Hans The murata K13 is the matching filter for the YK88S2. The band width is around 2.9khz . The K12 is less, around 2.5khz. Yaesu FT990 has one in it. Cost about 35.00 dollars over here ....... the YK88S2 & the Murata K13 both came in the TS-930 as standard equipment, & are popular with the Hi Fi SSB guys ........ John