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Winding ratio in IF transformer - RF Cafe Forums
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Charl
Post subject: Winding ratio in IF transformer Posted: Sun
Aug 05, 2007 3:42 pm
Colonel
Joined: Fri May 19,
2006 5:01 am
Posts: 25
Location: Netherlands
Hello everybody,
In building a radio, I am wondering: why do all IF transformers
I see have a near 1:1 winding ratio? This goes for both single-tuned
and doubled-tuned transformers. Couldn't they get more gain by using
say a 1:2 ratio? I could only see one objection: more windings ->
more self-capacitance -> self-resonance may get too near operating
frequency. Why don't they use more secondary windings?
Thanks
in advance,
Charl
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IR
Post subject: Posted:
Mon Aug 06, 2007 1:35 pm
Site Admin
Joined: Mon Jun
27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello Charl,
I guess that the reasons for that could be:
1. Maintaing
the same impedance along the IF chain.
2. 1:1 Transformers have
the widest BW available.
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Charl
Post subject:
Posted: Mon Aug 06, 2007 2:12 pm
Colonel
Joined:
Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Hello
IR,
Thanks for your reply. I really want to make sure I understand
this, so I hope you don't mind this follow-up.
1) How can you
talk of impedance when either side of the transformer is an LC tank?
2) What is the reason for this? (if this is textbook material, please
refer me to a good textbook :) )
Kind regards,
Charl
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IR
Post subject: Posted: Mon Aug 06, 2007 4:13
pm
Site Admin
Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello Charl,
Transformers
are located not only between resonance tanks but also between filters
and amplifiers to maintain the charactieristic impedance (1:1) or to
perform impedance transformation.
There are very good application
notes in RF Cafe elaborating about transformers design.
In general,
if you increase the number of turns or the frequency of operation in
transformer you reduce the flux. Therfore, reducing the number of turns
will give you a wider BW.
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nubbage
Post subject:
Posted: Tue Aug 07, 2007 3:20 am
General
Joined:
Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi
Charl et al
A good exposition on resonant single and coupled circuits,
ie tuned transformers, is given in Terman "Radio Engineering" Chapter
3.
If 2 windings are resonant and the number of turns is in a reasoanable
ratio, say 3 to 1, then the Q is approx the same. The dynamic impedance
of each side at resoance is 2*pi*f*L*Q
If Q is roughly equal on
each side, then the impedance ratio is the same as the inductance ratio,
which is the square of the turns ratio. This simplistic argument does
not take account of coupling coefficient, but this is fully shown in
the Terman textbook. If therefore the number of turns on the secondary
is increased, the load impedance will have to be very high for maximum
power transfer. Whilst the resistive part of many solid state device
impedances is high (IGFETs for example) the capacitive reactance is
very low at high frequencies. Thus the resonant secondary will be dominated
by high and temperature variable capacitance of the following stage
input load.
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nqureshi
Post subject: Impedance
of parallel RLC circuitPosted: Tue Jan 13, 2009 3:14 pm
Lieutenant
Joined: Tue Jan 13, 2009 3:06 pm
Posts: 1
can you tell me
how to calculate impedance of parallel RLC circuit? if I have a L=1.2uH,
C=72p, R=1.2k in parallel what will be the Impedance?[/quote]
_________________
RF circuit Design
Posted 11/12/2012
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