EMRFD Message Archive 10967

Message Date From Subject
10967 2015-04-08 08:38:13 Ronan McAllister Advice on textbook references to inter-stage, wide-band RF coupling
Greeting EMRFD,

I have been recently "reborn" into HAM, after a very long break in IT/software ; some decades ago I was an engineering tech and did some minor design work for the consumer audio electronics industry -- at that time all of my designs were direct-coupled and of course audio and in that situation we did not have the need for inter-stage impedance matching as the audio bandwidth wasn't all that wide/high.  Most of my "designs" were actually taken from manufactures data manuals, such as National Semiconductor's great op amp product lines wherein studying the plethora of examples for one part and bouncing that against my electronics textbooks allowed me to not only understand the original designs, but adapt/modify them for my use.

Recently I have been researching 40 M DSB QRP portable phone rig designs and have come across the use of balun transformers coupling a driver to a driven stage in a RF power amp stage wherein the balun serves to act as a RF choke to the B+ supply (one end supplies B+ to the driven element collector through one half of a bifilar winding,  and the other end (port) s AC coupled to the driven transistor).

Unfortunately without having been gathering QRP/ham circuit documentation and books for years, I sort of feel I'm looking for a needle in a haystack!

Any advice you can help is appreciated - I am looking for any technical explanations (other than the majority I find in Amateur Radio articles which sometimes lack enough depth) -- to help me understand how to analyze this for myself, so I can at least get close to a good match between stages on paper first, prior to "throwing" parts at a design and randomly winding transformers/toroids - I am looking for technical explanations, even textbooks but not so technical that I must understand the theory, transformations and differential equations...  I would like to use this design and modify it for my own purposes, transistors, etc.

IE rather than to simply "lift" someone's transformer design from an article/book, and "play" with components until I get it to work -- I would really like to know how to calculate my own transformers (in this configuration) at least to get me in the "ballpark".

Just the morning,  I found a great hint I have been looking for!   

In Wes Hayward's QST Apr 2006 pg 28-32 QST article "An Updated Universal QRP Transimtter" -- this EXCELLENT article contains a perfect example of what I am looking to find more inf on  (if you have a copy of this article, please look at the explanation on pg 28 and the schematic of T2/T3) -- this is very similar to other designs I have found although this design has two baluns rather than one I have seen in other more recent designs, the circuit description confirms how these work:

"Ferrite transformer T2 moves the 200 ohm output impedance seen looking into the Q2 collector to 50 ohm."...

"... The Q3 power amplifier input is matches with transformer T3.  The nominal 50 ohm of the driver is transformed to 12 ohm by T3"

On Wes's schematic T2, T3 windings are 10 and 7 bifilar turns respectively.

Initially when I saw this topology in another design, I thought it might be literally intended to do a balanced to unbalanced conversion (single-ended to differential) however I'm sure I was glossing over the real intent.

The 10:10 turns I see in several examples of these inter-stage ferrite core baluns (FT-37-43 for example) appear to be providing a 1:4 (or 4:1) impedance match between the driving stage and the driven stage.   From Wes's article I cite, this makes total sense -- 200:50 ohms is exactly 4:1 ratio, so with Wes's short yet meaningful explanation -- I am starting to experience a weakly-lit light bulb moment!!

Here is where I am : I hope (sans calculus expertise but with significant dogmatic attitude and a good basic electronics knowledge to the junior college level) to find enough similar examples of this inter-stage transformer design (even if it involves "cookbook" calculations) -- such that I can have at reasonable feeling when I build what I have designed on paper, it will work somewhat how I intend (that it won't fail miserably)!!!    I do NOT want to model, characterize these transformers to the 'nth degree, I am simply looking for as many references to additional information on this as I can.    BTW I feel I will be able to calculate the input/output impedance of transistor amplifier stages just fine (once I dust my brain off from too many years in IT)... To refresh my transistor design theory / knowledge,  I will be re-reading my old textbooks, EMRFD sections on this -- building simple amplifiers, coupling them (however my ham shack lacks sophisticated gear, I do have some basic equipment -- scope, DVM, freq counter; NO network or spectrum analyzer).    I'm very comfortable about transistor amplifiers, it is the use of the Baluns coupling stages in HF gear recently that totally threw me into a tail spin!!!    I'm not going to simply build someone else's design if I don't understand it... just me...

EMRFD itself is an excellent guide, although I am continuing to look elsewhere.  I am the type who learns best by looking at multiple examples of things similar to what I am trying to do.

I do have a schematic I made up in LTSpice of a PA RF stage (7mhz, two RF drivers, one PA -- 3-5W ) I "Lifted" from someone else's DSB QRP phone rig design without any technical explanation I was considering building) ; and I may muddle through it attempting to model a basic balun using coupled single inductors, however, without some of the theory and more background, I'm not 100% confident this "simulate and modify" (hunt and peck) method of design is the best path to follow.

In the meantime I will continue to research!   I am not expecting anyone here to tutor me in transformer design, two port analysis, transmission lines -- however, if anyone has such a good handle on this that they could point me to some useful references (again not too deep in theory) -- it would be greatly appreciated!!!!

Sorry for such a long e-mail, I wanted you to know where I am experience-wise.

Thanks in advance!


10969 2015-04-08 09:18:51 Jim Strohm Re: Advice on textbook references to inter-stage, wide-band RF coupl

The balun examples by Hayward in that QST article were straight from Guanella, and are reasonably well described by Sevick.  But -- a lot like EMRFD where you need to read and internalize all the info, you also need to read all of Sevick's books and similarly commit them to memory before any of it makes sense. 

The Granberg works on matching transformers are a lot more accessible, but you need Sevick before it all makes sense.  In the meantime (just the works I've mentioned are enough for an upper-division EE class) merely take it on faith that these "cookbook" designs work.  That'll get you past the "I don't understand" moments.

Here's a clue -- the impedance transform is the square of the turns ratio, which is relatively forgiving as to input and output impedances, and a transformer can be connected either way, for a transform up or a transform down. 

I won't give you a complete bibliography unless you ask, but these seminal authors will teach you just about everything you need to know about the subject.


10971 2015-04-08 10:56:36 Ronan McAllister Re: Advice on textbook references to inter-stage, wide-band RF coupl

Jim, that's perfect!

EMRFD: no additional suggestions needed ( unless you care to) to respond to my long winded posting -- I've got my reading list in place now.

Thanks again Jim!

10973 2015-04-09 10:09:09 wb8yyy_curt Re: Advice on textbook references to inter-stage, wide-band RF coupl

nice posting on your work.  you certainly have a decent test equipment stash to work with. 

yes those 'light bulb' pieces of data are strewn about out there, so capture and document them in your own electronic or paper notebook (preaching to myself here ....). 

please shoot me an email, as I may have something useful.  my address is available at qrz.com

73 CUL Curt WB8YYY