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AD8309 - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Below are all of the old forum threads, including all the responses to the original posts.

-- Amateur Radio
-- Antennas
-- Circuits & Components
-- Systems
-- Test & Measurement

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



Posted  11/12/2012

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