gilbert cell mixer implementation and balun - Page 1
Forum section: electronics / rf-microwave

author: promach
author_rank: Frequent Contributor
timestamp: April 03, 2019, 04:17:28 am
content: 1) How do I interfaceLC balunwithmixerusing differential current RF+ and RF- input signals ?2) For LC balun, how should I deal with the load impedance (RL) for LO+ and LO- which is almost infinite because it is mosfet gate ?3) How is the LC balun different from Marchand Balun ? see figure 3 ofA 60‐GHz double‐balanced mixer with negative resistance compensation for direct up‐conversion using 90‐nm CMOS technologyandAnalysis and Design of Lumped Element Marchand Baluns

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: April 03, 2019, 11:41:48 am
content: I haven't quite studied all of the images, but I'll try to answer some of the questions here. Some extra information could be usefull: What frequency are you working at? What technology/transistors?I assume that you are working with TSMC180 (as you have that lib loaded). I don't know how you got access to the TSMC technology. I do know that whenever TSMC gives you access, you have a big stack of NDA papers to sign. Either you did this, and you are breaking this NDA with the picture of LTspice, in which case I would suggest trying to remove it, or you didn't do this, and you have illegal access, in which case again, I wouldn't try advertising it too much. Not trying to scare you too much, but the technology people are crazy paranoid about these kinds of things, and I wouldn't want your lab or group to have access revoked.Also, LTSpice really isn't suited for this kind of RF design. You really want something that can give you nice smith-charts and loadpulls (spectreRF, Keysight ADS, NI microwave office,...)Quote from: promach on April 03, 2019, 04:17:28 amI don't quite understand what you mean here. The balun has a balanced output (ie, RF+ and RF- in your case). you just connect them to RF+ and RF- of your mixer.Downside of such a balun for IC application is biasing. One of your outputs has no DC link, the other does, which will lead to asymmetry. One way you could solve this is by putting a large resistance in parallel with the capacitors. This will form a DC path so both of the differential lines are connected at DC, giving easier balance. Ofcourse, simulate that well because its parasitics could pose an issue.Quote from: promach on April 03, 2019, 04:17:28 amThe load impedance will not be almost infinite. A mosfet gate at low frequencies can be approximated by a capacitor\$^1\$. What constitutes a low frequency depends on technology and application - I'd say on modern ICs low frequency would be up to a few GHz. This means that it is a high impedance at DC, but at RF it is a capacitor. You need to match to this capacitor. Sometimes you can do this with your bias inductors, or by doing a transformer co-design. To get this capacitance you need good simulation. Part of it will be from the transistor model provided (take into account the importance of the bias here! Changing the gate bias of a transistor also changes its input capacitance). Depending on the frequency, you get the remaining capacitance from a parasitic extraction of the layour (PEX), 2.5D EM simulation (eg, sonnet or Keysight Momentum), or 3D EM simulation (EMPro, HFSS, CST, COMSOL). Then you can figure out the location on the smithchart you are on and match it to a given impedance (say 50 ohm), and then you can just match your balun to the same reference impedance. (as you get more advanced and skilled, you stop doing that and instead just make the balun work optimally when it sees the load impedance presented by your transitors, or include a kind of 'matching network' to tie the two together. Getting rid of the transition to a given impedance and back gives you lower loss).Quote from: promach on April 03, 2019, 04:17:28 amI'm no expert on either LC or Marchand baluns and have never used either in my own designs, but I believe the marchand uses coupled lines.The LC balun you show, on the other hand, actually uses two filters: One low-pass and one high-pass. They both provide a similar phase-response, but with opposite sign. By then choosing the response such that at your target frequency, one provides +90 and the other -90 degrees phase shift, you get two outputs that are exactly 180 degrees out of phase: you have created a differential signal.I expect the issue here to be the quality of the components. On chip, at low frequency, capacitors are great. They have low ESR, and good Q as a result. Inductors, on the other hand, suck. They take up a lot of area, they are a pain to design, and they have very poor performance (large ESR since you usually need long bits of metal to get the amount of turns you need, and the substrate ain't great either). They also have some (potentially significant) parasitic capacitance with substrate, as well as distributed in between windings. This means that your LC balun will need two large, and poor performing capacitors at low frequency.At high frequency, the opposite is true: inductors start getting smaller and get higher Q (still nothing to write home about, but they get more and more worthy as you go into the tens of GHz). Capacitors, on the other hand, start to perform poorly. Large losses due to inductance and resistances everywhere. As a result, if you make an LC balun like that at high frequency, you tend to get shitty performance of capacitors.So, in the end, your LC balun will always have lousy performance - at low frequency because of the inductors, at high frequency because of the capacitors.Marchand baluns with tends to do a less shitty job because of the coupled lines being close to each other and thus you get less fields far away. I found this video on the topic:Alternatively you can also use a single transformer. This works very well and is common on a lot of RFICs.\$^1\$ At high frequencies, on integrated circuits you should add some gate resistance. This is one of the main limitations of gain at higher frequencies, as this limits the Q-factor you can get in your matching networks (as there is a resistor in front of your capacitors you can't get rid of).

author: OwO
author_rank: Super Contributor
timestamp: April 03, 2019, 12:16:11 pm
content: Quote from: TheUnnamedNewbie on April 03, 2019, 11:41:48 amhttps://user.eng.umd.edu/~newcomb/courses/spring2010/303/tsmc180nmcmos.libNow can anyone help me find the TSMC 28nm library files?

