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I.R
Post subject: AD8309
Unread postPosted: Tue Apr 13, 2004 3:05 am

Hi,

Our board has includes a limiter. The application is broadband:
30-500MHz. We use the AD8309 device. This device has an internal resistance of 1KOhm. Our source is 50Ohm. We therefore need to match between those 2 impedances. According to the datasheet the recommended way is to use a flux transformer. The impedance ratio is 16:1, the maximal bandwidth available for such impedances ratio is up to 130MHz (according to the numerous number of transformers we checked so far). To use a pad will dramatically reduce the SNR. Can anyone suggest an alternative way (besides cascading two 4:1 transformers)?

Thanks in advance
I.R


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Kirt Blattenberger
Post subject: Just some thoughts...
Unread postPosted: Tue Apr 13, 2004 11:43 pm
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Joined: Sun Aug 03, 2003 2:02 pm
Posts: 308
Location: Erie, PA
Unless I misunderstand your specs, the impedance ratio is actually 20:1, not 16:1. That means your transformer turns ratio should ideally be sqrt(20) = 4.47:1, or about 4.5:1. This can be implemented with a 9:2 transformer using integer turns.

Finding that transformer will be, as you stated, difficult. However, if you are going to need a large quantity, companies like Sprague Goodman and others can make a custom part for you. Alternately, if you only need a few, you could wind your own (keeping low IL across the band will be a chore).

http://www.spraguegoodman.com/890/890toc.html

Another approach might be a tradeoff of overall insertion loss where you implement part of the impedance transformation with a resistive pad and part with a transformer.

Yet another might be to use a high speed buffering opamp that can be set for a 50 ohm input and 1 kohm output and still maintain the dynamic range that you need.

Just some thoughts...


- Kirt B. :mrgreen:


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I.R
Post subject:
Unread postPosted: Wed Apr 14, 2004 10:10 am

Thanks a lot Kirt,

The Application Engineer of Analog Devices suggested us a way to overcome this problem (actually it is mentioned in the data sheet) - To connect resistor of 52.3 ohm in series with 4.7nH inductor, this gives a flat match of 50 ohm up to a frequency of 1GHz.

I was thinking like you of using a fast op-amp (amazing how fast these devices are becoming)...but the solution appears to be much simpler.

Thanks again,

I.R



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