Copyright: 1996 - 2024
BSEE - KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed
formulas and reference material while performing my work as an RF system and circuit
design engineer. The World Wide Web (Internet) was largely an unknown entity at
the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps
while tying up your telephone line, and a nice lady's voice announced "You've Got
Mail" when a new message arrived...
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AD8309 - RF Cafe Forums
RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media
platforms dominating public commenting venues. RF Cafe Forums began sometime around
August of 2003 and was quite well-attended for many years. By 2010, Facebook and
Twitter were overwhelmingly dominating online personal interaction, and RF Cafe
Forums activity dropped off precipitously. If the folks at
phpBB would release a version with integrated
sign-in from the major social media platforms, I would resurrect the RF Cafe Forums,
but until then it is probably not worth the effort. Regardless, there are still
lots of great posts in the archive that ware worth looking at.
Below are the old forum threads, including responses to the original posts.
|-- Amateur Radio
Gripes & Humor
-- CAE, CAD, &
Test & Measurement
Post subject: AD8309
Unread postPosted: Tue Apr 13, 2004
Our board has includes a limiter. The application
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
Post subject: Just some thoughts...
Unread postPosted: Tue Apr
13, 2004 11:43 pm
Sun Aug 03, 2003 2:02 pm
Location: Erie, PA
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).
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:
Wed Apr 14, 2004 10:10 am
Thanks a lot Kirt,
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.