RF switch that can pass DC (or something completely different?) - Page 1
Forum section: electronics / projects

author: wergor
author_rank: Contributor
timestamp: October 05, 2022, 04:04:57 pm
content: in my current project I need a precision 1PPS signal. the 1PPS signal may either come from an external source (TTL or CMOS level) or from an internal GNSS receiver module. since only either one or the other source will be used, I'd like to use one SMA connector for both:my problem is that I'm unable to find an RF switch that supports switching TTL / CMOS level DC signals, they usually require dc blocking caps at the RF ports. does anyone know a part that could work in this application, or maybe an entirely different solution (mayberelays)?

author: David Hess
author_rank: Super Contributor
timestamp: October 05, 2022, 04:32:55 pm
content: A relay will work, but is switching required at all? Something like a bias-t can separate the RF from the low frequency DC signal.

author: Gribo
author_rank: Frequent Contributor
timestamp: October 05, 2022, 06:12:07 pm
content: There are RF switches that can pass DC, but why do you need an RF switch? The 1PPS signal is a logic level signal. Mixing a GPS antenna trace and a digital trace will probably not work well.

author: wergor
author_rank: Contributor
timestamp: October 05, 2022, 11:26:13 pm
content: Quote from: David Hess on October 05, 2022, 04:32:55 pma diplexer might work as well.the bias-t is software controlled, so it does not necessarily be switched by the RF switch. logic high / low should still get through unaltered, if possibleQuote from: Gribo on October 05, 2022, 06:12:07 pmdo you have a model number? the ADG918 family seem to be able to pass DC, but operate up to max 2.75V only, which is suboptimal for my application.Quote from: Gribo on October 05, 2022, 06:12:07 pmboth are 50ohm traces (1PPS input is 50ohm terminated as well) but the line will never carry a logic level signal and a GNSS signal at the same time.

author: Gribo
author_rank: Frequent Contributor
timestamp: October 06, 2022, 05:49:28 pm
content: The GPS RF signal is -100dBm under best case conditions, you don't want anything on that trace except the antenna, an LNA and the GPS module. If you work only with logic level signals, you don't need an RF switch. Let the module deal with that signal.RF switch that's rated for DC (Not really recommended by the mfg):https://cdn.macom.com/datasheets/MASWSS0179.pdf

author: wergor
author_rank: Contributor
timestamp: October 06, 2022, 07:18:41 pm
content: Quote from: Gribo on October 06, 2022, 05:49:28 pmlike this?U508 is a SAW filter with a 1575 - 1605 MHz pass band. just need to figure out how to terminate the U501 input without messing up the termination in the SAW pass band.Quote from: Gribo on October 06, 2022, 05:49:28 pmI've seen that before, but it needs control voltages below 0 for operation without AC coupling. I'd like to avoid that additional complexity, if possible.edit: fixed image

author: Gribo
author_rank: Frequent Contributor
timestamp: October 06, 2022, 07:43:17 pm
content: You insist on doing this the wrong way and complicate your design for the sake of one SMA connector.Your GPS module has a 1PPS output - It uses the GPS module's logic levels (3.3V?) - this output doesn't require an RF switch. If you want to support both 5V and 3.3V there are buffers that can do the level conversion.Any noise on that 3.3V rail will couple to your GPS RF input, the PSRR of U501 is not infinite. Good luck.

author: wergor
author_rank: Contributor
timestamp: October 06, 2022, 09:44:24 pm
content: Quote from: Gribo on October 06, 2022, 07:43:17 pmI created this thread to find out if this idea was feasible (or sensible)Quote from: Gribo on October 06, 2022, 07:43:17 pmgood point.