GPS (~1.5 GHz) amplified splitter design - Page 1
Forum section: electronics / projects

author: nsayer
author_rank: Regular Contributor
timestamp: September 05, 2015, 10:09:12 am
content: I've installed a half-way decent outdoor GPS antenna on my roof and want to design an amplified splitter for it. I'm just a bit out of my depth with some of the RF issues (I should know this stuff better being a ham and all).I'm designing it around the MAX2641. My thought is to use one of those for each output channel, as it has a ~30 dB reverse isolation spec, which would be good. But that implies a 7 dB (this is a four port design) loss followed by a 15 dB gain, which doesn't sound like a great idea to me (doesn't that amount to just raising the noise floor 8 dB?).Ok, then a separate one just for the input? 15 dB gain, followed by 7 dB loss, followed by 15 dB gain again? Now the noise figure is ~3.2 dB (from two of them cascaded) and a total gain of 22 dB. Is that a reasonable design?The other question I have is about the DC portion. Each receiver port needs to present a 200 ohm DC load to convince the receiver there's an active antenna present. On the input side, I want to DC-couple a 5 volt antenna power supply (which will also power all of the amps) from a 7805 and a wall wart. For the DC coupling inductors, is the value of them terribly critical? It seems to me that once you get high enough relative to the frequency of interest that it should just be like there's nothing there at all, right?The big challenge will be designing the striplines and splitter circuitry. It's undoubtedly going to be quite fiddly.

author: KJDS
author_rank: Super Contributor
timestamp: September 05, 2015, 10:32:46 am
content: I'd build three identical boards with the amp followed by a two way splitter. Noise figure should be fine like this.For the splitter, use Sonnet to see if you're in the right ball park.For the inductors, try coilcraft,http://www.coilcraft.com/apps/finder/rffinder.cfmand it should provide a reasonable range of possibilities. No idea what they're like supplying to the US, but for the UK it's easy to buy a few direct.

author: ConKbot
author_rank: Super Contributor
timestamp: September 05, 2015, 07:50:30 pm
content: Well first, look at cascaded noise figures, there are a few online calculators and pages that explain the calculations. I.E.http://www.microwaves101.com/encyclopedias/cascade-analysishttp://www.microwaves101.com/calculators/859-cascade-calculatorThe major factor in system noise figure is the first amp, and I'm assuming your antenna is active, or you have LNA very close to it? If not, thats step 1.Assuming:A 30dB preamp in the antenna (typical) and a noise figure of 1.5,10dB of coax loss (and a corresponding 10dB noise figure, since its a passive element) + -7dB from the splitter (7dB NF), but combine the 2 into a single 17dB loss, 17db NF assembly since they are passive.A 15dB lna after the splitter, 1.5 for the NF again.Punch that into the calculator, or work the math by hand, and you get a noise figure of 1.56 dB. Only 0.06 dB of noise came from the second amp.And to see why the amp being at the antenna is so important, switch the order, put the coax first. -10/10 for the coax, +30/1.5 for an amp thats indoors, and your noise figure is then 11.5, so your CNR just took a huge 11.5 dB hit, meaning that a 'good' 40 dB CNR sat is only going to be 28.5 (on the low side of OK) and a 30 db CNR sat, is going to be a useless 18.5 CNR to the RX.As far as powering it, I'd use a newer, high PSRR LDO for feeding the amplifiers instead of a 7805, but if you though enough capacitance at it, it shouldnt be much of an issue either way.Also, all of this is just assuming youre a glutton for punishment, and want to do this for "fun"Otherwisehttp://www.prostudioconnection.com/Symmetricom-090-58537-01-GPS-1-4-4-Way-Active-Ante-p/361322807013.htmEven DC loads each RX too, like you need. The noise figure is 5dB, but look back at the cascade thing again, and you'll see that since thats not the first amp, thats not that big of a dealDont forget the lightning arrestor and good grounding too.

