Ultra low insertion loss SPDT 1.4GHz RF switch - Page 1
Forum section: electronics / rf-microwave

author: srce
author_rank: Regular Contributor
timestamp: October 21, 2021, 11:12:53 am
content: I'm looking for an ultra low insertion loss SPDT RF switch for 1.4GHz.Can anyone recommend anything better than an InfineonBGS12PN10? Insertion loss is given as around 0.17dB at 1.4GHz.Input power is very low, Z0=50Ohm, can supply any voltage and cost not really important either.

author: MartinL
author_rank: Regular Contributor
timestamp: October 21, 2021, 12:43:12 pm
content: What are you trying to do? Whatever it is, if it depends on getting better than 0.17dB loss in a switch, then I think you probably need a different approach. That's already pretty spectacularly low. It's getting down to the range where, even if you found a part whose datasheet claimed a lower figure, I'd be surprised if you were able to reliably measure a difference between them that would be distinguishable from the noise of individual manufacturing variations between units and boards.Here's an electromechanical one with SMA ports and a 12V solenoid drive for $127 - but it still only promises to get you down to 0.15dB:https://www.digikey.com/en/products/detail/radiall-usa-inc/R570312000/10521679

author: srce
author_rank: Regular Contributor
timestamp: October 21, 2021, 02:32:07 pm
content: Quote from: MartinL on October 21, 2021, 12:43:12 pmA switch for a Dicke Switch radiometer. The switch goes in front of the LNAs so minimising loss is critical. Any ideas for a different approach?

author: MartinL
author_rank: Regular Contributor
timestamp: October 21, 2021, 09:16:01 pm
content: Hmm, interesting. Not a topic I'm familiar with I'm afraid.But poking around the literature a bit, I seethis paperwhich proposes the use of a "dual path" LNA, such that no separate switch is needed:Looking at the circuit proposed, it's basically just two separate frontend LNAs, simultaneously connected to the input of a second stage LNA, with only one powered up at a time.They're working on-chip at 100GHz, but I don't see why the same approach couldn't be applied in a board level design at 1.4GHz.The presence of the inactive LNA would have some effect on the load impedance seen by the active one, and the source impedance seen by the next stage. But it should be possible to design a matching network in the middle such that everything sees the correct impedance when either one of the two LNAs is active. You would want to choose LNAs with suitable shutdown behaviour - ideally a high output impedance when off.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: October 22, 2021, 05:56:09 am
content: I think ADI has some MEMs switches that could work but your application should be extremely low power so there are likely lots of candidates. I think PIN will be lower loss than FET switches, I can't tell the technology of the Infineon switch. We have such as switch in the Fieldfox but I don't know the technology. In fact we have it in the PNA-X but we found the improvement using it wasn't worth the trouble (you have to know the temperature very well of the load side of the switch, and our instrument drift was lower than the temperature drift). I'll ask and write back. will this be room temperature of cryo?

author: srce
author_rank: Regular Contributor
timestamp: October 22, 2021, 08:33:46 am
content: Quote from: MartinL on October 21, 2021, 09:16:01 pmYeah, there are some other examples where it's done at the board level with the switch after the first LNA. This obviously has lower noise, but there's perhaps more possibility for gain variation between two different ICs (which is problematic for calibration) or if there is a thermal difference between them. I may give both a try to see how they compare.

author: srce
author_rank: Regular Contributor
timestamp: October 22, 2021, 10:57:45 am
content: Quote from: Joel_Dunsmore on October 22, 2021, 05:56:09 amI was going to try some basic cooling with a Peltier module. Don't have any experience with cryogenics. If there is an off-the-shelf cooling solution not much bigger than around 10cmx10cmx10cm I'd be interested to hear about it.

author: srce
author_rank: Regular Contributor
timestamp: October 22, 2021, 02:21:01 pm
content: Quote from: Joel_Dunsmore on October 22, 2021, 05:56:09 amHad a quick look - there's theADGM1304which looks like it has similar insertion loss to the Infineon part (although hard to tell precisely from the graph), however, being MEMS, it has a limit to the number of times it can switch. In the literature, Dicke Switches switch at 100-1kHz (not quite sure I'll need this frequency), so a lifetime of 1 billion switches, is only a few months.

author: MartinL
author_rank: Regular Contributor
timestamp: October 22, 2021, 02:30:15 pm
content: Quote from: srce on October 22, 2021, 08:33:46 amOne way to reduce variations between the two frontend LNAs would be to use a part with two independent LNAs on one die. MaybeGRF2078?

author: srce
author_rank: Regular Contributor
timestamp: October 22, 2021, 04:47:17 pm
content: Quote from: MartinL on October 22, 2021, 02:30:15 pmYep - the downside is that they seem to have a higher NF than single channel parts. Will probably take a bit of experimentation to see which trade-off is the least important.

author: TimFox
author_rank: Super Contributor
timestamp: October 22, 2021, 05:08:44 pm
content: Quote from: srce on October 21, 2021, 02:32:07 pmWhat is the noise figure of that radiometer? The attenuation added before that input will degrade the LNA noise figure by the attenuation value.

author: srce
author_rank: Regular Contributor
timestamp: October 22, 2021, 05:36:55 pm
content: Quote from: TimFox on October 22, 2021, 05:08:44 pmThe datasheet figure for the 1st LNA is about 0.2dB, so even though 0.17dB is pretty good for a switch, it's a significant degredation in the overall NF.

author: TimFox
author_rank: Super Contributor
timestamp: October 22, 2021, 06:49:40 pm
content: Remember that the NF gives the noise added to the (theoretical minimum) noise from the source impedance, not the SNR itself.

author: awallin
author_rank: Frequent Contributor
timestamp: October 22, 2021, 06:54:34 pm
content: Quote from: srce on October 21, 2021, 11:12:53 ammechanical coaxial relay from e.g. mini-circuits?https://www.mini-circuits.com/WebStore/dashboard.html?model=RC-1SPDT-A18rated to 18 GHz (!) - so 1.4 GHz is a low frequency and datasheet p.4 shows something under 0.05 dB for 1.4 GHzthey might have only the SPDT switch without box, psu and ethernet/usb interface also as a product.

author: SQ9MT_PL
author_rank: Contributor
timestamp: October 30, 2021, 08:17:57 am
content: A good (expensive) mechanical relay will always perform better than a solid state one.However, such relays are more and more often used in measuring electronic systems and it is "not from yesterday" the best if it is possible for you to present the system in which it is supposed to work.The question is what parameters you want to achieve and choose the price-quality ratio. Well, not to mention that such a mechanical switch will be large due to its design.