How to design the front end of a radio

Discussion in 'Homebrew and Kit Projects' started by KG6NEI, Jun 8, 2008.

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  1. KG6NEI

    KG6NEI Ham Member QRZ Page

    Hey guys,

    I am looking to see if anyone knows of a website that goes over the basics of creating the front end portions of an fm radio (front end meaning the radio itself, up to the feedpoint of the antenna). Any sugesstions?
     
  2. VK2TIL

    VK2TIL Ham Member QRZ Page

    His Eminence The Receiver

    W7ZOI's series of articles entitled "His Eminence The Receiver" is still relevant nearly 40 years later. It is a great discourse on the requirements for a "good" receiver.

    The articles don't seem to be available on the 'net (they are in QRP Classics, your library may be able to get it) but this page;

    http://www.geocities.com/capecanaveral/lab/3955/N1OOQ.html

    seems interesting.

    FM, AM, CW, SSB...... The principles are the same, although the design has to consider the particular conditions likely to be encountered; those conditions will be different in each case, ie 160 m CW, 2 m FM etc.
     
    Last edited: Jun 9, 2008
  3. N4CD

    N4CD Ham Member QRZ Page

    The best book on the sbuject, as far as I know, is Communications Receivers - Principles and Design, by Ulrich Rohde and T. Bucher.

    There are people who spend their whole lives doing nothing else but designinig the 'front end' of receivers.

    You have to decide how much money, time, effort you want to expend in making your receiver front end.

    You start with the design of the front end filter - and that depends upon the application. That rejects 'out of band signals' and minimizes the bandwidth of noise. YOu want the lowest losses. Gold plated cavities are nice if you want the ultimate for VHF/UHF.

    Then, you might want a nice RF amplifier run off fairly high voltage for the highest 3rd order intercept point. 48v DC might be nice here.

    Then, more filtering, gain allocation, more filtering, attention to oscillator noise/phase noise, superb injection chain.....etc.

    Followed by highest spec IF strip and detector for the mode of interest, then low noise audio chain.

    In the process, you need to add in a superior noise blanking system, and probably DSP noise reduction and maybe digital filtering in the IF.

    If you aren't interesting in pursuing all the math in the Rohde book and ultimate design prinicples, the W7ZOI book provides a basic cookbook for a decent front end design. They are a bit dated now with the emergence of digital filtering, but you can incorporate that into the designs once you understand the principles, and get up to speed on DSP (digital signal processing).
     
  4. K8ERV

    K8ERV QRZ Member QRZ Page

    Just design a good rear end, then turn it around. Works for me-----

    TOM K8ERV Montrose Colo
     
  5. AC0FP

    AC0FP Ham Member QRZ Page

    If your looking for something for free, I can't help you. If your interested in gaining knowledge get the ARRL handbook. All of the receiver design knowledge you will be able to use is contained in this book. ;)
     
  6. AB8RO

    AB8RO Ham Member QRZ Page

    I think that's a bit overstated. I have probably 10 or so handbooks from 53 to 05 and I don't think that any of them or even all of them put together contain many of the insights that you can find in "experimental design for the radio amateur" or w1fb's "practical rf design manual." The handbooks are good and every ham should have one or two (or more) but other books have insight that's missing or buried in the weeds in the handbook, so to speak.

    Another book that's golden is "the art of electronics."
     
    Last edited: Jun 9, 2008
  7. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    There are several books on the subject but the basic principles for an FM radio front end are noise figure and selectivity. Most high performance FM receivers use helical resonators in the front end coupled to a low noise cascode preamp. Helical resonators can have an unloaded Q as high as 10,000 which gives you a good bit of rejection. GaAs FETs have very low noise at VHF and above and are an excellent choice for preamps.

    Also, helical resonators get pretty large at frequecies below 144 MHZ so you won't see them in HF gear. Most high end HF rigs use a Cohn type of preselector ahead of the first RF amp and mixer.

    Other things to consider are mixer injection level vs noise. Typical HF mixers use high level injection for strong signal handling but more oscillator injection equals more noise in the output in the form of phase noise (mixing products). For a VHF receiver a simple diode mixer will work and is fairly stout but has some loss. An active quadrature mixer has gain but is prone to overloading unless you design it for high level injection and again with the phase noise issue. I would opt for a diode mixer in an FM receiver. Your going to have plenty of gain in the IF to make up for the loss.

    Have fun and try a few different circuits out. The best advice I can give you is that once you establish a signal to noise ratio at the first mixer that's it so build a good solid first filter and amplifier ahead of the mixer and you pretty much have it whipped.
     
  8. NA5Z

    NA5Z Ham Member QRZ Page

    And if you like "free", download "AppCad".

    The tab for system noise figure alone is worth the price of admission.
     
  9. WY6K

    WY6K Ham Member QRZ Page

    Doesn't seem to work on Vista. Anybody know any fixes?
     
  10. VA2GK

    VA2GK Ham Member QRZ Page

    Tom, I love your sense of humor, really :D:D:D
     
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