Amidon toroids in RF HF oscillator, 2 or 6 material?
Amidon specifies material 2 for 2-30MHz and 6 for 10-50MHz
However the next site specifies material 2 for 250 KHz-10 MHz and material 6 for 3MHz-40 MHz
The material 2 http://toroids.info/T68-2.php
The material 6 http://toroids.info/T68-6.php
Older amidon datasheets specify it differently www.amstzone.org/Amidon-Jan01.pdf
Could I use the 6 material for the 7MHz oscillator? I like it because it has the lowest thermal stability, although the material 2 comes close.
The same applies to the 7 material. Some sites use it for 1-25MHz, and in the amidon they refer to 3-35MHz. But some of the other amidon documents in the same page refer to 5-35MHz. Go figure out...
Perhaps either of options would work somehow, but what I know is that: never use a ferrite toroid as a resonator mainly due to its poor thermal characteristics. There are many industrial thermometers that use exactly these toroid properties.
I read that some people even tried boiling the cores, claiming it stabilized the material to temperature changes.
Myself, I would stay away from boiling electronic components before I will use in my circuits.
All these cores (2, 6, 7) are not ferrite toroides, they are iron powder. They have good thermal stabilities (30-40ppm).
Ferrites are indeed thermally unstable, not to be used in resonant circuits, but in other circuits like in broadband amplifiers, SMPSUs etc.
Not quite right. -2 material has e.g. +95 ppm, low μr ferrites have considerably lower TK (<10), but it may be still unwanted high for oscillator applications, so they won't be used without air gap.
These Amidon cores are probably (?) fairly similar to the Micrometals range with which I am much more familiar.
Both companies use the same colour codes, and appear to offer very similar material specifications.
Anyhow... the -6 material will have marginally higher Q at 7 Mhz, but the biggest advantage will be better temperature stability. In fact -6 material in the Micrometals range is the best of the whole range for thermal stability.
Radio amateurs use a lot of -6 material, and it seems to work pretty well in VFOs. You can also make up a proportion of the resonating capacitance with some negative temperature coefficient ceramic capacitance.
This all works quite well, but it can become slightly tricky because of the several different thermal time constants involved. If you thermally shock the whole system, it can drift first one way then the other before finally settling down.
All jolly good fun.
I have recently found the 7 material (white) from amidon, but it seems to me it is a relatively new material. It has even better temperature stability than the 6 material and it is specified from 3.5MHz to 3o MHz or so. So it seems more ideal on first look.
Have you got any experience or information about it's Q?
I have only ever used Micrometals products, and the Micrometals published data I have here does not include a white material.
Definitely well worth a look, as temperature stability is a big issue with a VFO.
As you already mentioned, temperature compensation using NPxx capacitors in oscillators is more important and can lead to more stable oscilaltors than using toroids with less ppm/C. But since compensation is difficult to be done, I think it is a good idea to use components with good temperature stability.
If anyone has more info or have ever used the 7 material please let me know.
I agree, reducing the problem at the source with a more stable core material is the best solution.
Temperature compensation is a bit of a band aid, but a very slight amount of compensation added to the final design can add that last little bit of perfection.
The trick seems to be to slow down the rate of temperature change such that everything inside the thermally insulated VFO box pretty much tracks any very slow (over several hours) temperature variations.