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Review: Hantek DDS 3X25. Anyone own one?
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saturation:
Except for wirewound and carbon composites, where only certain special types or conditions can be used, most film types are easily used to the GHz, its more a question of better and best, and then what is good enough to reduce parts cost.


--- Quote from: grenert on March 17, 2011, 08:49:03 am ---How about these MELF resistors?  Vishay claims they're good to 10 GHz+
http://www.vishay.com/docs/28718/melfhf.pdf

They say low inductance, but no mention of capacitance.

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saturation:
Yes, just slightly amplified, it just shows that 75 vs 50 ohm for this application, the differences are not so striking.  But note the frequency count of your image capture, its 10.0210 MHz versus 10.0000 MHz on mine, for the same waveform.  The Rigol might be reading glitches as the slight difference, as frequency rises those glitches could cause more effects that can be eliminated with proper connections. 

 
--- Quote from: Mechatrommer on March 17, 2011, 01:41:30 am ---
--- Quote from: saturation on March 16, 2011, 08:00:19 pm ---Most striking is a negative glitch is half the magnitude.

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its there, i just amplified it using improper connection (ringing?) ;)

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saturation:
As always, alm, you pose the most interesting questions.  Thanks for those spec sheets.  Yes, saw the mention of 'high frequency' again.  The other part of resistor articles from those references say that the frequency response deteriorates at increasingly higher resistance ratings but moreso for carbon composition resistors, CCR [excluding standard wire wound for obvious reasons.]   Probably the best thing we can do is get a spec sheet from a CCR manufacturer, I'm sure they won't deceive us!

http://www.koaspeer.com/pdfs/RC_SS-263_R4.pdf

See the frequency spec with its X axis being MHz*MOhm.

I think that if a device worked with variable frequencies, one could compensate for the changes in impedance by choosing resistors of lowest values, whenever possible.  This causes higher power consumption but more consistent response.

Likewise, if the device worked at a narrower range of frequencies and required high value carbon composition resistors, then even if actual resistances are reduced, a designer could work with that predictable characteristic and simply choose a higher ohm resistor to compensate for the reduction.

Here's a US Navy document regarding application of CCR in USN electronics:

http://www.google.com/url?sa=t&source=web&cd=17&ved=0CDsQFjAGOAo&url=http%3A%2F%2Fwww.everyspec.com%2FUSN%2FNAVSEA%2Fdownload.php%3Fspec%3DTE000-AB-GTP-010_R1-CHG-A.030031.pdf&rct=j&q=variations%20of%20carbon%20composition%20resistor%20high%20frequency%20use&ei=lnuCTZXOA474gAeBy7jbCA&usg=AFQjCNGTf2y90OvaLGY6o3qA0mLGOYEyPg&cad=rja


In this rather large image, a curious enthusiast note that the only way to confirm a CCR is really to break in open, stating that some CCR sold since the 1970s actually contained film technology:

http://www.w8ji.com/beverages.htm


--- Quote from: alm on March 16, 2011, 11:59:36 pm ---Thanks for the info, I wasn't aware of the parasitic capacitance. Most sources I've seen (eg. this and this, just random datasheets) seem to consider them superior to metal film for high frequency (thin film is obviously superior). The first datasheet recommends carbon composition for a part that has transition times of 20-30ns, not exactly GHz speed, but no 10MHz either.

I find it suspicious that the graph has only one curve for carbon composition, I can't imagine parasitic capacitance having the same influence at say 1ohm and 1Mohm. I also doubt that all carbon composition resistors are useless beyond 10MHz, lots of equipment from the seventies and earlier was full of carbon composition (wirewound was inductive, and metal film expensive), many of it can go beyond 10MHz. I'd have to check if for example the Tek TM500 series (which uses lots of carbon composition and multiple plugins go up to 250MHz) use metal film in all places that need to pass high frequency. I believe carbon comp was frequently used as termination resistor, but if they'd only work up to <10MHz, they'd be almost useless, since you need fairly long cables to even notice the transmission line effect at that frequency.

I just checked, the Tek PG-502 (250MHz pulse generator with a rise time of <1ns) uses two ~50ohm carbon composition resistors (as indicated in the electrical parts list) as switchable rear termination, with no reactive components nearby to compensate for parasitics. It would need about 350MHz bandwidth to reproduce the 1ns edge, can't imagine that the termination has a zero impedance (extrapolating from the graph you posted) at those frequencies. These were just the first resistors I checked, there are many more carbon composition resistors in there (and in similar equipment of that vintage). There are some 1% metal film resistors, but they're there because of accuracy/stability. This doesn't mean that carbon comp is superior to metal film for HF, but it does suggest that carbon comp works well beyond 10MHz.

Maybe there are different ways of producing a carbon composition resistor? That would explain the conflicting data, although I've never seen something like a 'low-capacitance carbon comp resistor'.

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ElektroQuark:
Please, reduce image. Thank you.
saturation:
Its linked, not my own.  I've removed it, you can see it at the website.


--- Quote from: EdoNork on March 18, 2011, 08:22:45 am ---Please, reduce image. Thank you.

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