The fundamental-type crystal filter, as opposed to a filter using 3rd- or 5th-overtone crystals, has a better shape factor and is less susceptible to intermodulation under strong-signal conditions. Roofing Filters: These are SMD fundamental-mode monolithic crystal filters (MCF), manufactured by Daishinku. Again, we believe that relay (rather than diode) switching will be used here, for the same reasons as outlined in 2. It is likely that both preamps will use high-current MMIC's, optimised and tuned for their respective frequency ranges. In addition, up to 18 dB of attenuation can be inserted in the RF signal path ahead of the first mixer. RF Preamp/Attenuator: The IC-7800 offers two selectable RF preamps one for the HF range, and the other for 6m. We are confident that this concept will be extended in the IC-7800, with physically larger inductors in the preselector, RF BPF, mixers and amplifiers. RF Inductors: One of the IMD-reduction measures employed in the IC-756Pro II front end was the redesigned RF BPF with inductors wound on larger cores, to avoid saturation at high signal levels. In this way, the preselector in the 7800 is somewhat protected by the BPF one recalls the possibility of generating intermodulation products in capacitors at extremely high signal levels." Saturation of inductor cores will also cause intermodulation. He says that normally, the signal flow through selectivity elements is always from broader to narrower circuits. Why is the RF BPF before the RF Preselector? Here is a comment contributed by Clemens Paul, DL4RAJ: "I have discussed this with a friend of mine who is a retired R&S HF engineer. The roofing filters following the first mixer are switched by NEC µPG2009TB GaAsFET switches. The use of relay switching can significantly improve IMD2 and IMD3 dynamic range. It is noteworthy that all high-end commercial and military HF transceivers use relay-switched RF bandpass filters - as do some amateur transceivers such as the Elecraft K2 and K3. Switching diodes can generate intermodulation products under strong-signal conditions PIN diodes insert loss ahead of the first active RF stage, thus degrading system NF. The filters are entirely relay-switched, to eliminate switching or PIN diodes from the RF signal path of the receiver. RF Bandpass Filters (RF BPF): Our take on the RF BPF bank is that it will consist of third-octave filters with contiguous -3dB points, to allow general coverage. The new Icom Japan IC-7800 pages indicate a 100% relay-switched approach this is mechanically simpler, and thus potentially more rugged and reliable. The alternative to varactor tuning is a 2-stage synchronously-tuned BPF using relay-switched fixed capacitors and inductors, with either motor-driven air variable capacitors or additional relay-switched fixed capacitors for fine tuning. A 6dB fixed attenuator at the preselector input served to minimise parametric mixing (in which a strong RF signal can modulate the varactors and generate unacceptable intermodulation products.) This attenuator also significantly degraded the receiver system noise figure (NF). The JRC JST-135 HF transceiver employed a varactor-tuned preselector. RF Preselector (Digi-Sel): Initially, we assumed that the tracking preselector is not varactor-tuned. Onlythe HF range (exclusive of 6m) will be considered here. The new Icom Japan IC-7800pages tend to confirm the concepts presented below. Figure 1 shows the RF bandpassfilters (BPF), preselector and preamplifier (AMP) ahead of the first mixer shown inthe IC-7800 receiver block diagram in Icom's IC-7800 Technical News handoutfrom Dayton 2003.