ESR ceramic capacitors - Page 1
Forum section: electronics / rf-microwave

author: salviador
author_rank: Regular Contributor
timestamp: June 03, 2020, 10:00:20 am
content: Hello ,I'm struggling with a VLF resonant preamp circuit (77Khz). I have a 160uH inductor and was putting ceramic capabilities to make it resonate at that frequency.I realized by measuring the bandwidth that I have two different abyssal results using two different types of capacitors.Using Blue ceramic capacitors I have an optimal passband of 800HzUsing ceramic disc capacitors I have a bandwidth of 4-5KhzWhy? Are there any capacitors to use for LC resonant circuits?

author: JohnG
author_rank: Frequent Contributor
timestamp: June 03, 2020, 12:35:38 pm
content: Welcome to the world of ceramic capacitors!One possible explanation is different dielectrics. A good place to start is here:https://en.wikipedia.org/wiki/Ceramic_capacitor. If you want ceramic capacitors to use in a resonant network, use Class I dielectric (read link above for further info), NP0 or C0G are stable, low loss, and common. Note that funny things like lead type can affect Q to some extent.You may also want to look at polypropylene film caps, or polystyrene if you can find them. Mylar (polyester) have higher losses.There are many, many types of capacitors, and the most stable, linear, and low loss ones generally have the smallest capacitance per volume for a given voltage rating.Hope this helps.John

author: salviador
author_rank: Regular Contributor
timestamp: June 03, 2020, 01:34:04 pm
content: I made two measurements with the VNA of the two capacitorsYou can see the differences

author: radiolistener
author_rank: Super Contributor
timestamp: June 03, 2020, 01:45:32 pm
content: Quote from: salviador on June 03, 2020, 10:00:20 amBecause your Blue capacitor has lower loss than Red one. It means that Blue one has higher Q and it affects Q of your filter. Q affects bandwidth. So, more loss = wider bandwidth.bandwidth = 1 / Qwhere bandwidth is relative value, to get absolute value just multiply this value by working frequency Fc, or use this equation:bandwidth in Hz = Fc / QIn order to get narrow bandwidth you're needs to use high Q capacitors and high Q inductors for your filter.

author: TimFox
author_rank: Super Contributor
timestamp: June 03, 2020, 03:22:54 pm
content: As a rough generalization, some dielectrics (ceramic and plastic film) have a Q value determined by the dielectric itself, which is a very weak function of frequency. In physics, this is known as the imaginary component of the dielectric constant.Others, such as various electrolytics, have a parasitic ESR, and that gives a Q value that decreases with frequency as the reactance drops: for a series model, Q = Xc/ESR.The "good" dielectrics for RF are Class I ceramic (NP0 or C0G) and mica, which have Q values around 1000. The really good porcelain dielectric, used by ATC, can have Q around 10,000. Polypropylene film also has Q around 1000, but the self-inductance limits its use at RF."Mediocre" dielectrics include Class II ceramic (X7R) and polyester film ("Mylar"), which have Q values less than 200.If you make an L-C resonant circuit, the Q of the resonant peak is the "parallel combination" of the two Q values: 1/Qtot= 1/QC+ 1/QL. With "good" capacitors, the Q is dominated by the lower Q value of the inductor. Adding an external load to the network reduces the Q further. To calculate that, you should convert the losses of the C and L into equivalentparallelconductances and add the conductance of the load, calculating the resulting Q with the reactance of L and C (equal at resonant frequency). The lemma (found in textbooks) for converting between series and parallel models for a reactance and resistance is very useful here and is left as an exercise for the reader.

author: salviador
author_rank: Regular Contributor
timestamp: June 04, 2020, 02:00:38 pm
content: I've never tried porcelain dielectric capacitors. I would like to try them and see if the Q improves significantly

author: TimFox
author_rank: Super Contributor
timestamp: June 04, 2020, 02:43:29 pm
content: The ATC porcelain capacitors are great, but you will have great difficulty obtaining inductors with Q values high enough for them to make a difference (compared with good ceramics). Also, they are available up to about 5 nF.Seehttps://atceramics.com/multilayer-capacitors. The true porcelain capacitors have a temperature co-efficient of "P90" (+90 +/- 20 or 30 ppm/K). Their construction gives them good performance into the GHz range. The table on the referenced page gives the ESR at VHF: at audio frequencies, the Q usually exceeds 10,000.