Porcelain multilayer high Q capacitors, for RF high power and microwave
The history of high Q porcelain capacitors has followed the evolution of solid-state technology. There is a trace in the late 60's with the first projects of Motorola and RCA regarding UHF transistor power amplifiers, in fact for the HF and VHF bands they are already used types "mica Unelco" but they gave no satisfactory results in UHF. In the presence of high-power tubes (valves) the involved impedances were moderate - high, so it was worked with high voltages and low currents. With the power transistors there is low or very low impedances on the base and collector, in such applications matching and tuning circuits, have to transform the impedances from very low to 50 Ω, it means to work with high current and low voltage.
With this new type of design was necessary to have a low ESR capacitors (Equivalent Series Resistance), this is indeed the main parameter to consider the entire evolution of high-Q capacitors. The loss in a capacitor is indicated by its tan δ, or dissipation factor. The ESR shows the equivalent resistance in series to the capacitor, which dissipates power as heat and introduces attenuation.
|comparison table of 3 different types of caramic capacitors, 30 pF @ 300 MHz|
|Specification||non adequate capacitor||0805 capacitor SMD (COG class 1)||high Q porcelain capacitor|
|DF (tan d)||0,028||0,008||0.0028|
|Q (1 / DF)||35||118||350|
|ESR (Xc / Q)||0.5 Ω||0.15 Ω||0.05 Ω|
|dissipated power||@ 1 A RF current||0.5 W||0.15 W||0.05 W|
|@ 3 A RF current||4.5 W||1.35 W||0.45 W|
This table clearly shows the behavior of 3 different types of capacitors in presence of medium power (1A) and high power (3 A), it is strongly suggested to avoid a dissipation power of only 1 W in a capacitor.
The ESR acts as a double effect, first because it dissipates power and introduces attenuation, second because the power dissipation is a cause itself of self-heating and so of a further deterioration in terms of electrical characteristics that decrease the life of the capacitor.
|Dissipated power values for Murata capacitors , SMD COG class 1 standard types but suitable for RF (values suggested by Murata)|
|type (case)||GRM 39 (0603)||GRM 40 (0805)||GRM42-6 (1206)||GRM42-3 (1210)|
|single capacitor dissipated power (source Murata)||100 mW 200 mW max||125 mW 250 mW max||145 mW 290 mW max||225 mW 450 mW max|
|With ATC 100B is acceptable a 3 W of dissipation power thanks to the fact that it can work up to 125 °C|
|high Q porcelain capacitors, some specifications taken from manufacturers catalogues|
|max current @ 1 GHz||ESR @||working power(indicative values)||voltage range available in stock|
|150 MHz||1 GHz|
|ATC B 3 x 3 mm||3p9||5 A||0.050 W||0.12 W||500 W in HF-VHF 250 W a 1 GHz||up to 100pF = 500V 110 - 200pF = 300V|
|10 pF||5 A||0.045 W||0.1 W||220 - 470pF = 200V|
|39 pF||6 A||0.040 W||0.1 W||510 - 620pF = 100V||not available because of too low voltage|
|100 pF||8 A 100 MHz||0.035 W||0.1 W||> 680 = 50V|
|390 pF||0.030 W||0.1 W||
510 - 1000pF = 300V (estended voltage, available)
|ATC A 1.5 x 1.5mm||1 pF||0.8 A||0.25 W at 1 GHz||70-100 W @ 1 GHz 30 W @ 10 GHz||from 50 to 200 V depending on type and availability|
|3p9||1,5 A||0.2 W at 1 GHz|
|10 pF||2 A||0.15W at 1 GHz|
|47 pF||3 A||0.09 W at 1 GHz|
|common specifications for types A and B|
|Q||> 10.000 @ 1 MHz --- Q > 20.000 / C(pF) @ 100 MHz|
|90 ppm / °C , tradotto in pratica questo significa che con una variazione termica da +20°C @ +120°C , cioè di ben 100°C , la capacità si sposterà al massimo dello 0,9 %|
|insulation resistance||@ 25°C= 10 ³ GW @125°C = 10 ² GW|
|operating temperature range||-55°C / +175°C 0.1 to 330 pF, -55°C / +125°C 360 to 1000 pF|
|life cycle||2000 hours @ +125°C @ 200% of Vmax|
ATC 100 capacitors are usually mounted with the electrodes parallel to the plane ( printed circuit board ), the label corresponding to the capacitor value is parallel to the printed circuit board.
We tested with the network analyzer the SRF of a 62 pF case B capacitor parallel oriented . For this capacitor's value is guaranteed an SRF > 900 MHz , in fact the network analyzer shows an SRF of 1550 MHz . The picture below shows the improvement of SRF with vertical orientation at 2.7 GHz.
The same with 4.7 pF case A , for this capacitor's value is guaranteed an SRF > 4 GHz , in fact the network analyzer shows an SRF of 7.6 GHz . The picture below shows the improvement of SRF with vertical orientation at 12.3 GHz.
These two tests demonstrate how it is possible to improve the self resonant frequency in multilayer capacitors, we suggest this improvent only for ultra wide band applications , where you have capacitors with the SRF into the frequency range of the application . For narrow band applications this improvement is not necessary .
The SRF effect is due to very complex mechanisms related to the phase differences of the varios electrodes in multilayer capacitors.
|62pF case B|
|4.7pF case A|
|The multilayer structure provides a very low inductance series and ESR resistance series and a considerable RF current because the result is like a parallel of many capacitors. They are built with high-purity ceramic (porcelain), they have a remarkable thermal stability even under extreme environmental conditions even with temperature and humidity variations. ATC capacitors are even used in low noise front ends from VHF to microwave used togheter with low noise GaAs-FET you can reach outstanding performances.|
The 800B series is the latest addition to the ATC production, it is characterized by a very low ESR (equivalent series resistance) due to the use of silver electrodes. It is designed mainly for applications beyond 200 MHz where the improvement of the ESR and heat transfer are particularly remarkable and involve an increase of the maximum current.
Among the key features of the 800B series there is also a better SRF (self resonance frequency) and NP0 thermal stability, the "standard" isolation voltage is equal to 100B and 700B series, also footprint size is exactly the same.