How to Make GHz BW Differential Signal output to Single Ended Coax Cable? - Page 1
Forum section: electronics / rf-microwave

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 11, 2019, 12:36:12 pm
content: i have non inverting and inverting CML output like in attached picture (DC coupled cyan trace, in-circuit 100nF AC coupled), 350mVpp+ on each output 50 ohm terminated. i want to get 700mVpp+ single output from them, preferably through single 50ohm coaxial cable, as finding differential input to single end output opamp at this bandwidth is not feasible. we are talking 14GHz bandwidth and would like to maintain 35ps risetime @ 700mVpp+. any RF simulation SW for this? can Sonnet the Free version do this kind of challenging task? or any hint to the circuit example? my google-foo failed miserably, thanks.

author: capt bullshot
author_rank: Super Contributor
timestamp: July 11, 2019, 12:46:00 pm
content: To me, the problem looks like it could be solved by a properly constructed balun. Please don't ask me about the proper construction, since 14GHz is above my skills. Just wait for T3sl4co1l (shht, don't say the word transmission line transformer) to appear

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 11, 2019, 01:00:51 pm
content: i believe one guy in this forum did this without balun component. just pcb element and some possibly inductor and capacitor on MAX3949 11Gbps diff output laser driver. attached is cropped view pcb on related area from his website. i will be blessed if he will reply as he has made this circuit commercial. it will be interesting to know what RF SIM SW he is using (or if any equivalent) and how to go about it.

author: capt bullshot
author_rank: Super Contributor
timestamp: July 11, 2019, 02:22:51 pm
content: Yes, this pulser is well known to me (and this forum I think) - but I've never went into the details of how he matched the IC output to the output connector. Maybe you just ask him in his threadhttps://www.eevblog.com/forum/projects/yet-another-fast-edge-pulse-generator/525/AFAIR, he made quite a number of PCB design iterations to achieve the best results. Anyway, I wouldn't be surprised if he just used one side of the differential output and simply terminated the other (resulting in half the amplitude) - and most of the magic is in making the traces as short as possible.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 11, 2019, 05:24:41 pm
content: Do you need it to be DC coupled? How much do you care about DC performance? Getting true DC - 14 GHz performance will be a challenge.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 11, 2019, 05:57:50 pm
content: this is mainly for pulse/square generator, similar to Leo's pulser except i'm trying if its possible cheaper using 2 layers board and driven from arduino so we can easily program the pulse output to our heart desire. afaik Leo's pulser cant be reprogram both in SW and HW. and GBP99 for sma version is a bit high for my taste, so i'm just trying my luck and for learning purpose. not so luck i guess with this ADCMP572 purchase. but i know now the pulse and voltage rail are not oscillating with my 1st batch pcb thats good it increases my confidence level a bit.i should be aware of CML structure before ordering this comparator (ADCMP572). i thought its as innocent as the LM393 output but i was wrong. reading back the MAX3949, i think bullshot is right, the outputs voltage swing is near rail to rail like LM393 (if i read it correctly at VOH VOL spec) so Leo probably just terminated the other (inverting) output of it and just send the non inverting output to BNC/SMA connector, without the need for balun. searching "ghz pcb balun" in google also turned out several interesting informations, so i learnt something as well. now i think i need to order MAX3949, make another pcb for it and learn again. it also 3X cheaper than ADCMP572. if anything i should aware about this laser driver IC "specialty" i will be glad if anybody can give a hint before i make another order from digikey.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 11, 2019, 06:08:54 pm
content: Quote from: ejeffrey on July 11, 2019, 05:24:41 pmi prefer the output to be square signal, VOH = Vcc, VOL = 0V thats the ideal. but this CML is very different/special, the pulses ride on very large common mode voltage, about 2V CM (Vcc = 5V) as you can see in the attached pictures (non invert and inverting output). so i need them AC coupled like in the OP with a smd capacitor to bring it down near 0V level. Leo's pulser is also AC coupled, +V to -V output, so to me it also less than ideal. i need ideal square signal (digital like) with as fast risetime as possible to check dso BW.

author: David Hess
author_rank: Super Contributor
timestamp: July 12, 2019, 02:47:22 am
content: A 2 transistor differential input transconductance amplifier can be used for fast differential to single ended conversion. Below is an example from the Tektronix 7834 which I selected because it is relatively simple. The output transistor cascodes are optional and of course polarities can be reversed so NPN RF transistors can be used. This configuration is also found on active probes.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 12, 2019, 04:17:57 am
content: thanks david that can be a good reference.

