Re: Any DIY balun info for S-video to Cat5 conversion?

On Sat, 03 Sep 2005 08:25:08 +0200, martin griffith
<martingriffith@xxxxxxxxxxxxx> wrote:

>On Sat, 03 Sep 2005 09:56:17 +1000, in sci.electronics.design Tony
><tony_roe@xxxxxxxxxx> wrote:
>snip
>>All true; but this IS "sci.electronics.design", not
>>"sci.electronics.buy", so it's a valid question.
>
>and "cost" is part of the design process.

Learning can be part of the design process too, weigh the value of the
learning against the costs.

There is lots of balun design info out there, and here on my drive I
find an old Signal Integrity list message from Ray Anderson on the
subject, with book references which are likely to be more useful than
SED for this particular design.

(BTW Ray Anderson is the administrator of the Signal Integrity list
and a very well respected engineer, and no one chimed in with any
additions or corrections.)

-------------------

Designing 1:1 Baluns for Receivers
Date: Fri, 28 Oct 1994 09:38:32 +0800
From: Raymond.Anderson@xxxxxxxxxxxx (Ray Anderson)
Message-Id: &lt;9410281638.AA01322@xxxxxxxxxxxxxxxxxxxx&gt;
To: qrp-l@xxxxxxxxxx
Subject: designing 1:1 baluns for receivers


Dave W6MIK recently asked me a question about designing small
baluns for use in a receiving converter between 40 and 70 MHz.

I've put together some notes on this and thought there might be
interest amongst the other homebreweres on the list about this
topic.

I'm certainly no expert on this topic, what I've put down comes
from several of the common references on the subject. If there are
any errors they are probably my own and I retain copyright to them :)
If anyone on the list knows more about this than I do, ( and I sure
hope
someone does) feel free to make corrections and comments.

72's de Ray WB6TPU
raymonda@xxxxxxxxxxxxxxxxxxxx

--------------------------------------------------------------------------
Here is a bit of information on building small baluns using
toroids or 2-hole cores:

.
out &lt;--------------------mmmmm----------+-- GND
|
+----------------------------+
(balanced) | .
+-------------mmmmm----------+
|
out &lt;-----+----------------------------+
| .
+-------------mmmmm------------&lt; &lt;= Unbalanced
Input

(dot on end of inductor indicates phasing)

1. The device is wound with trifilar wire.
(use 3 different colors if you can to help
eliminate confusion)

2. First, determine impedance (Zo) the balun will be
used with. Zo=sqrt(Zin * Zout)

3. Rule of thumbs says the inductive reactance Xl of each
winding should be 4 to 5 times Zo.
(So for 75 ohms in/out Xl=300 ohms to 375 ohms)

4. Determine the inductance for the Xl at the lowest
frequency you want to use the device at.
(for Xl=300 @ 40 MHz L=Xl/(2*PI*F)= 1.2 microHenries)

5. Select the core material: for HF to low VHF Amidon type
63 or 61 Ferrite material would be appropriate.
These have mu values of 40 and 125 respectively.
At lower frequencies you would want to use something
with a mu of several hundred. At higher frequencies
a mu of 5 - 10 using powdered iron material might be
appropriate.

6. Select the core size: FT23 is .23 inch diameter which is
one of the smallest sizes commonly available. Bigger parts
can handle more power before saturating as a rule. Choose
the size that fits your application.

7. Look up Al value for the core material/size in a
catalog/databook. core FT23-63 Al=7.9

8. Calculate the number of turns required.
N=sqrt(L/Al)*1000 (L is in mH)
So N=sqrt(.0012/7.9)*1000 = 12.3 turns

9. Wind 12 turns trifilar according to diagram above.


The same logic works if you are going to use double hole
"binocular" balun cores. The number Al will be different
but is obtained from the core manufacturers literature.

With small cores you need to use small wire. #28 or #32 or even
#40 might be appropriate for FT23 size core. The higher the
mu value and the lower the frequency, the more turns you are
going to need.

There are other topologies for 1:1 baluns, but all the calculations
are still the same, just different number of windings and connections.

We haven't considered power levels or core saturation since we
have assumed we are making the balun for a low power receiver
application. For higher power applications there are additional
things that need to be considered or else the core will saturate
with magnetic flux, and the coupling between windings will decrease
and the thing won't work correctly (and may go up in flames).

See Sevicks books on Transmission line transformers, The ARRL
Solid-State Design Book, and Haywards book on RF design for other
good info on designing these type baluns. The Amidon catalog
and Ferroxcube catalog amongst others also have some good info
as well as the Al values for their particular cores.

Ray WB6TPU
.

Relevant Pages

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