Concerning RF Structures On Integrated Circuits - Page 1
Forum section: electronics / beginners

author: JoeN
author_rank: Frequent Contributor
timestamp: February 14, 2017, 08:52:48 am
content: I'm sure many people are familiar with this site:https://zeptobars.com/en/. They do acid immersion and imaging of IC dies. An interesting category of ICs are ones that have onboard radios. All of these that I have seen so far seem to have easy to identify RF structures that are circular, like the ones in this image (zeptobars's latest teardown). What I don't understand is why there are commonly so many of these. Why do they need so many? Is each structure tuned somehow to a different wavelength? Or how is tuning achieved in any case? Anything you can point me to to read up about this would be interesting to me. Thank you.

author: tron9000
author_rank: Frequent Contributor
timestamp: February 14, 2017, 09:27:05 am
content: inductor traces maybe?

author: rfeecs
author_rank: Frequent Contributor
timestamp: February 14, 2017, 04:20:19 pm
content: Most of those structures are coupled coils. You can think of them as transformers. Some of them are just inductors.They might be used in baluns, for balanced mixers as an example.Try googling "rfic planar transformer".

author: rfbroadband
author_rank: Unknown
timestamp: February 15, 2017, 05:11:04 am
content: they are used for example in VCOs, the VCO freq. is then tuned with on chip varactors and capacitor banks. RF Filtering is another application. Very often you need multiple VCOs to cover the desired frequency range, so you need several inductors (different sizes) foe each VCO. The inductors are designed with EM tools.

author: JoeN
author_rank: Frequent Contributor
timestamp: February 15, 2017, 05:17:43 am
content: At first I was thinking they were some sort of antenna. After I posted this I had a second thought that they might be inductors. Thanks for the confirmation. It's sort of interesting. It looks like you can have one inductor, or maybe about 20,000 transistors, in the same area. Your pick. Very inefficient, but I suppose better than a bunch of external components. They sure are prettier than the seas of transistors for logic and SRAM, though.The huge yellow area I assume is the SRAM.

author: helius
author_rank: Super Contributor
timestamp: February 15, 2017, 05:38:58 am
content: The yellow "basket weave" areas are just covering up the rest of the circuit. This is done as a tamper-resisting measure, so that the chips can be used in security applications.edit: it may also be to improve RFI since the cores will emit noise in the microwave band

author: JoeN
author_rank: Frequent Contributor
timestamp: February 15, 2017, 05:43:25 am
content: Quote from: helius on February 15, 2017, 05:38:58 amI was thinking that might be the case, given it doesn't look like the other transistorized areas. However, don't you think that area is probably mostly SRAM? This IC has a lot of SRAM (520KB). Or do you think it is the logic also? This is a dual core IC. I was thinking the cores might be the twin structures in the bottom-right quadrant of the "uncovered" area in the upper-left. But heck if I know, I know almost nothing about this. It just got my interest. The photos on this site are all interesting.

author: helius
author_rank: Super Contributor
timestamp: February 15, 2017, 05:51:59 am
content: 520KB = 6 * 520 * 1024 * 8 = ~26M transistors (including address decoders, column and row drivers, etc). Each 32-bit core is probably ~4M. So yes, most of the area is SRAM.The structure of SRAM is very specific, it must always be rectangular, and needs ancillary structures next to it. You can't see it because they covered it with a shield.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2017, 06:14:34 pm
content: Inductors and transformers. Likely resonating with nearby MOS or MOM capacitors, so call them IF transformer cans if you prefer!The quality factor is quite poor, something like 15 at best. They can't provide very much selectivity this way, and it is very expensive on die area -- area better spent constructing high performance digital logic.Size corresponds to frequency range, so I would suppose they designed the architecture this way:- Biggest coil for 2.45GHz (or whatever it was) antenna coupling. This would have the power amplifier, and receiver via T/R switch, connected to it. It looks like it's a matching balun (two ends, and a center tap, going to pads; the higher impedance end, also going to pads seemingly!).- Transmitter may be direct synthesized, using whatever modulation scheme is necessary (if this supports Wifi everything, then that includes QAM modes). How exactly this is done, depends, but it could be a pair of quite-high-speed DACs followed by linear amplifiers, or it could be gate drivers and an array of different size power transistors (otherwise known as a "power DAC"). There may be pulse shaping to reduce harmonic distortion, or they may be relying on the output tuning (and whatever filtering is employed externally, if any?) to filter it out.- Smaller coils are for higher frequencies. They are probably up-converting to a high IF. This makes gain more expensive, but transistors are tiny and cheap, coils are not! There should be a VCO (with one coil), a mixer, an IF coil and IF amps, then another mixer (possibly using a second oscillator), which downconverts to low IF or baseband, where an ADC, and hardware DSP, can finish things off.I could have that upside-down, and they actually do quite a lot of amplification and filtering on the transmitter (which would be important to minimize spurious emissions and IMD). In that case, there could still be a high IF, or the ADC might be direct conversion (sampling at 2.45GHz +/- some difference, capturing the whole band as equivalent-time samples).BTW, that "weave" pattern is probably connected to the supply, or something like that; it would make an excellent bypass capacitor. Something supremely important with switching frequencies unsuitable for board-level bypass!Tim

author: rfeecs
author_rank: Frequent Contributor
timestamp: February 15, 2017, 07:13:53 pm
content: With regard to the front end architecture, it is probably a standard I/Q transceiver. Here is an example:https://www.researchgate.net/profile/Chang-Tsung_Fu/publication/260872447_A_24GHz_WLAN_transceiver_with_fully-integrated_highly-linear_18V_284dBm_PA_34dBm_TR_switch_240MSs_DAC_320MSs_ADC_and_DPLL_in_32nm_SoC_CMOS/links/00b7d532937e2c194b000000/A-24GHz-WLAN-transceiver-with-fully-integrated-highly-linear-18V-284dBm-PA-34dBm-T-R-switch-240MS-s-DAC-320MS-s-ADC-and-DPLL-in-32nm-SoC-CMOS.pdf