EMRFD Message Archive 8717

Message Date From Subject
8717 2013-06-10 10:28:41 Alan Ibbetson LF spurs using Sortabalun as PA output transformer
I am making a 40W class C PA using switching FETs as part of a 5.2MHz
60M project. I am using a drain-drain load of 50 ohms and I tried using
the T5 output circuit of fig 34 on p62 of the old Solid State Design
book. This circuit couples the push-pull output to the single-ended
harmonic filters with DC blocking capacitors and a common mode choke,
also sometimes described as a "sortabalun".

My PA was beset by low level spurs at -60dBc about 75KHz away from the
main output.

I've realised the problems are caused by the series resonance of the DC
blocking capacitors and the inductance of the sortabalun, around 75KHz
in my case. It doesn't seem to make much difference whether miniature
coax or twisted pair is used.

The problem does seem real and not caused by circuit strays or other
defects of implementation in as much as LTspice predicts this behaviour
using perfect components, although the spurs are hardly detectable in
class AB when the drain waveforms are less able to excite the LF series

I changed the output circuit to one using a conventional transformer,
like fig 2.101 in EMRFD, and the spurs have vanished.

A sortabalun does not seem to be a good choice of output circuit for a
push-pull class C PA. Or have I overlooked a simple fix that would allow
this admittedly very convenient topology?

73, Alan G3XAQ