author: David Hess
author_rank: Super Contributor
timestamp: July 13, 2019, 05:07:16 am
content: I gave that example as a place to start but there are some subtle aspects to it.The RC networks between the emitters of the input transistors peak the response for better bandwidth. Note that parasitic inductance in series with the emitters will limit potential bandwidth to not much higher than 200 MHz when through-hole parts are used. (1) Surface mount parts with a tight layout will do considerably better but might not get to 1 GHz.The cascode output transistors serve two functions. The obvious one is preventing voltage swing at the collector of the input transistors from limiting bandwidth but they also produce a constant voltage across the input transistor collector and load. When the idle current (current with zero output voltage) produces a voltage across the RC network which is equal to the collector voltage of the input transistors, then changes in signal level in either direction produce the minimum change is power dissipation in the input transistors reducing thermal imbalance.An equivalent circuit using current feedback operational amplifiers is possible also. The OPA860/OPA861 are particularly suited for this but there are faster conventional devices available now. The AD8130 difference amplifier could also be used up to about 250 MHz.(1) Plus through-hole 1 and 2 GHz transistors are no longer readily available. It would be fun to see what kind of performance 2N3904/2N3906 transistors would produce in this circuit but 100 MHz would generally have used 650 MHz parts like the MPSH10/MPSH81.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 13, 2019, 05:12:50 am
content: thanks for the more detailed info David. i'm aware the BW for the above circuit is only 400MHz while i require in the order of 14GHz. but i already have few stock rf transistors to play with, or maybe in another project such as BFP840 85GHz Ft and BFP640 40GHz Ft, i will find out later...

author: David Hess
author_rank: Super Contributor
timestamp: July 13, 2019, 08:21:05 pm
content: Quote from: Mechatrommer on July 13, 2019, 05:12:50 amI did not realize you needed that much bandwidth. Parasitic elements from the package and layout will limit performance so faster designs must be integrated. The inductance in series with the emitters is especially problematical.Faster implementations are integrated and may use an Ft doubler circuit configuration but offhand I do not know of any available integrated parts above a couple hundred MHz. The differential to single ended conversion circuit in the Tektronix 1 GHz 7104 oscilloscope shown below does both.Still, it would be neat to see how far a surface mount design using the smallest common surface mount transistor packages could be pushed.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: July 18, 2019, 08:40:08 am
content: If you can keep the pulse-rate constant, perhaps making a delay line with 1/2 of your pulse period combined with a simple Wilkinson power combiner can work for you. After all, if it is just a pulse waveform, the positive is the same as the negative with a 1/2 period shift.The problem would be that you really only care about the rising edge, and as a result, any tiny mismatches in delay would cause your rising edge to be ruined. Because of the BW you want (up to DC!) I think passive combiners will not perform well here, unless you can do a kind of 'multi-band' thing where you split the signal into multiple frequency bins, balun, then combine. All this, however, might give okay-ish frequency response, but for rise time phase response is also vital, as any phase shift over frequency will cause your risetime to be ruined (even though all the frequencies are still present).It could be easier to simply add a gain-stage to double that output power instead of trying to combine it. Alternatively, you could consider using some differentially-driven, high speed logic gates (line drivers, perhaps?) to pull it off. But now that I think about it, that is kinda the same as a gain-stage anyways.The hard thing with making amplifiers could be stability. A transistor that has gain up to 40 GHz is very likely to have crazy gain at low frequencies. You amplifier might be good at 40 GHz, but if you give it gain up to DC it could oscillate. In addition, this gain stage could also hurt your phase performance which is vital here.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 18, 2019, 04:05:44 pm
content: Use a buffer.Since the outputs are current mode, you don't gain anything from driving 50 ohms differential, versus using just one output. So there's no point in, say, bridging between the two outputs with a piece of coax and stuffing ferrites along its length to keep the common mode clean.You may have something to gain with a 100 or 200 ohm differential load, in which case you need a balun, and that'll be a TLT of some sort. Not beyond the realm of possibility but you're on your own as far as flatness in this much bandwidth.By "may", I mean that the output power goes up with load resistance, until voltage saturation is reached; but, at the same time, bandwidth falls, because the pin capacitance dominates that much more (or more generally, the load impedance mismatches the pin impedance, and this manifests as lumped L or C, or more complicated structures if the bandwidth is high relative to the pin structures).Tim