Some time ago, I said I'd post a description and schematic of an
excellent frequency doubler using a quadrature hybrid directional
coupler to drive a diode DBM. It is now available on ko4bb.com
following Didier's recent work on the site (Thank you, Didier, for
this wonderful resource):
<http://www.ko4bb.com/manuals/download.php?file=05_GPS_Timing/4_App_Notes_and_Articles/Frequency_doubler_quadrature_DBM.pdf>
When carefully constructed, this circuit has exceptional inherent
balance and, consequently, very low feedthrough of the input
frequency and its harmonics. It also avoids most of the flicker
noise created by push-push doublers. For many uses, no output
filtering is needed. (The schematic includes a filter/amplifier, in
case one needs a signal with even lower harmonics.)
In addition to the frequency doubler (which is drawn for 5MHz/10MHz,
but can be easily scaled), I included the seminal article on
lumped-constant quadrature hybrid splitter/combiners. More
information on these extremely useful but relatively unknown devices
can be found in Chapters 3 and 9 of Wes Hayward's "Experimental
Methods in RF Design," in the discussions of single sideband and
image-reject mixers.
(EMRFD is a must-have book for anyone playing with RF hardware, IMO,
the same way Horowitz & Hill's "The Art of Electronics" is for
general electronics knowledge. Hayward's "Introduction to Radio
Frequency Design" is also extremely useful.)
Best regards,
Charles
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