Low microphonics SMD inductors / capacitors for RF - any suggestion? - Page 1
Forum section: electronics / projects

author: stefan_o
author_rank: Regular Contributor
timestamp: July 03, 2018, 03:16:53 am
content: Hello,I've got several RF circuits, which are unfortunately quite shock sensitive. I found a few caps and inductors in the PLL that cause massive RF spikes with even by the slightest touch (with RF alignment tool). To reduce this I would like to replace them with better caps/inductors (the ones used were probably good, but the devices are 20 years old).I'm looking for:220nH inductor, size 0603 or 08052p7 capacitor, size 0603The circuit operates at up to 850MHz, so I need an inductor with high SRF. Currently there is a wire-wound type, not sure if a MLCC would be better (and if there ones with high SRF).Any recommendations which ones I should use?Best regardsStefan

author: JS
author_rank: Frequent Contributor
timestamp: July 03, 2018, 04:44:54 am
content: For small value caps, always COG/NPO types, as close as ideal as it gets, MLCC are for higher values, but good ceramics are harder to find and pretty expensive for those higher values.For inductors, sometimes all you need is some potting, caps are more likely to be a problem, so I'd try changing the caps and applying some glue to the insuctors to damp any vibration. Most likely soft gummy glue, like hot snob or silicon, but you should use neutral silicon as acetic is quite corrosive.JS

author: stefan_o
author_rank: Regular Contributor
timestamp: July 03, 2018, 11:27:05 am
content: NP0/C0G are of course used for this low value caps. There is no high-value cap in the RF circuit. The question is, is there any special ultra-low-microphonics series by any manufactor that I should use? Dave made a video about different types (as a follow-up about that capacitor cracking), for example ones with flexible ends. All the shock is coming from the PCB, the caps are inside an RF shielding box.

author: JS
author_rank: Frequent Contributor
timestamp: July 03, 2018, 12:51:09 pm
content: You could mount the whole pcb on rubber mounts to damp any vibration, that should work better than just the caps onto flex terminals as higher mass allows for lower cutoff freq of the system.The other option is to have cutoffs on the pcb around the sensitive circuit, like those LTZ1000 voltage reference, of course not practical for an existing device and probably a bad idea for HF anyway.JS

author: stefan_o
author_rank: Regular Contributor
timestamp: July 03, 2018, 01:12:48 pm
content: Unfortunately rubber mount of the PCB is not possible, as everything is screwed by the shielding case. I've already looked for many different options and changing parts seems to be the only option.Low frequency, like shaking the device is also not a problem, high frequency impacts are the problem.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 03, 2018, 02:03:50 pm
content: I would not expect NP0 caps to be vibration / stress sensitive. There might be other higher value caps at the supply decoupling that could be a problem. It is usually not that easy to verify which part is actually sensitive, as the stress / sound is spreading. Also semiconductors can be sensitive and also vibration of the shield /case can have an effect.Inductors with ferrite core can also be rather sensitive - they can work as ultrasonic transducers.

author: stefan_o
author_rank: Regular Contributor
timestamp: July 03, 2018, 04:36:54 pm
content: Quote from: Kleinstein on July 03, 2018, 02:03:50 pmI tested by touching the parts with an RF alignment tool. With the very little force I applied there was no reaction by touching mostly anything (I tested all caps and inductors). Parts like the varicap diodes or the PLL chip itself caused nearly no reaction at all. But touching one inductor gave massive RF spikes (frequency hub), also touching one capacitor placed directly next to it. Other caps gave little reaction and also other inductors some (they are all very close to that one inductor, so it could be mechanical coupling).As it is the same with several devices, it is not actually one faulty inductor.They are wire-wound inductors, are there better alternatives? (220n, 0805, SRF > 850 MHz)Quote from: Kleinstein on July 03, 2018, 02:03:50 pmDo you know of inductors that have minimized that problem?

author: JS
author_rank: Frequent Contributor
timestamp: July 03, 2018, 04:43:30 pm
content: Air core inductors or pcb layout ones, depending on freq range. I'm not a HF guy but those two options are often used in HF stuff...JS

author: stefan_o
author_rank: Regular Contributor
timestamp: July 03, 2018, 10:54:54 pm
content: Quote from: JS on July 03, 2018, 04:43:30 pmAir core are much lower value (in the range up to 50nH) and PCB layout ones are not an options, as it is an existing device.But there are many different types of SMD inductors, what could be recommended?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 03, 2018, 11:17:02 pm
content: 220nH, is that an RFC, not an actual resonant inductor?You might relax your SRF constraint on it then, because the first resonance is parallel mode, hi-Z. You don't get a better RFC than at resonance.Obviously if you need wideband or multiband over or beyond that frequency, the L and C components matter.What types, brands and families have you tried for this component? Wirewound? Multilayer? Wurth? Bourns? Coilcraft? etc.Tim

author: stefan_o
author_rank: Regular Contributor
timestamp: July 04, 2018, 01:47:38 am
content: Quote from: T3sl4co1l on July 03, 2018, 11:17:02 pmI have not yet tried anything to replace it, as I would have to buy an appropriate part first. It is an existing device about 20 years old. Currently there is an wirewound SMD inductor, for me it does not look any special at all. But I never designed RF circuits myself, so I have no idea if a special type of inductor is used. The service manual only lists "SMD coil 220nH". I attached the part of the circuit: The signal to be modulated comes from R31 (bottom left), the output to the RF amplifier is between C43 and C44 (bottom right) . The inductor causing the problem is L2 (close to the bottom, slightly left from the middle) and the caps C27 and C29 next to it.For me L2 is just for DC grounding the varicap diode, it's not part of a resonator.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 04, 2018, 02:10:37 am
content: Hmm, I would think L3 would have the same or greater sensitivity. Maybe it's just broken, or loose?It could also be the nearby capacitors, C77, C39, C26, etc., if they are microphonic.Tim

author: stefan_o
author_rank: Regular Contributor
timestamp: July 04, 2018, 03:27:08 am
content: Quote from: T3sl4co1l on July 04, 2018, 02:10:37 amI tested L3, it does not have much effect. I tested 5 devices (all I have), it's all the same, it's highly unlikely that they all broke at the same spot. Touching C27 or L2 causes the biggest spikes. I found some other 220n SMD inductor (wire wound), replaced it, but no difference. Unfortunately I do not have any 2p7 caps here to test.