Copyright: 1996 - 2024
BSEE - KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
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design engineer. The World Wide Web (Internet) was largely an unknown entity at
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Driving low impedance - RF Cafe Forums
RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media
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August of 2003 and was quite well-attended for many years. By 2010, Facebook and
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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: Driving low impedance Posted: Mon Nov 23, 2009
Joined: Mon Nov 23, 2009 1:21 am
I'm no RF expert and could use some help.
I need to drive a single-turn coil over the range of 5 to 200 MHz. I
am using a DDS to generate my sine wave and have used a PGA IC meant
for driving CATV cable to drive the coil. Since the amp is meant for
driving 75 ohm loads, I have quite a voltage drop when driving the low
impedance of my coil. I would like to put more power into the coil and
wondered how I might do this. I can get almost 1 VRMS across the coil
now but would like get about 2.5 VRMS. I want a fairly flat amplitude
versus frequency curve into this coil as it is used to excite a system
that has it's resonances sensed. It seems like RF amps are all designed
for the standard 50 or 75 ohm loads. Any help is appreciated.
Post subject: Re: Driving low impedancePosted:
Mon Nov 23, 2009 4:10 am
Joined: Fri Feb
17, 2006 12:07 pm
Location: London UK
you will probably have more success by using a high-current differential
line driver, also used for CATV, but not the single-ended unbalanced
These are designed for high current drive into a twisted
pair cable, so you connect the one turn coil across the two balaced
Also recall that discrete component audio amplifiers used
a complementary pair emitter-follower circuit to drive 4 ohm loudspeakers.
If you design a similar arrangement but using HF/VHF techniques for
component choice and circuit layout, it should be possible to drive
significant current through the coil and achieve the objective. I have
used this idea to drive a low resistance bridge to measure low values
of resistance accurately.
It might be more helpful to think in terms
of current and mmf rather than voltage in this low impedance situation.
At bottom, life is all about
and blowing out.