Post subject: WB FM detection via delay
line & C-loaded transmission line Posted: Thu Jun 11, 2009 5:17
Joined: Mon Apr 20, 2009 2:04 pm
I'm still trudging through my video
link in a world that left WB FM behind (sigh...).
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
1. Has anyone gotten some experience with
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?
Post subject: Re: WB FM detection via delay line &
C-loaded transmission linePosted: Sun Jun 14, 2009 11:32 pm
Joined: Sun Jan 11, 2004 4:44 pm
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
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.
I'll be interested in seeing your result.