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: April 03, 2019, 12:32:32 pm
content: Quote from: OwO on April 03, 2019, 12:16:11 pmI wasn't aware that these were publicly available like that. I just know what kind of paperwork I used to go through to get access, and how scared people were of it leaking (eg, no copies, screenshots, or parameters should be saved on any device that is not whitelisted, no mention of the technologies we have access to, no discussion with anyone who has access to any competitor technologies, etc), hence my comment.

author: promach
author_rank: Frequent Contributor
timestamp: April 03, 2019, 02:28:45 pm
content: QuoteWait, why one of the outputs has no DC link ?QuoteStrange, I never came across this though. Care to explain why ? because of Cgs (fan-in plus fan-out) ?QuoteRemember that this is mixer. Therefore the LO+ and LO- will be of course be sinusoidal wave that changes in (AC+DC) amplitude. So , what is your advice ?QuoteThanks for the link. I will look into it.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: April 04, 2019, 06:06:17 am
content: Quote from: promach on April 03, 2019, 02:28:45 pmI meant there is no easy DC link between the two outputs. I made a quick crude sketch here to show what I mean:You would perhaps like both outputs to be biased with one single bias line. This is not easily possible with this LC balun because there is no DC link between the two. You might want to bias them through the single-ended input, but then the capacitor in the bottom path blocks DC. The other option is to bias them through the AC ground (which is where the inductor in the bottom path and capacitor in the top path connect to) but then in the top path, the capacitor again blocks DC. This asymmetry is challenging, and could be a reason why you woulnd't want to use such a topology.In the Marchand balun, both RF lines are connected to RF ground. This means you can simply try adding some capacitance or such to enforce this RF ground, and then bias them gether. Added bonus is the fact that the input and outputs are fully isolated, which means whatever you are using to drive your balun can have a DC level on that output. (see picture)Quote from: promach on April 03, 2019, 02:28:45 pmI have never really heard someone talk about it in terms of fan-in and fan-out. Those seem more like digital terms, where you look at an amount of capacitance you want to charge. Here, you need to think about it more as impedances. The root cause of the capacitance is mostly the same: parasitic capacitance of the feeding network, gate-source, gate-bulk and gate-drain capacitance.This is a combination of all of the parasitic capacitances. Main ones are usually gate-source and gate-bulk (though often those are shorted together so it is just one big capacitance). However, the gate-drain capacitance is also important. It can provide a feedback path for the RF signal, which can cause instability at high frequencies and there is little you can do about it except for taking great care in layout (it also provides a feed-forward which can complicate your impedance matching a bit - changing the output/input impedance also impacts the input/output impedance).Quote from: promach on April 03, 2019, 02:28:45 pmWhy would it change in DC amplitude? You want your DC bias to be stable and fixed.And true, you will see a varying capacitance on your input. This is just the nature of a design, you need to take this into account. This could be a why you tend to do large-signal simulations too (which I think LTSpice can't do?) since this changing capacitance can cause harmonic generation and asymmetry between your LO+ and LO- which could result in higher LO leakage. You simply need to match so that you get best performance, and this will not be as good as the performance would be if the capacitance to AC ground seen at the gate were fixed. That is just the nature of RF design.Final note: depending on the type of mixer, you might want to use a higher power LO that is more like a square wave (Gilbert can still be used as a linear mixer, but is not commonly used in CMOS, as you have a lot of transistors stacked up on each other reducing swing - passive mixers are more common, where you might want to switch them hard to get low conversion loss).

author: promach
author_rank: Frequent Contributor
timestamp: April 06, 2019, 12:30:20 pm
content: Quote1) Why 100um and 200um for the Marchand balun ? Do you think if I would be able to simulate a self-drawn marchand balun using software likeopenems,QucStudioorZ0lver?I do not have the money to run ADS on linux.ForA Low Power and High Conversion Gain 60-GHz CMOS Up-Conversion Mixer Using Current Injection and Dual Negative Resistance Compensation Techniques,2) How to implement TL1 and TL2 (inductor implemented using transmission line) inspice software?3) How to derive the conversion gain expressions ((2) and (4)) for the up-conversion mixer circuit with and without negative resistance compensation in the paper ?

author: promach
author_rank: Frequent Contributor
timestamp: May 05, 2019, 09:43:37 am
content: @TheUnnamedNewbieIf you look at the S11 waveform fromthe same video you linked previously,why there is a deep dip around 12 GHz ? Is this normal ?

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: May 07, 2019, 09:02:09 am
content: Quote from: promach on May 05, 2019, 09:43:37 amIf that dip was not there, the balun would not work. That dip is the point where the balun is matched to the input, IE, the signal is not reflected back. You would want that around your target band, IE< in this case, it would seem the balun is designed for operation around 12 GHz.

author: promach
author_rank: Frequent Contributor
timestamp: May 09, 2019, 02:42:30 pm
content: What do you guys think about thisresistive network Marchand Balun?I am having problemderiving the maths expression for coupling factor, C. Any help ?

author: bncengr
author_rank: Newbie
timestamp: May 12, 2019, 04:49:11 pm
content: How does the Keysight TC260 accomplish this? The datasheet has a shematic.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: May 13, 2019, 05:12:09 am
content: The tc260 is single-ended input. The luxurynof using a process which is tuned to RF circuits I stead of digital logic.