EMRFD Message Archive 2846
Message Date From Subject 2846 2009-04-02 20:50:18 JT Croteau Toroid Mixtures for VFOs Gentlemen,
I've read in several sources that a micrometals mix of -6 is preferred
for VFO circuits because of their low temperature coefficient and
their ability to play nicely with NP0 capacitors. However, I note
that a -7 mix has an even lower temperature coefficient. I'm
wondering if there would be an issues with using a -7 mix in VFOs
around 3 and 7 MHz?
JT Croteau, N1ESE
2850 2009-04-03 06:00:07 Bill Noyce Re: Toroid Mixtures for VFOs Yes, -7 would be marginally better than -6. When your sources were
written, the -7 mixture might not have existed yet, or might have been
hard to find. For the 3 MHz VFO, you'll need a lot of turns -- it
might be easier to get the inductance you want with -2, but its tempco
is worse... In any case, use a relatively large toroid (t68-x?).
Larger wire can give higher Q, but you need to be able to wrap it
tightly to minimize microphonics and drift.
-- Bill, AB1AV
2851 2009-04-03 06:14:59 Chris Trask Re: Toroid Mixtures for VFOs >For temperature stability, the -7 material will have a slight edge over -6. It is also useful over a broader range. Micrometals doesn't have any Q curves for the -7 material, so the loss characteristics are a bit unknown. Losses would affect the phase noise performance of the oscillator.
>I've read in several sources that a micrometals mix of -6 is preferred
>for VFO circuits because of their low temperature coefficient and
>their ability to play nicely with NP0 capacitors. However, I note
>that a -7 mix has an even lower temperature coefficient. I'm
>wondering if there would be an issues with using a -7 mix in VFOs
>around 3 and 7 MHz?
I don't have any cores of the -7 material available. Perhaps someone who does could wind some coils on it, -6, and -2 and make some Q measurements.
,----------------------. High Performance Mixers and
/ What's all this \ Amplifiers for RF Communications
/ extinct stuff, anyhow? /
\ _______,--------------' Chris Trask / N7ZWY
2852 2009-04-03 06:17:17 Robert Cerreto Re: Toroid Mixtures for VFOs In additi 2853 2009-04-03 07:42:23 JT Croteau Re: Toroid Mixtures for VFOs 2855 2009-04-03 12:20:02 Wes Hayward Re: Toroid Mixtures for VFOs Hi all,
The temperature coefficients of various cores is always of interest. As Chris and others have mentioned, the TC is slightly better for the -7 than for the -6, but the difference is 35 ppm versus 50 ppm. From my experience, this kind of difference is easily lost in the slop that can come from winding the toroid with a wire size that is too large. For oscillators, it really is worth while to use a size that is smaller than intuition would dictate. I usually use #26 or #28 for oscillators. The hit on Q is not all that much. As always, lore is to be avoided -- measure instead!
The thing that is not found in much of the literature regards Q. The Micrometals Q curves feature various mixes to go with various "recommended" frequency ranges. The Q curves often have surprisingly narrow peaks. If one were to build a filter for the 160 meter band, we would always pick the -2 material. But is this always the best choice? I have been very surprised with some coils I have wound on -6 material, thinking I would get the ultimate inductor at 7 MHz. (Yea folks -- I know there are other bands.) When I then did measurements, I often found that these -6 inductors were spectacular for Q at 3 MHz. I've seen some over Qu=400. I presently have a portable CW transceiver (Whoops -- it's at 7 MHz) that uses a 2 MHz VFO. I use a T50-6 core for the VFO inductor with C0G capacitors that was then temperature compensated.