Adding a mechanical filter into a is often a relatively simple job. The physical tasks are sometimes more difficult than the electrical. The electrical tasks are:
Suitable filters can be found for sale here. Ebay is also a possible source. Mechanical filters expect to be driven and to be terminated by the proper source and load resistance. Older filters with transducer coils also require the proper loading capacitors to resonate the coils to the IF frequency. It's also desirable to have an idea of the insertion loss of the filter to know if an amplifier will need to be added to supply additional gain. Of course you need to know the bandwidth of the filter to know if it's a desirable filter to add to your receiver. Most filters will be either high impedance (100 Kohms) or low impedance (2Kohms).
If modifying a modern solid state receiver, the receiver will usually be equipped with one or more ceramic filters already. If the receiver has a single filter, the design decision will be whether to replace the existing filter with a fixed mechanical filter for better bandwidth and or shape factor, or to add an additional filter which can be switched in or out of the circuit, leaving the existing filter in place.
If adding an additional filter, some method of providing switching must be selected. This might be by adding a toggle switch, either with a drilled hole in the front or rear panel, or for receivers with a door in the top, perhaps inside. An alternative might be to use an existing front panel switch with an unneeded function - dial light or ANL off or on, for example and use that to select the filter instead, either wired directly, or using the switch to power a relay or perhaps diode switching circuitry.
If replacing an existing mechanical filter, remove it and wire to its input and output terminal pads. If adding a switched filter in series with an existing ceramic, there is usually a series capacitor between the filter and the following IF amp. If the capacitor is removed, its two connection points become the inputs and outputs for the added filter/switching circuitry. Since the added filter should be connected with shielded cable, a nearby ground point will have to be found for the cable shield.
If adding a low impedance filter to a solid state radio, the circuitry driving the present filter may match the added filter correctly without changes. If the added filter has a low insertion loss, no other circuitry may be required. More often, an added stage of gain is required. Figure 1 Figure 1 shows an amplifier schematic which has worked well. RL and CL represent the required filter termination.
If using a high impedance filter in a low impedance solid state receiver, the input resistor should be placed in series with the filter, as shown in Figure 2. Even if additional gain is not required, the amplifier is useful to interface between a high impedance filter and low impedance receiver circuitry.
A lightweight filter such as the F455FA21 can often be held in place with double sided foam tape. Heavier filters will require a suitable mounting bracket.
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Copyright © 2004 John Kolb Last revised Apr 28, 2004