The following is an argument for installing 9 MHz and 455 kHz filters with equal or comparable bandwidths and shape factors in an Icom HF or multi-band transceiver:
Assuming that the filter passbands overlap (PBT or Twin PBT in centre position), the composite adjacent-channel selectivity at the stopband (typically -60 dB) will be superior to that of one filter alone.
In addition, for PBT or Twin PBT to operate correctly, the passbands of the two filters should at least be similar. Thus, for example, the FL-80/FL-44A pair ensures a much tighter and more aggressive PBT action than FL-80/FL-96. Pairing 250 and 500 Hz CW filters (in any order) will make the PBT action very sloppy.
A pair of 500 Hz CW filters in a radio equipped with PBT or Twin PBT is a very acceptable compromise. Whilst the adjacent-channel selectivity with the effective bandwidth reduced to 250 Hz via the PBT is not quite up to that of a pair of 250 Hz filters, it is still very satisfactory in many cases.
In a receiver, the filter closest to the first mixer does most of the work in terms of determining the IF bandwidth and filtering out the unwanted sideband, out-of-band signals etc. If only one filter is installed, it should be the 9 MHz filter. Note that in the original, unmodified IC-765, the 9 MHz filter is bypassed when enabling IF Shift. This allows strong out-of-band signals to ride down the IF strip and overload the third mixer (9 MHz/455kHz). These undesired signals degrade close-in dynamic range.
Conversely, in an analogue transmitter which shares the filters with the receiver, the first filter should be as close to the balanced modulator as possible, to ensure adequate suppression of spurious products. This means that the transmitter should always have a 455 kHz filter.
Thus, we end up with a transceiver fitted with crystal filters at both IF's. View a list of popular Icom IF filters, and their passband curves.
Copyright © 2002 A. Farson VA7OJ/AB4OJ. All rights reserved.
Page last updated: 11/04/06