absorptive filter design - Page 1
Forum section: electronics / projects

author: xzswq21
author_rank: Frequent Contributor
timestamp: December 03, 2017, 07:30:47 pm
content: HelloI have a high speed sampling circuit. is there any software to design a absorptive filter?Actually absorptive filter improves VSWR.Thanks

author: ejeffrey
author_rank: Super Contributor
timestamp: December 03, 2017, 09:59:25 pm
content: Here is one paper on reflectionless filters with approximately gaussian pulse response:http://alenka.ece.gatech.edu/wp-content/uploads/sites/463/2015/06/RLC_Low_Pass_Filter.pdfThe basic idea is that you model a lossy transmission line, and turn it into a lumped element equivalent.I have used similar filters. They can have excellent performance in terms of low reflection and excellent attenuation in the stop band, even up to very high frequencies. The downsides are a rather shallow initial rolloff and that they require a lot of components -- maybe 20-30 components for a reasonable design.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 04, 2017, 09:27:09 am
content: http://jeroen.web.cern.ch/jeroen/reports/crfilter.pdfDunno if Nuhertz filter tool will do that, but... probably?Tim

author: dmills
author_rank: Super Contributor
timestamp: December 04, 2017, 12:26:51 pm
content: I have just been playing with there to go with a LTC2107, more progress would have been made if my VNA had not packed up when I went to use it (8753C now showing self test 10 fail which is the digital IF, I am thinking one of those Intersil muxes so annoying)....You are basically designing a diplexer with a terminated high pass port, and diplexer design is well documented, also look at "mixer port termination" as this is a well trodden path for diode ring mixers (all of which are termination sensitive), and a sampler is basically a mixer.Incidentally, if you are working at under 400MHz, LTI have got Soshin to add a differential diplexer to their standard range for exactly this use which is well behaved up into the GHz bands, I cannot find a European distributor however.Regards, Dan.

author: xzswq21
author_rank: Frequent Contributor
timestamp: December 04, 2017, 06:37:45 pm
content: Quote from: dmills on December 04, 2017, 12:26:51 pmcould you please explain about Soshin?it's not important about the location.I need an absorptive low pass filter with 50MHz and 100MHz cut-off frequency to put at the input of a high speed smapling board.is there any software?do you recommend any inductor series for the filters? from coilcraft or Murata? which series?Thanks

author: dmills
author_rank: Super Contributor
timestamp: December 04, 2017, 07:06:23 pm
content: Soshin Japan have a SMT part HMD6117J which Linear Tech got them to create for use in front of high speed ADCs to provide the broadband termination out to the GHz bands. They are theoretically available, but... If you find a source of small quantities please let me know?There is also a linear tech app note showing a discrete designhttp://www.linear.com/solutions/7817which goes over at ~100MHz or so.You can design such things by designing a lowpass and a highpass reflective network, terminating the highpass into the target source impedance and ensuring that the input goes high Z out of band for both filters (Series cap on the highpass, series L on the lowpass part), the real trick is making the things well behaved waaay out of band (By 4Ghz or so parasitics can be bigger then the intended behaviour).I would also note that if you have a active ADC driver, the absorptive filter only really needs to absorb above the frequency at which the driver output impedance starts to rise, this filter is not primarily to control aliasing but to soak up the sampling transients.For magnetics, I like the johanson dielectrics parts, and remember to keep the copper a little way away from the windings or capacitance will kill you. Measurements are pretty much mandatory if going with a discrete design.Regards, Dan.

author: xzswq21
author_rank: Frequent Contributor
timestamp: December 04, 2017, 08:24:51 pm
content: Quote from: dmills on December 04, 2017, 07:06:23 pmDearYes, I have a active driver (FDA) before the ADC.you mean it's better to not use a absorptive filter at the input of FDA? but it's better to put a absorptive filter between the FDA and the ADC?why coilcraft inductors are much more expensive than Murata or Johnson? usually Analog Devices and other companies use coilcraft inductors.I want to use Murata capacitors in the filter.Thanks

author: dmills
author_rank: Super Contributor
timestamp: December 05, 2017, 12:15:16 pm
content: You use an absorptive filter anytime out of band impedance match matters, so diode ring mixer ports, ADC and DACs are obvious contenders.Generally on an ADC you also want to limit the noise bandwidth at the sampler input to minimise the out of band noise folded back into the baseband anyway, so making this filter absorbative is a good move, this will usually put the corner down where the driver still has plenty of loop gain meaning it still has a well controlled output impedance.For this reason an absorbative filter between the ADC driver and the ADC is a reasonable plan, not the only option, but a reasonable one if you are going for ultimately high SFDR.Now if the ADC driver is being fed from a termination sensitive source (Like say a diode ring mixer) then you may also want a similar thing on the mixer output port, particularly if your anti aliasing filter presents an unfortunate load to the mixer out of band.As to the coilcraft love fest, no idea, but ultimately you just need to pick something having sufficient Q and a high enough SRF (Think 5-10GHz ideally). A VNA measuring a real circuit is a good thing here, the datasheets are often optimistic on small SMT parts as they tend to ignore the effect of the board ground plane on the Q and SRF.

author: xzswq21
author_rank: Frequent Contributor
timestamp: December 28, 2017, 10:08:26 am
content: can I use 0603 SMD size in such filter? (for my application the frequency is up to 50MHz)for 50 Ohm resistors in the filter I have used 49.9 ohm %0.1 resistors from Panasonic.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 28, 2017, 11:56:24 am
content: Yes. Observe power ratings.Tim

author: xzswq21
author_rank: Frequent Contributor
timestamp: December 28, 2017, 06:58:21 pm
content: Quote from: T3sl4co1l on December 28, 2017, 11:56:24 amLinear Technology uses RF resistors from Vishay company in their designs (I mean between the Driver and ADC), it's a little expensive (each resistor is $2.5)but I didn't choose it.unfortunately most of resistor's datasheets don't include frequency response information.for the capacitors and inductors they have used 0402 SMD size, but I have chosen 0603 SMD size.

author: dmills
author_rank: Super Contributor
timestamp: December 30, 2017, 07:21:22 pm
content: Physically smaller is generally better except in inductors.I went for 0402 for resistors and caps and 0603 or 0805 for the magnetics, but some poking around with a VNA will reveal all.Regards, Dan.

author: KE5FX
author_rank: Super Contributor
timestamp: December 30, 2017, 09:09:26 pm
content: Quote from: dmills on December 05, 2017, 12:15:16 pmDefinitely, and not just "ideally" -- this is a pretty big deal. ADC manufacturers spend a lot of time talking about the full-power bandwidth or the 3-dB bandwidth of their sampling front ends, but they don't talk about what happens after that, which is massively important for SFDR. You can build a perfectly usable 6 GHz sampler with an 80 MSPS ADC... and in fact that's what you're going to get, whether you want it or not.

author: dmills
author_rank: Super Contributor
timestamp: December 30, 2017, 11:29:03 pm
content: Yep, been there done that!I have seen situations where spacing the driver a LONG way from the ADC pins actually helped because it meant the reflected transient arrived outside the sample window, kind of case specific that trick however.Fast ADCs are a barrel of laughs to try to squeeze performance out of, if I have another digital weenie tell me that the clock is 'digital' there will be damage done.regards, Dan.