On 11/28/2014 10:08 AM, Dave M wrote:
Rick, Thanks for the brief review of MiniCircuits stuff (I'm not connected with them in any way except as a customer). Since you've characterized some of their parts, perhaps you could help answer a question that someone else posted, and one that I would like to have answered as well. Have you measured the effects of DC current in the windings of an RF transformer, such as is seen if the transformer is in the collector circuit of an amplifier? If so, could you provide a generalization of the effects, such as changes in frequency response, losses, etc.? Many Thanks!, Dave M
The very tiny cores on MiniCircuits transformers will start to saturate at hundreds of mA. The effect is that the magnetizing inductance drops, which matters more at low frequencies than high frequencies. I try to avoid feeding DC to an amplifier through a transformer winding. Instead I use a separate RF choke for that. However, it would probably work OK for, say, up to 25 mADC for a small signal transistor, but why take a chance. If you are using a DC feed through a transformer winding, be careful not to accidently short circuit it, causing the full available current from the power supply to flow through the transformer. This can actually magnetize the core and permanently damage it. Saturation via DC is much more deleterious than saturation via AC. It is easy to calculate the flux density using Ampere's law, which is one of the four Maxwell's equations. H = I/(2piR). Since R (radius) is in the denominator, cores saturate from the inside first before the whole core is saturated. Multiply H by mu, (as any time nut knows) to get B. If R is 1 mm, and I is 628 mA, then H = 10 ampere turns per meter. If mu-relative is 1000, then B = 4piX10^-7 X 1000 X 10 = 125 mT. That is a hefty 1250 Gauss. Some materials may be affected at 1/10 this flux density. Now a days, a lot of RF is differential, in which case you are free to feed DC through the output transformer without worrying about this issue. I worked for several companies where those 6 hole cylindrical chokes were ubiquitous. I specifically characterized those and established a maximum current rating of only 100 mA. Of course, many production designs exceeded this limit and "worked" anyway. I actually observed someone try to put 20A through one of these. The tantalum capacitors on the "cold" side of the bead actually exploded due to RF current. Rick Karlquist N6RK _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
