Low-Pass/Antialias Filter for AD9850 Design & Test - Page 1
Forum section: electronics / projects

author: alterbaron
author_rank: Contributor
timestamp: May 09, 2016, 07:17:17 am
content: Hi all,I'm slowly working towards designing and building a USB scalar network analyzer based around the AD9850 DDS chip. This larger project is inspiring a series of smaller projects in which I design and test smaller components of the final system.My small project for this weekend was to attempt to design and test an antialias filter for the AD9850 chip. There's a nice page on antialias filters for this chiphere, but I wanted to try my hand at the design, just for fun.For this application, I want a low-pass filter with a corner frequency of 50 MHz, and an attenuation of 60dB in the stop-band, specified at 125 MHz (the DDS clock frequency).I obtained a first guess for the filter design using the freeFilsynfilter design software. After some component tweaking, I obtained the following circuit:The theoretical response of this circuit is as follows (assuming ideal components):Of course, this is completely unrealistic, as real capacitors and inductors are nowhere close to ideal. In reality, the stop-band behavior will depend quite a bit on component parasitics.Speaking of which, my chip inductors and capacitors are no-name models from eBay, so I have no idea what those parasitics may be. Oh well, may as well build it and see!I etched a small board from double-sided copper clad on which to build up the project. 0805 chip inductors and capacitors were used in the construction of the filter.I then tested the filter using my HP8656B signal generator and HP8566B spectrum analyzer. Under PC control, this pair of instruments can serve as a passable scalar analyzer. Here's the measured filter response:The corner frequency is just under 50 MHz, and pretty spot-on. There's a steep roll-off until about 90 MHz, after which the filter becomes gradually less and less effective. Attenuation is a little better than 40dB at 125MHz. Definitely not ideal!I'm thinking I should try to measure my inductors and capacitors to try to get a sense of their parasitic properties. I'd guess that the parasitic capacitance of my inductors, for example, is very significant in the stop-band, and may well be larger than the capacitors they are paired with!Thanks for reading, and as always, please share any thoughts or suggestions for improvement!

author: poorchava
author_rank: Super Contributor
timestamp: May 09, 2016, 07:26:40 am
content: You don't need a truckload of those parts, only a few pieces to make a single unit of test equipment. I would definitely go for we'll documented name brand components from Farnell, Digikey, TME or whqtever other company like that you have handy. There are special HF inductors for e reasonSent from my HTC One M8s using Tapatalk.

author: dmg
author_rank: Regular Contributor
timestamp: May 09, 2016, 10:05:58 am
content: When these kind of filters (with series LC resonators) have an almost spot-on passband but fail miserably at the stopband showing early reentry that turns them useless it's almost always a symptom of parasitic inductance at the shunt capacitors. Your layout seems to be single layer and there's a relatively thin copper trace between shunt capacitos and that's a source of inductance. You don't need too much of it, less than a nanohenry is enough to screw things up. You need a solid ground plane and several vias to ground from every shunt capacitor. Many small vias are better than fewer larger vias in this case, better if you can fill them and plate copper over them, to have an as uninterrupted as possible ground plane. Also, splitting the shunt capacitors in two paralel caps, oriented each one towards a side (one to the left of the connectors, the other to the right) helps a lot. That should improve things a lot. I've had to design similar filters and when requirements are tought you end up having to simulate the thing with CST or other 3D EM solvers.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 09, 2016, 02:44:18 pm
content: The inductors have some self capacitance. So the caps parallel to the inductors should be a little smaller, especially for the 2.2 pF this may be important.The filter is relatively low order, so even if not perfect a high order filter could give more high frequency attenuation. It sometimes makes sense to have one of the inductors without any parallel cap, to get the zero as high as possible for the give inductor. The exact placement of the zeros is not that important, except for the lowest one.

author: alterbaron
author_rank: Contributor
timestamp: May 10, 2016, 03:38:34 am
content: Hi all,Thanks for all the awesome advice!I had a play around with the filter this evening to see if I could coax some better performance out of it. First thing I did, mostly just out of curiosity, was replaced the shunt capacitor C3 with a couple of 68p capacitors in parallel. This had very little effect on performance.Quote from: Kleinstein on May 09, 2016, 02:44:18 pmI'll have to give this a shot. Would be interesting to measure the self-resonant frequency of these inductors to get a sense of the magnitude of the parasitic capacitance too.Quote from: dmg on May 09, 2016, 10:05:58 amAmazing advice!First of all, the board was double-sided copper clad, but was only etched on one side, leaving a ground plane on the reverse side. Considering the small size of the board, I thought it would be enough to have the two ground planes tied together by the SMA edge connectors.Apparently this was not nearly enough! Being to lazy to drill vias, I instead tried bridging the top and bottom ground planes with copper tape along the edge of the board. Suddenly, the stop-band attenuation near 1 GHz improved by around 30 dB! I soldered the copper tape along both sides of the board, making the connection permanent.A log-frequency sweep of the improved filter over the range from 1-990 MHz gave the following:The filter now meets the initial specification, with better than 60dB attenuation at 125 MHz! Attenuation in the stop-band is better than 50dB until 600 MHz, where for some reason it suddenly degrades by about 10dB, before gradually improving again.I was really surprised by just how large an effect bridging the ground planes had on filter performance. On the final PCB, I'll be sure to put some vias right up next to the shunt capacitors!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2016, 04:15:40 am
content: Sexy!Are those multilayer inductors, or what? They might have more capacitance than desirable, too.Well... capacitance and inductance there are what you /want/, but the inductor probably has internal modes that aren't quite what you want, which manifest as funny business in the gigs.And yeah, vias are good.Tim

