Protecting GPS L1 input, TVS diode? - Page 1
Forum section: electronics / projects

author: lincoln
author_rank: Regular Contributor
timestamp: May 06, 2015, 08:09:37 pm
content: Hello,I don't know much about RF. I'm using a GPS module for a project that will be for a fixed installation and I want to protect the GPS antenna input form ESD strikes. The input circuit is an SMA connector, DC bias is fed from an inductor to 22pf NPO cap like this:I have added a 22p DC blocking cap between the module and DC bias, the antenna I'm using works best at 4~5 volts. The MFG of the GPS said this was ok.Form my research it looks like I need :- unidirectional protection- Cd < 1pf- operating voltage of 5Vnice to have: package no smaller the 0402,Original I build up a prototype using an Infineon part that was sold through digikey and specifically mentions GPS use. I don't have proper rf analysis tools. The way I tested it was to compare signal / noise data form the GPS module during an a/b compare.Well now the Infineon part doesn't seem to be distributed through the retail channels any more so I am looking for an alternative. Littelfuse has a number of parts that look like they should work.-http://www.littelfuse.com/~/media/electronics/datasheets/tvs_diode_arrays/littelfuse_tvs_diode_array_sp3031_datasheet.pdf.pdf-http://www.littelfuse.com/~/media/electronics/datasheets/tvs_diode_arrays/littelfuse_tvs_diode_array_sp3030_datasheet.pdf.pdfWhat do you think? Alternative parts?Thank you,Link

author: kjs
author_rank: Regular Contributor
timestamp: May 06, 2015, 08:13:24 pm
content: There's almost no way to protect the antenna input besides what the IC manufacturer already did. TVS diodes which would help have that much capacitance that the RF signal will be shorted out and the very low C ones are more for psychological feelings.What you can do (but most likely already done in the module) is an inductor from antenna input to ground. Has to be high impedance in the GHz range and not have series resonance below 2GHz. Naturally that has to be isolated from the 5V antenna feed point with a capacitor (between pin18 and your 22pF to GND).Inferno decided that the ideal business model is to sell several billions of parts all at a hefty loss to only 50 customers and not another billion parts to 500 customers at a very significant profit. That's why they never had a positive ebit after they were spun off without any debt from Siemens (semiconductor group).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 06, 2015, 11:14:12 pm
content: On the upside, there shouldn't be much energy in ESD at >1GHz.Is it not possible to have the antenna elements enclosed in plastic (in whatever manner -- enclosure, overcoat, PCB planar, etc.)? Or is the cable an ESD concern? (Coax cables almost always mate shield-first, so that shouldn't be a problem?)Tim

author: kjs
author_rank: Regular Contributor
timestamp: May 07, 2015, 12:58:55 am
content: We have several designs with GPS modules in production and never had an ESD issue with them. They are ublox modules so your mileage may vary. The only part I would worry about is your 22pF capacitor which may go South with ESD.What helps you too is that most GPS antennas have the "radiator" connected to ground too (naturally high impedance for the RF).Lightning is a different story for externally mounted antennas and receivers and a TVS diode won't help there. That's why our high end models for outdoor use all transfer via fibre-optics. Replacing the receiver is cheap compared to replacing the equipment which is hooked up inside......

author: lincoln
author_rank: Regular Contributor
timestamp: May 07, 2015, 07:58:18 pm
content: Hello,Thank you for your input, looks like it was a bad idea.