UHF REJECT FILTER FOR R/C RECEIVERS


                 

      When installing an ATV transmitter on a radio controlled vehicle, some
 difficulty is often encountered with interference to the R/C receiver. This
 can be caused by the TV signal getting into the receiver and causing funda-
 mental overload of the receivers front end. The signal can get in via the
 72 MHz antenna, connecting leads, or via direct pickup on the receiver board.
 The use of shielding and bypassing can deal with the latter two problems, but
 there remains the problem of antenna pickup. A suitable filter is shown below
 that should help the problem considerably.

              .08 uH    .145 uH      .145 uH    .08 uH
        INPUT---L1---------L2-----------L3---------L4----- OUTPUT
                      |           |           |
                      |           |           |
         50 ohms     C1          C2          C3           50 ohms
                      | 39 pF     | 43 pF     | 39 pF
                      |           |           |
         GND ---------------------------------------------- GND


      This filter is a 0.1 dB Tschebychev lowpass with a rejection of 90 dB
or better at 430 MHz, and 0.2 dB insertion loss at 72 MHz. The 3 dB cutoff
frequency is 120 MHz. Capacitor and coil details are given below.
                                                                      
      C1 = C3 =  39 pf NPO chip (preferred) silver mica or NPO useable*
                                                                     
         C2   =  43 pf NPO chip (preferred) silver mica or NPO useable*

      * Note: Capacitor leads to be 0.1 inch (2.5 mm) or shorter

      L1 = L4 =  6.5 turns # 22 enamelled closewound .140" (3.55 mm) ID

      L2 = L3 =  11  turns # 22 enamelled closewound .140" (3.55 mm) ID

      Note that a #28 drill can be used as a mandrel to wind the coils on.
Use drill shank, and remove the insulation and pre-tin coil leads before
soldering into the circuit. The filter is symmetrical and input and output
can be interchanged. Layout configuration can be like the schematic.

      The above performance figures are theoretical off the computer and the
actual rejection you get in practice depends on component layout and the deg-
ree of stray coupling between sections, and the components used. Chip cap-
acitors are ideal, though good quality silver mica or NPO ceramic capacitors
will be satisfactory if leads are kept very short, <0.1 inch or 2.5 mm. Also,
capacitor lead inductance will affect stopband performance. Ideally the filter
should be shielded to prevent the receiver side from picking up signal. It
does little good to lock the front door and leave the back door open. But you
should be able to achieve 60-70 dB or better, which is still pretty good, and
should be sufficient to reduce the interfering signal to a level the receiver
can deal with.

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