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 2:21 am
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
Post subject: Re: Driving low impedancePosted: Mon Nov 23, 2009
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 50ohm type.
These are designed for high current drive into a twisted pair
cable, so you connect the one turn coil across the two balaced outputs.
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
Sucking in and blowing out.