :cool:
     
  2. W3JN

    W3JN Ham Member QRZ Page

    There's a pretty good book by Joseph Carr on receiver design, written for beginners. Has plenty of projects.

    The best reference source would be one of the old Radio Handbooks (aka "the West Coast Handbook") or just about any ARRL handbook. If you're interested in building a tube receiver, go for a late 50s/early 60s edition.

    For more advanced theories, Dr. Ulrich Rohde (of Rohde and Schwarz) has written a couple books on the subject.

    Finally, Google is your friend! There's a ton of info on HB receivers on the net.
     
  3. KI6J

    KI6J Ham Member QRZ Page

    "...maybe someone can give me an explanation here about what makes a good receiver and what makes a better one?"

    Frequency STABILITY, signal SELECTIVITY, and incoming signal SENSITIVITY. I think there are a few others, like noise floor and dynamic range, but they might be related to the first three. To improve these, you must add circuit COMPLEXITY.
     
  4. AB8RO

    AB8RO Ham Member QRZ Page

    IMNSHO, you won't really learn much from a crystal receiver. Doug Demaw suggests that a regen is great starting point. Many suggest starting with a direct conversion receiver.

    To some extent what you're looking for will come with understanding. Give it time. Build a simple receiver without worrying too much about what comes next. Then ask what are its limitations? You usually trying to balance a number of competing qualities.

    Although there are other considerations, Doug Demaw, in his book "Practical RF design Manaul", highlights Dynamic Range, Selectivity, and Sensitivity, and describes how each quality would affect the performance of a communication receiver. I think that book is great and very readable, however, it does not include complete projects.

    I also like "Solid State Design for the Radio Amateur", and "Experimental Methods in RF design." Although both have many projects, I think that you get the most satisfaction out of extracting the elements and "designing" your own.

    I'd start by building an audio amplifier, then an on frequency VFO or VXO and you are most of the way to a direct conversion receiver. You could then think about what changes would be necessary to make it a single conversion superhet. What would be the advantages of doing so? For example, how do you improve selectivity in a direct conversion receiver vs a superhet. What are the advantages and disadvantages of each method.
     
  5. NC4JB

    NC4JB Ham Member QRZ Page

    Lots of good info to go on, thanks folks!

    I do have some basic schematics for direct conversion AM receivers. As far as I understand I can change the intended frequency range by adjusting the LC values? Maybe someone can correct me on that, but if it's true then I know there are tons and tons of schematics online for regen and superhet receivers that I can modify to encompass the frequency range I want, then experiment with to add features and improve performance.

    I have the ARRL 2010 handbook but it seems to sacrifice detailed information on the building blocks in favor of more information about digital frequency synthesis and phase locked loops, which is all fine and dandy but I don't want to build radio-on-a-chip rigs :rolleyes:
     
  6. AB9LZ

    AB9LZ Ham Member QRZ Page

    Heh, that question probably has been the subject of many a doctoral thesis and pretty hard to answer in one paragraph.

    The ever popluar NE-602 - SA-612 based direct conversion RX's are a good place to start. They (unlike the regen's) use the basic building blocks of most modern receiver designs, and can be decent performers.... i.e. regens can be pretty fickle at times.

    If you are new to electroncis, I'd shy away from the tube stuff until you get a few solid state projects up a running. Even with published designs, there is still quite a bit of trial and error / trouble shooting involved in getting homebrew stuff to work... and being able to touch a *live* circuit without hurting yourself is something to think about as you'll have your fingers all over that thing trying to get it to work.

    Cost is another factor, building tube gear these days is expensive if you don't have a deep junkbox to rely on.

    73 m/4
     
  7. NC4JB

    NC4JB Ham Member QRZ Page

    Do you know where I can get NE-602s? I've used one before in a Ramsey kit and it seemed pretty straightforward, but I've been unable to locate anywhere that sells them in a DIP package. Mouser and Digikey were no help.
     
  8. W3JN

    W3JN Ham Member QRZ Page

  9. AI3V

    AI3V Ham Member QRZ Page

    Like has been posted, a good receiver is a balance of many things.

    Frequency stability, slow drift is obviously bad, but fast drift- or phase noise is bad too. think of phase noise a a very fast drift of the vfo, for some fraction of time you simply are listening to a different frequency!

    Sensitivity-more sensitive is better, untill you run into "KTB" noise, then you cannot improve sensitivity further.

    Strong signal handling- in general a receiver that uses amplifiers that can handle more power (signal) will be harder to overload, we do this by using amplifiers that have more dc power going thru them AND local oscillators that provide more power to the mixer, the tradeoff to more lo power is that signal will tend to escape from the receiver (be radiated) and interfere with other receivers, that's what the FCC "part 15" notice on the back of yout clock radio is about. Also, the more current needed in a amplifier will be directly opposite what you want for backpacking, a low power draw set. (To use a TX analogy, you cannot expect to get 1 watt from a 1/2 watt battery!)

    Selectivity- should match the "occupied bandwidth" of the transmitted signal, and this selectivity should be as close to the antenna as you can afford (as opposed to being on the speaker end of things), by stopping off frequency signals (and noise!) from ever getting to the circutry, you prevent the strong signals from overloading that circuitry!

    Then you get into features,

    RIT
    If shift
    Noise blanker
    Squelch
    Multi mode/multi band
    Digital display
    Computer interface

    There is nothing quite like the thrill of using a set you designed and built yourself.

    Good luck in you journey!:)

    Rege
     
  10. AI3V

    AI3V Ham Member QRZ Page

    Yep, this is about it.

    Keep in mind that if you make HUGH QSY- say trying a 160M design on 70CM that things won't scale well.

    Try "Dan's small parts" He's got the "good stuff" for experimentors:cool:

    Rege
     
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