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https://designers-guide.org/forum/YaBB.pl Design >> RF Design >> large inductance values at RF frequencies https://designers-guide.org/forum/YaBB.pl?num=1283212989 Message started by vp1953 on Aug 30th, 2010, 5:03pm |
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Title: large inductance values at RF frequencies Post by vp1953 on Aug 30th, 2010, 5:03pm Generally we do not find inductors (discrete) that have values in the range of several 100 nH, Q of between 20 and 40 and SRF in the 10-20GHz? If such inductors are indeed available, I would like to know about them. If such inductors were to be available, what are some applications that could use them today in moderate to large quantity? |
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Title: Re: large inductance values at RF frequencies Post by RFICDUDE on Aug 30th, 2010, 5:43pm I think a more curious question is why in the world would you need 100nH at 10GHz? The reactance alone would be 6283 ohms and it would only take 253 fF of capacitance to resonate with the inductor. The parasitic capacitance of a discrete component or even the board capacitance would make such an inductance value impractical. A more practical way of thinking about the problem is to calculate the minimum inductance necessary to act as a choke for a particular circuit operating at a particular frequency. You can use the order of magnitude difference method as a way to approximate a value. |
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Title: Re: large inductance values at RF frequencies Post by loose-electron on Aug 31st, 2010, 10:46am agree with the above. Your parasitics will kill you in many situations. Too much distributed wire to wire capacitance makes the large values somewhat useless as a tuned RF circuit. Do a search on DigiKey for whats available in discretes. |
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Title: Re: large inductance values at RF frequencies Post by vp1953 on Sep 1st, 2010, 5:08pm Hi RFICDUDE and L_E, Thank you for your input. So you guys have a good point that a 100nH inductor is pretty much useless at 10GHz. But what about at lower frequencies say 1GHz? Are there applications that could use a high quality inductor with large Q's, and costs about the same (or less) as currently available discretes (obviously such an inductor could replace current discretes)? One example of an application could be that a inductor with several 100n of inductance can be used to make a narrow bandpass filter that could potentially be used in place of a saw filter in select applications. |
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Title: Re: large inductance values at RF frequencies Post by RFICDUDE on Sep 1st, 2010, 7:37pm Sure there could always be a use for a good choke in the GHz band, but I'm not so hopeful that SAW/BAW filters can or would be replaced by high Q LC filters. It really depends on the size and performance of the filter. SAW/BAW filters provide 25MHz bandwidth with 45-50 dB rejection at 45MHz offset in the 800MHz band and higher. A LC design would have to provide that kind of performance with either lower cost or size to be competitive. A small tunable filter that provides similar performance at lower cost is equally as interesting. |
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Title: Re: large inductance values at RF frequencies Post by loose-electron on Sep 2nd, 2010, 7:59pm Go get an accurate model of the inductors of interest - You will quickly find that the parasitics will make it a pointless exercise. Need to look at the complete RLC network that you are thinking about. It does not look like a simple inductor. |
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Title: Re: large inductance values at RF frequencies Post by vp1953 on Sep 3rd, 2010, 2:53pm Hi L_E, Quote:
Can you be more specific? As RFICDUDE pointed out, it would not be very useful at 10GHz but could be a good solution for 500MHz. Are there other reasons that could preclude usage at lower frequencies? Quote:
Most on chip inductor model that i have seen are far more complex than a simple RLC network. The inductor could possibly be represented as a three element RLC network BUT only at a single frequency |
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