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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]

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