author: TSL
author_rank: Regular Contributor
timestamp: September 05, 2015, 08:29:21 pm
content: If you're not doing this for the exercise and you just need a splitter, consider picking up one of the many HP or Symmetricom units available on eBayThis Symmetricom one can be had for ~$89UShttp://www.ebay.com/itm/171919985970or HP cheaper...http://www.ebay.com/itm/HP-58516A-GPS-L1-1x4-4-Way-Active-Antenna-Signal-Splitter-Divider-N-Female-/171840402156a quick search of Ebay will show others.

author: nsayer
author_rank: Regular Contributor
timestamp: September 05, 2015, 08:54:36 pm
content: I do have an active antenna - it's. Gilsson marine one I've pole-mounted outside above the roof.I saw the Symmetricom new units, but for that price I could do it myself.You're right about LDO selection. I didn't literally mean a 7805... I have a D2PAK one I use for a lot of projects. I don't remember the number offhand.I don't know why my eBay searches never find what I want. But I dunno, there's something to be said for designing one (and learning more along the way) rather than buying a hand-me-down.

author: ConKbot
author_rank: Super Contributor
timestamp: September 05, 2015, 10:19:38 pm
content: Well as long as youre cable loss is under ~20dB ish (so there is ~10 db of gain left from the first LNA) then you should be pretty good. The 2641 has a separate power pin (cheater), and maxim was kind enough to give you input and output matching for 1575MHz, so get some high Q caps and inductors and go nuts. For matching the amps in and out, a VNA would be a luxury, a SA/TG would be nice, but you could probably do it just looking at your satellite CNRs in your RX software. Though you'd have to deal with CNR changing on its own.As far as adding and removing DC bias, etc, as long as the Z of the coupling cap is low enough and the Z of the feed inductor is high enough, youre right, it wont be too critical to the values. Just mind your parasitics. (ESL of caps, SRF of inductors) If youre feeling lazy about that, mini-circuits wideband chokes cover 50M-10G(!) and are only $2.75 ea if you buy 20. You'd obviously only need 5, but with a frequency range like that, you can use them for just about anything else you want while the microwave bug has bitten you.

author: edavid
author_rank: Super Contributor
timestamp: September 06, 2015, 01:11:19 am
content: Do you actually need the output amps? Seems like overkill.

author: nsayer
author_rank: Regular Contributor
timestamp: September 06, 2015, 03:41:36 am
content: They are there to isolate the receivers from each other. No LO leakage / cross-pollution.

author: nsayer
author_rank: Regular Contributor
timestamp: September 06, 2015, 04:13:33 pm
content: One more question:Given that I do have the output amplifiers, do you think I need to be fussy about how I split the output from the first one? Do I need to actually create a proper stripline network to split the output, or would it be likely to be good enough to just make the shortest possible trace to tie the first output to all the secondary inputs?The board is going to be the smallest possible size that fits all of the components. I'll be using edge-mount SMA-F jacks and an SMT barrel connector for power.

author: KJDS
author_rank: Super Contributor
timestamp: September 06, 2015, 04:17:09 pm
content: If the stage two amps each have 50 ohm inputs, then putting four in parallel is unlikely to please the first amp.

author: nsayer
author_rank: Regular Contributor
timestamp: September 06, 2015, 04:28:30 pm
content: In principle, I have the opportunity to alter the input impedance by changing the coupling cap specified on the data sheet. If each of the four were 200 ohms...?

author: KJDS
author_rank: Super Contributor
timestamp: September 06, 2015, 05:56:51 pm
content: Quote from: nsayer on September 06, 2015, 04:28:30 pmThen you'll probably screw the gain and noise figure, and unless you can get them all within a tenth of a wavelength, you'll screw gain and gain flatness too. 200 ohm microstrip I think requires a negative line width.

author: nsayer
author_rank: Regular Contributor
timestamp: September 06, 2015, 08:05:38 pm
content: Ok, well, that means cascaded Wilkinson dividers - which is fine.