WB FM detection via delay line & C-loaded transmission line - RF Cafe Forums

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Mike_From_Plano
Post subject: WB FM detection via delay line & C-loaded transmission line Posted: Thu Jun 11, 2009 5:17 pm

Captain

Joined: Mon Apr 20, 2009 2:04 pm
Posts: 14
Hello everyone,

I'm still trudging through my video link in a world that left WB FM behind (sigh...).

Anyway, it occured to me that an off-the-shelf mixer IC and a 1/4 wave delay would make for a nice detector. I popped in an SA602 + delay into Spice, and it worked great, as long as I didn't over drive it. It was extremely linear over 70MHz +/-10MHz.

Now, I have two basic questions:

1. Has anyone gotten some experience with this?

2. I don't care to have 3.57ns of coax laying about in my case, nor do I care for a couple of feet of meandering transmission line on my PCB. Has anyone had any luck with distributed C loading along a shorter transmission line. From the equations, it looks as though I can pepper the line every 1/2 inch with 10pF caps, and reduce the length to about 6 inches with only a lower Z and some ripple in the UHF as the cost.
Does this sound rational? Has anyone tried this?

Thanks,

Mike

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VSWR
Post subject: Re: WB FM detection via delay line & C-loaded transmission linePosted: Sun Jun 14, 2009 11:32 pm

Captain

Joined: Sun Jan 11, 2004 4:44 pm
Posts: 8
Howdy Mike form Plano,

I've seen FM detectors that used 1/4-wave delay lines (first time in my college communications book). Those kinds of books usually dealy strictly with theory and not with actual implementation like you've got going there. I like your idea about trying to load with some caps to try to shorten the delay line length to a reasonable length. The only real requirement is that you get a 90-deg phase shift between the mixer ports (with minimal distortion).

http://books.google.com/books?id=C1C6IBiCoXsC&pg=PA167&lpg=PA167&dq=fm+detector++%22delay+line%22&source=bl&ots=v2DI_qa2Gj&sig=u77FcuybI-JAGEi6c9lImID8caI&hl=en&ei=QL41SsbJBY-Ntgfyn4n5Dg&sa=X&oi=book_result&ct=result&resnum=6#PPA167,M1

Check out this patent. It gives component values for you. You might be able to come up with something better, but at least this shows that somebody's thought along the same lines.
http://www.google.com/patents?vid=USPAT4535296

I'll be interested in seeing your result.

Posted 11/12/2012