Thanks Ian
My external audio filter is a passive LC design by W3NQN that I built as a kit back in the 1990's . No longer available as a kit but it is well described on the web-site (www.w3nqn.com). However it is quite bulky with the AF inductors (and has 3db insertion loss) so possibly not first choice for portable operators! I have just purchased the SOTAbeams 'Laserbeam' digital filter and will be trying that out soon (also incorporates a fixed narrow filter so may have to tweak the uBITx for this as well).
Also following up your other advice, many thanks.
Chris
