EMRFD Message Archive 235
Message Date From Subject 235 2006-12-15 10:17:35 brianars Using EMRFD with other RF Design Books I've enjoyed catching up on posts from the past few months on here and
have a question that I believe many of you will have great insight on.
I'm a big fan of EMRFD and have even exchanged a few technical
E-Mails with Wes about this book.
Over the years, I've also collected my own copies of Intro to Radio
Frequency Design and Solid-State Design for the Radio Amateur. I must
admit that I only thumbed through them a bit, probably because I
picked them one up when I was in high school (didn't understand the
theory) and the other while in college (didn't have much time). But
EMRFD is the first of these three that I've dug into, playing around
with modeling and building these circuits, and learning some great
stuff. I'm sure others on this list have these other books as well...
So my question is: How do you (or would you) approach using all three
manuals together? On a recent post, I saw that it was pointed out
that EMRFD was not intended to be an encyclopedia. This is more of
where the Handbook comes in. And while SSD and IRFD approach
-slightly- different topics, I'm looking for thoughts and ideas on how
folks use these books together.
Do you start with EMRFD, then refer to SSD and IRFD for a more
detailed explanation? Do you do the opposite? Or what?
Thanks for the ideas, and I look forward to the responses.
Brian - N1FIY
236 2006-12-17 16:08:57 Stan Re: Using EMRFD with other RF Design Books Hi Brian,
You do raise an interesting point - where does one start?
The Solid State Design and the ARRL handbook are good references. I
can tell you how I got started in RF design. I would take a known
working circuit and then attempt to determine how every component was
selected by the designer. I had to dig deep but that technique
works. Start with the LC networks, then the resistors and then the
caps. There is enough information in the SSD, EMRFD and handbook to
figure out all of the components in a simple HF solid state
Many of the simple transmitters you see use 50 ohm input for the LPF
and 50 ohm output, this is the wrong approach they should attempt to
determine the output RF collector impedance and then design a LPF to
match that value to their 52 ohm output connector. But you will
find quickly which guys actually design their LPF and which copy from
someone elses circuit. The main difference between LPF are the
selection 3db frequency and ripple. Try different values, also down
load the AADE filter program (its free) to check your results the
cpu thinks is correct.
W1FB does a good job indicating what rule of thumb he uses in most of
his circuits. i.e. The RF choke if you use one is usually 10 times
the value of the output RF stage impedance. Anything close will
work. The bypass caps across a resistor is usually will have an Xc
of 10x the value of R.
Give you something easy to check. A simple op amp audio amplifier
will have a feedback resistor paralleled by a cap, the 3db rolloff
frequency will be the freq at which the Xc of the bypass cap is the
same value as that resistor.
But start with simple circuits and then ask questions. You will
note even after doing it for many years I still ask questions on the
elists. i.e. I have asked on this list WHY
237 2006-12-17 17:39:08 Rick Re: Using EMRFD with other RF Design Books Good comments except for the following:
> Many of the simple transmitters you see use 50 ohm input for the LPFCurrent wisdom is that a power amplifier device (bipolar or FET) is not matched, it is
> and 50 ohm output, this is the wrong approach they should attempt to
> determine the output RF collector impedance and then design a LPF to
> match that value to their 52 ohm output connector.
loaded. For just over 1 watt with a 12 volt supply, 50 ohms at the desired frequency
presented to the collector by a capacitor input 50 ohm LP filter is exactly what experienced
and knowledgeable designers often use. That is a popular power level for simple
transmitters, and the circuits have been widely copied back and forth by experienced and
inexperienced experimenters alike. No matter how talented the designer, there are only
so many standard value components, so 40 meter 50 ohm low-pass filters all end up
looking pretty much alike.
For other power levels or supply voltages, a different load impedance is presented to the
collector. At HF and VHF, the low-pass filter is still usually designed for 50 ohms, and the
impedance transformation done using a broadband transformer. Using the LP filter to
perform the impedance transformation from 50 ohms to the desired collector load is
occasionally done for single-band transmitters (and often in cell phone PAs), but even
then it is the filter that presents a load to the collector, not the other way around.
Another reason to use 50 ohm low-pass filters is that you can measure them with just a
signal generator and power indicating device. If you choose to do the impedance
transformation in the low-pass filter, you should really have a spectrum analyzer on the
bench to look at the harmonic levels out of the complete amplifier.
Best Regards to all,
251 2006-12-19 06:33:49 brianars Re: Using EMRFD with other RF Design Books Stan...
Thanks for the reply. You certainly got me thinking about this a bit
Rick - Thanks for your input as well.
The way I've handled this recently is that by moving through EMRFD, if
I want to dig a bit deeper into the theory, I see which of the other
books has more theory. I'm thinking there must be a better way...
Any other thoughts