Testing inductors and Practical LC filter limitations? - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: November 28, 2015, 05:26:09 pm
content: Right not im trying to build a simple LC band pass for 29mhz, im filtering the 28.8 tcxo for an RTL SDR. The thing is doing the simple LC filters is netting me inductors in the pH range... I have no idea how to measure something that small, I ordered some of thesehttp://www.analog.com/en/products/rf-microwave/direct-digital-synthesis-modulators/ad5934.html#product-overviewhoping maybe they would help me analyze small inductors with a sweep.Should I just be looking in to different kinds of filters?? If so is there a place the can explain them and a calculator? Im going to have to build one for a a 130mhz LO soon too, and also 130 to 155 band stop.

author: dmills
author_rank: Super Contributor
timestamp: November 28, 2015, 07:31:07 pm
content: If you are ever getting pico henry answers you are probably doing it wrong.Rework the capacitor values and try again, I would expect inductors in the hundred nH to maybe ten or so uH region and caps measured in tens or hundreds of picofarads.You are trying to design a bandpass? What impedances, bandwidth and stopband?Regards, Dan.

author: alterbaron
author_rank: Contributor
timestamp: November 29, 2015, 09:12:06 am
content: Consider that the inductance of a 1cm piece of straight, 1mm thin wire is around 6nH.It's difficult to get a well-controlled inductance much smaller than that.Similarly, it's difficult to get a controlled capacitance of less than 1pF.Remember that it's the product of L and C that determines the resonant frequency of an LC circuit.So, roughly speaking, you may increase L by a factor of 10 so long as you reduce C by a factor of 10 as well. Or vice-versa.Measuring Inductors:The easiest way, IMO, is to use a capacitor with a known capacitance, a 10k resistor, an oscilloscope, and a function generator. Connect your unknown L and known C in parallel, and connect one side of the parallel circuit to ground. Connect the other side to your function gen output through the 10k resistor. Set the function gen to output a relatively low frequency pulse train or square wave (~1kHz is OK here).Probe the terminal in-between the parallel LC and the resistor with a 10x 'scope probe. You should be able to see the classic "ringing" of an LC circuit on the scope. (Each edge of the input waveform causes the LC circuit to ring.) If it's too small to see, crank up the function gen output. You may also switch to a 1x 'scope probe, but be mindful of probe parasitics!On the scope, measure the time T between two subsequent peaks on the ringing waveform. 1/T = f0 = 1/(2*pi*sqrt(L*C)), the resonant frequency of the LC circuit. Solve for L.Try this out with some hand-wound air-core coils, and make sure that the result you get roughly matches up with theory before moving on to building filters with hand-wound coils. It'll give you a better feel for making inductors.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2015, 09:51:58 am
content: If you're having trouble with orders of magnitude, consider:The value of L or C will never be far (within an order of magnitude, i.e., x10 or /10) from the characteristic values of the application. Namely, that cutoff frequency and system impedance correspond to inductance and capacitance: L = Z / (2*pi*F) and C = 1 / (2*pi*F*Z).6pH is 50 ohms at 1.3THz (a frequency which probably doesn't even conduct very well, in the conventional sense), or 1mohm at 28MHz.And as mentioned, yeah, 6pH might be typical for via or trace inductanceson a microchip, and is all but negligible (i.e., down in the noise of manufacturing variations) for PCB level circuits.So, get a feel for realistic values of Z and F, L and C, and how they pivot around each other. It's just ratios. Always do a sanity check, find typos...Tim

author: rfbroadband
author_rank: Unknown
timestamp: December 12, 2015, 06:06:13 pm
content: provide a few details and I can synthesize one for you rather quickly:Order : ?Bandwidth  ?Stop band attenuation ?Filter Type : Elliptic, Chebyshev etc ?Source/Load Impedance : 50Ohm ?

author: tggzzz
author_rank: Super Contributor
timestamp: December 12, 2015, 06:11:57 pm
content: Have you done a sanity check with something likehttp://www-users.cs.york.ac.uk/~fisher/lcfilter/

author: rfbroadband
author_rank: Unknown
timestamp: December 12, 2015, 06:41:47 pm
content: attached a picture of a filter using ideal components....please be warned....the moment you use 'real components' it may 'fall apart'. The challenge is in selecting the passive components so that the desired filter response will be maintained.I hope this helps.