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. VA2GK

    VA2GK Ham Member QRZ Page

    Or you can also get a carbon earphone, a 1N60 diode, some magnet wire and a roll of toilet paper.

    Oh, wait... he said FM :D
  2. KC7YPJ

    KC7YPJ Ham Member QRZ Page

    throw vista in the trashcan where it belongs then install xp?
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    The ARRL sells 3 or 4 current books on radio design.

  4. K8ERV

    K8ERV QRZ Member QRZ Page

    The TP roll will have to wait a few days.

    Carbon Earphone?

    TOM K8ERV Montrose Colo
  5. K8ERV

    K8ERV QRZ Member QRZ Page

    Me too, but dog doesn't. She likes food better than jokes. Bummer.

    TOM K8ERV Montrose Colo
  6. KC4UMO

    KC4UMO Ham Member QRZ Page

    This place would be a bore with out Tom. I love his comments.
  7. AB8RO

    AB8RO Ham Member QRZ Page

    Here's a more specific question that I'm fumbling with but related to the OP's question.

    If we have a receiver that's meant to be fed with a 50 ohm antenna and the impedance looking into the first stage is higher, say 700 ohms, then we have to do some transformation as well as perform some bandpass filtering. While W7ZOI suggests the DTC is the minimum and the TTC is preferred he points out that many receivers use just a simple single tuned resonator. I've seen several ways to do this including a tapped capacitor with a resonating inductor. A transformer with a resonating capacitor. A transformer with a resonating capacitor and a tapped secondary. An inductor with a resonating capacitor where the inductor is fed via a tap close to ground.

    There must be trade offs involved with each choice, yes?

    So related to this is choice of inductor/capacitor. You can use toroids and fixed capacitors if your filter doesn't need to be peaked. You can use toroids and variable capacitors otherwise, or, fixed capacitors and slug tuned coils. I prefer the later but only because I find slug tuned coils easier to work with than toroids.

    What are some of the decisions you make and why? When would you choose transformer matching over a tapped inductor or capacitor? Do you always use the same basic input design? Do you calculate things, or, like one of my old elmers did, just grab something and dip it til it fits. How important is precise impedance matching on receiver front ends as opposed to transmitter outputs? What real world differences would you expect to see between a simple resonator, a double tuned circuit, or a triple tuned circuit?

    Doug Demaw gives a simple breakdown of the split capacitor resonator/impedance transformer in his "practical RF design manual" on pages 108-110. He talks about choosing a bandwidth to determine the loaded Q of the resonator. I understand that some of the choices above, i.e. to tap an inductor are to not load the resonator excessively in order to preserve Q. He further explains on page 27 that loaded Q and insertion loss are inversely related hence it's often necessary to endure higher insertion loss in order to ensure better Q. Of course this loss might mean additional amplifier stages which means more noise so we exchange one desirable property for another.

    What isn't clear is how one chooses the right balance. In the example on page 109 he uses a bandwidth of 1Mhz for a 5Mhz circut yielding a Q of 5. Is that a reasonable value across HF? Should one expect that a 2Mhz bandwidth at 10Mhz is a reasonable figure?
  8. AG3Y

    AG3Y Guest

    Yea, but try to FIND a copy of XP now days ! I think MS recalled them so sales of Vista wouldn't be hurt. I'm staying way away from Vista!

    :eek: :eek:
  9. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    All of the modern commercial radios use broad band transformers in the front end to match the 50 ohm input to the high impedance input of the first RF stage. As far as impedance matching goes, maximum power is transfered when the impedances are matched. Most old tube receivers had an antenna trimmer. This was an L network designed to match low impedance input to the grid of a tube stage.

    This same circuit will work for a solid state rig and provides a small amount of pre-selection also that improves signal to noise ratio. The tapped inductor circuit will work but is usually narrow band to maintain a useable Q. I generally calculate an input circuit based on the desired bandwidth using min and max frequency for omega and add 10 to 15 percent for component slop.

    This will work for any design but for homebrew receivers operating at Fo I prefer to use a peaked L network so that I can maximize signal input. If you build a 70 MHZ front end and down covert you can get away with broad band transformer matching. That's what most modern radios use or a wide ceramic filter.
  10. AB8RO

    AB8RO Ham Member QRZ Page

    Ok, so if I'm reading this correctly it suggests that the DeMaw example is wide for any ham band application. This is confirmed a bit by the example on page 95 of solid state design which gives an example for 20 meters with a loaded Q of 37.4 and a bandwidth of 374 khz. This is much narrower than the DeMaw example and the SSD suggests that it's appropriate for the entire 20 meter band.
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