Transmitter

Discussion in 'Wireless & RF Design' started by tazntex, Aug 6, 2009.

  1. tazntex

    Thread Starter Member

    Sep 29, 2008
    27
    0
    Hello to all,
    I am trying to get this circuit to produce rf on 433.92Mhz. I am using an rfm0301a resonator. L1 is the pcb trace antenna. I have injected a 1k square wave into R3 and all I see on my scope at L1 is my square wave but on the spectrum analyzer I have nothing. From the datasheet R2 is supposed to be 470 Ohms but a friend said make it 10. Looking at the datasheet I thought this circuit was basically a colpitts and calculating L/C/F I am way off. Any suggestions?
     
    Last edited: Aug 6, 2009
  2. fanie

    Active Member

    Jan 20, 2007
    63
    0
    You get little RF transmitters as a module. They are factory tuned to be precise on frequency and it is far easier to use than trying to get your circuit right. All you need to do is pump your data out.
     
  3. tazntex

    Thread Starter Member

    Sep 29, 2008
    27
    0
    Thanks for the reply.
    Yes, rf modules are great items and take up very little landscape and the price of those are great, but trying to build a circuit from scratch and learning in the process is priceless. I am just trying to figure out how to calculate the values for the components used in this circuit.

    Thanks again
     
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    At VHF frequencies and using PCB traces as components, the calculations will be a nightmare. From experience building such things commercially, just the make of circuit board material can have enormous effects.

    For a prototype, you may be better off building 'dead bug' style - use a solid piece of copper clad board and stick the components to it upside down, with only ground wires actually soldered to the copper.

    Keep every connection as straight and short as possible and put a 0.1uF ceramic cap directly from any power/VDD/VCC terminal to the copper ground to avoid parasitic resonances.

    This eliminates most of the stray capacitance involved in circuit board traces and gives your circuit a chance of working.

    (ps. your circuit file appears to be a netlist dump from some unspecified program?)
     
  5. tazntex

    Thread Starter Member

    Sep 29, 2008
    27
    0
    Thanks again for the response. The file attached was LTspice drawing. Attached is the datasheet with the circuit I was inquiring about. It is the example of a typical AM transmitter. Although they give you an example of component values for this circuit I am wondering if there is a formula I could use to calculate the exact values.

    Thank you
     
  6. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    The basic formula for resonant frequency is
    1 /
    2 x pi x (square root ( L x C))

    L is in Henrys and C in Farads, the result in Hz.

    At a guess the series capacitance of c1 + c2, plus the transistor in parallel, may be about 4pF
    Working that back from 433.92MHz gives: 3.36 e-8H or .0336 uH; 33nH for L1

    Working through with 4pF + 33nH gives 438MHz so I think these values are theoretically OK.

    Make C1 a 1-10pF ceramic trimcap and try 12 or 15pF for C2, then see if you can find (or wind) a 33nH inductor.

    If you are really neat with the assembly, it may work - but as it says in the data sheet, "C1, C2 and L1 are the most critical components and will require value optimization for each PCB layout or other design change".

    I'm guessing values, I just hope they are somewhere near.
     
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