SLIM-MCF-106
Construction Page

(Iremoved this page from the web on 8-3-09, I don't think it was every referred to.)
SLIM-MCF-106, Crystal Filter, size-A
    Download SK-MCF-106, Schematic, in ExpressPCB software.
    Download LAY-MCF-00, Layout, in ExpressPCB software.
    Download PLSLIM-MCF-106, Parts List in text format.
(The Express extensions will download in a text format.  Save and change the name to .pcb or .sch)

     The SLIM-MCF-106 is based on a generic design that will accommodate a variety of multipole, Monolithic Crystal Filters.  The filter shown in the design (X1) is distributed by U.S. Electronics.  However, their minimum quantity is 1000.  This has a center frequency of 10.695 MHz.  The bandwidth is 2.2 KHz and has a loss of -4.5 dB.   It has a 600 ohm inut/output impedance.
    This module, using the 10L024, is a "blind" design, and is really a place holder until I can find a good, inexpensive MCF that the home builder can easily acquire.  I have been using MCF's that were removed from old single side band CB radios.

SK-MCF-106, Schematic of SLIM-MCF-106
schematic of Crystal Filter

LAY-MCF-00, Layout of SLIM-MCF-106 Rev 0
    This layout was fabricated by ExpressPCB in Jan 08, for the MSA/FG/VNA Varification Assembly.  I built the module as designed, but used a filter taken from a Radio Shack CB SSB radio with a part number of FL-096.  Testing showed a performance flaw in my design.  The input section and output section are too close together to get really, good isolation performance.  I could only get -84 dBc input to output isolation.  The "pass-thru" seemed to be through the FR4 material, right through the line of ground vias.  I would consider this a fault of the pwb material, not my basic layout design.  I have used this scheme with closer connectors and have never had this amount of poor isolation.  Therefore,  I modified the layout to closely resemble the pictoral that is shown next, as:
LAY-MCF-00, Layout of SLIM-MCF-106 Rev 0
layout mcf-106 Rev A
    I  retained the same filter taken from a Radio Shack CB SSB radio with a part number of FL-096.  It is probably made by Uniden, but I cannot confirm that.  The size is called "L2".  Now, the isolation is quite good, approaching -100 dBc.  See the following Graph taken during tuning.  The filter is mounted on the bottom side of the pwb with the leads extending through to the top layer.  The perimeter of the Crystal filter is soldered to the bottom ground plane.  The top layer has a full perimeter shield fence and another shield fence separating the two sections.  Two "lids" are soldered to the top of the fences.
    A size "L1" filter can be used in this layout, which is smaller.
SLIM-MCF-106 Rev A, using FL-096 Crystal Filter
isolation plot
    This crystal filter impedance is close to 1000 ohms, with 6 pfd internal shunt capacitance.  Therefore, L1 and L2 are 3.3 uH and C1 and C2 are approximately 56 pf.  The input vias and output vias are actually ground vias to the ground planc.  They must be cut away from the ground plane on the bottom of the board.  Use a sharp drill bit to isolate the vias from the ground plane.
    No matter what monolithic filter you use, the input impedance and output impedance will not be exactly as the manufacturer claims.  Also, the input impedance and output impedance will not be the same.  It will be close enough to give good results using standard inductors and capacitors.  If the builder would like to critically tune the filter, the impedance matching inductors and capacitors need to be variable.  The fixed input capacitors, C1 and  C2, could be decreased in value by 15 pfd and a small (2-20 pfd) variable placed in shunt with the fixed capacitors.  The inductors, L1 and L2, could be wound as toroids and be adjustable by removing or adding turns.  This critical tuning can improve the bandpass ripple by as much as 1 dB and decrease insertion loss by, perhaps, .5 dB.