Ignoring most of the previous posts in this thread (sorry - I deleted them) it
occurs to me that maybe the problem may be the relatively low impedance load on
the buffer, whose finite output impedance therefore causes some waveform
distortion and loss of 5th harmonic. So how about a hi-Z parallel tank coupling
circuit; for 17.2MHz (say 4p7 || 18uH) into a load of 100-500 ohms (say a 1k pot
+ 100n to Gnd, to see the effect of load variations), then into another 74AC
buffer?

Tony (remove the "_" to reply by email)