What is the expected level into the Tx bi-amp stage at 45MHz? I don¡¯t have a feel for this and can¡¯t check it as my uBITX is in pieces at present.
If it's a relatively high level then the 45Mhz amp stage maybe into limiting.
But, my testing on a proto build of these stages, shows that the bi-amp as it is, although fitted with a BFR106 in 1st stage, is indeed, severely limiting at levels above about -13dBm.
?
Ie at -10dBm input level, 2nd harmonic is ~21dB down, 3rd is -22dB
And at -13dbm input, 2nd harmonic is -33dB, 3rd is -32dB.
Around a 10dB improvement in harmonics for a 3dB input level drop!
?
This is probably the reason why fitting the extra filter mod works reasonably well, as it's actually just reducing the gain of these stages, as Allison has surmised before.
If this is actually the case, then a simple ~3-4dB (or more?) pad at the input may be a solution although better would be to re-calc the Bi-amp stage for lower gain.? Currently the gain is about 18-19dB.
Glenn?? -vk3pe
