Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if
you would like to post something on RF Cafe's
page, please do.
Below are all of the forum threads, including all
the responses to the original posts.
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.Posted 11/12/2012