author: dmg
author_rank: Regular Contributor
timestamp: May 11, 2016, 08:52:02 am
content: In these particular kind of filters inductor parasitics aren't *that* important, parallel parasitic capacitance can affect mainly the passband and magnetic coupling between them will affect ultimate rejection, but they aren't as sensitive as filters that only have series inductors instead of LC tanks. These filters are absorptive, whatever doesn't get shunted eventuay goes through so one must pay attention to gruound return path and shunt caps. Once you get rid of steep reentries and tame the stopband to acceptable levels you can fine-tune the passband. Also, for these applications is not uncommon to have a secondary low Q and low order filter after the main antialias filter, something with a nice slowish rolloff, higher cutoff frequency and smaller components than the antialias. That will tame the overall frequency reponse up to higher frequencies to delay the reentry until you no longer care about it.Oh, by the way be sure to have well-defined source and load impedance, have them be as resistive as possible within your interest frequency range, because mismatches have horrendous consequences for the passband in these filters.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2016, 07:37:48 am
content: Quote from: dmg on May 11, 2016, 08:52:02 amNo, it is reflective.If you need to make the absorptive kind, this is an excellent reference:http://jeroen.web.cern.ch/jeroen/reports/crfilter.pdfBut yes, the ground path is critical, regardless!If you like, you can model imperfect grounds as inductance in series with the shunt capacitors. So your equivalent circuit will have L||C series sections (as intended), and L+C shunt sections (C as intended, L as parasitic). Which is a notch filter! The notch width and depth being determined by the ratios of inductances, and such.Since those inductances were uncontrolled in the first trial, you basically had a really poorly optimized notch filter.QuoteYes! When you need very wide band filtering, it is excellent practice to cascade filter designs.The main caveat is: you need to provide loss (damping, resistance) to keep the impedance stable, out at frequencies where you can't be sure what the main filter's impedance is doing (due to normal reactance, or due to parasitics).For example, if you have an LCLC (4th order) filter, you can't just cascade a smaller (i.e., higher Fo) LC filter off the end, because it will simply resonate with the last C of the first filter. (For explanation's sake, call the components of the first filter L1, C2, L3 and C4, and of the second filter, L5 and C6.) It'll resonate at an unpredictable frequency and bandwidth and peak height, because you don't precisely know the ESL of C4.However, if you make L5 lossy (say by taking 2/3rd of it and putting an R across that, and leaving the 1/3 as low-loss), you can guarantee at least modest damping, in return for a much softer roll-off -- but this is perfectly okay, because we're already above the major roll-off, and any additional poles don't have to be pretty.Or you can put the smaller LC in front of the larger LCLC, but now you need to worry about that small C resonating with the large C and putting a dip in the passband. Same idea applies, though: dampen it with a suitable R+C, and you lose some insertion loss but keep it flatter.Damping methods are kind of illustrated with the constant-resistance filter I mentioned at the top, though that's done with more nuance (and numerical solutions!).The conventional "can't be arsed" solution is: use a modest (usually 3 or 6dB) attenuator before, after, and inbetween each filter stage. This provides, well, 6dB worth of isolation from any reflected power, in any direction -- but this gets wasteful rather quickly, so must be used sparingly!An attenuator is simply pure loss, so it should be apparent that, if you can focus that loss only to the needed frequency range (and you can identify what those ranges are, based on your components and their estimated parasitics), you can stand to save a lot on passband insertion loss.QuoteOr more precisely, have them match the circuit impedances, whatever they happen to be.I think DDS generators usually provide current mode outputs, which are usually suggested to terminate with resistors. So you'll have a Norton equivalent source. There will be some internal pin capacitance to ground, too. Both suggest using a cap-input filter section. The pin capacitance must be subtracted out to get the correct component value, and you're good.If the filter source is terminated, then the load end can be terminated, or open or shorted. (An inverting op-amp input is ideally a virtual ground, but it probably won't be a very good one at frequencies where LC filters look good, so you probably won't use a shorting load. An open load could be a non-inverting amp stage; again, the amp input capacitance will have to be accounted for. A terminated load could be either kind, as long as you have resistance in the right place.)This is not complete madness: a filter can be constructed with only one end at nominal impedance (or with unequal impedances on either end). It does require pulling filter coefficients from a different table. Such tables are in here:http://kvetakov.net/~mira/literatura/Electronic%20Filter%20Design%20Handbook%204th%20Ed.pdfTables are given for different ratios of source and load resistance; obviously when source and load are unequal, some power is always reflected because of the mismatch, but the filter response (relative to the expected passband loss) is still correct. In the infinite/zero case, all power is reflected, but thevoltageorcurrentresponse (at the open/shorted end) is still correct.You could also go for the case where the DDS output is high resistance, and the amp/buffer/driver side of the filter has nominal resistance. This might be less safe, because around Fo, the voltage swing is double (though that's easily solved by designing the filter for half Zo), and the output resistance of the generator may be poorly specified (namely, how much Early effect / channel length modulation effect there is, among other things, depending on how they designed the output stage in the IC).Tim