Clipping RF power to protect equipment? - Page 1
Forum section: electronics / rf-microwave

author: chrishamel1989
author_rank: Newbie
timestamp: February 21, 2019, 03:37:05 pm
content: Hi all,Is there anything that would allow me to clip a range of RF signals? I need to clip some signals from wifi so I can avoid overloading my spectrum analyser. I know of power limiters but I'm not sure if those just cut out all the signals or do they clip the signal and leave the other emissions alone? My current solution for what I'm doing is using a notch filter for a wide measurement and taking a narrower look with higher attenuation. I am missing some emissions to the noise floor.Thanks for your time,Chris

author: radiolistener
author_rank: Super Contributor
timestamp: February 21, 2019, 04:03:27 pm
content: if you're need to measure high level signal, then you're needs for RF attenuators.If you're worried about burnout of your equipment, because there is possible high power pulses on the input or high voltage static discharges, then you can protect your equipment with limiter diode.You can find ready solutions for such purposes on very high frequency, but they are not cheap.Here is some info about it:http://www.skyworksinc.com/uploads/documents/200480C.pdfI found some on aliexpress, but didn't tested it, so I cannot say if it works good.Here is a link:https://www.aliexpress.com/item/PIN-Diode-RF-Limiter-10M-6GHz-10dBm-20dBm-0dBm-for-amplifier-SDR-short-wave-receiver-Spectrum/32936655735.htmlif you will try buy it, let us know how it worksFor frequencies 1 GHz and below, you can make it yourself just with two high speed low noise diodes connected in anti-parallel.For example HSMS2812.But note, these diode limiters make signal distortions, especially when signal is strong (when voltage is near to diode open level).

author: tkamiya
author_rank: Super Contributor
timestamp: February 23, 2019, 01:13:01 am
content: I use cell phones and wifi right next to my spectrum analyzers. I never ever see any interference. By nature of the device, spectrum analyzers are well shielded. Maybe you have a faulty cable or connectors? SA also have nearly 100db or more of dynamic range. If you are having real issue, notch filter can be of use. You can make one using coax as a stub.

author: DaJMasta
author_rank: Super Contributor
timestamp: February 23, 2019, 08:43:21 am
content: Notch filter, limiting bandwidth, and added attenuation are the usual go-tos because they're really the best option, as mentioned, actual diode protection produces a huge amount of harmonics when the signal is clipped and is generally undesirable for this kind of application.Could the noise be coupled in through another source? Perhaps through the power/signal cables, or maybe if there's a peripheral or something attached? It should be designed to reject that stuff, but maybe if the noise is strong enough and the protection is mediocre or compromised, something could get through.If you're using an antenna, you could try a directional one and avoid aiming at wifi and cell sources, and if you're using a fixture or cabled test setup, your shielding is probably inadequate. It's also worth mentioning that the peak instantaneous RF power of commercial communications devices are basically always below the input specification of an SA's frontend - putting out a maximum of a few watts if you're right next to the antenna and receive it all. Doesn't take a lot of distance before almost any consumer radio is just going to be noise and won't be risking even a fairly sensitive passive antenna.

author: radiolistener
author_rank: Super Contributor
timestamp: February 23, 2019, 01:59:59 pm
content: Quote from: DaJMasta on February 23, 2019, 08:43:21 amlimiting diodes is intended to protect equipment input from random unexpected high pulses.If you want to measure signal with high pulses, then there is only one way - is to use RF attenuators.Yes, it reduce signal level, so the high voltage pulses will be within your equipment working range.Of course you will need for high dynamic range equipment to see all low level signals after attenuator.There is no other way if you want flat frequency response, because any kind of filter will make output level depends on frequency.

author: radiolistener
author_rank: Super Contributor
timestamp: February 23, 2019, 02:05:51 pm
content: Quote from: DaJMasta on February 23, 2019, 08:43:21 amUsual communication transceiver has 50-100 W RF power. This is +50 dBm.Handheld VHF transceivers and cell phone has 5 W RF power. This is +37 dBm.Does SA input really can deal with so high power on the input? (with no damage)Previously I thought that the safe level for such kind of equipment is not more than 10 dBm.

author: ogden
author_rank: Super Contributor
timestamp: February 23, 2019, 02:13:31 pm
content: Quote from: radiolistener on February 23, 2019, 02:05:51 pmIf it is not connected directly to output of transceiver - sure. Connectors are very bad antennas. Note that OP talks aboutemissions, most likely he does not connect SA to signal sources but antenna.

author: radiolistener
author_rank: Super Contributor
timestamp: February 23, 2019, 02:38:29 pm
content: Quote from: ogden on February 23, 2019, 02:13:31 pmIf you're using 3.6 MHz antenna and your neighbor ham transmit on that frequency from 200 meters away with 2 kW transmitter (for DX QSL), you will get for about 63 dBm transmitter power - 29.6 dB free space loss = 33.4 dBm from your antenna.It's almost the same as direct connection to 5 W transmitterAs I know, some guys using even more powerful transmitters (10 kW and even 100 kW). That's not legal for ham radio of course, but possibleAnd there are a lot of commercial transmitters with 10-500 kW (AM broadcast, FM broadcast, TV broadcast, and other)

author: DaJMasta
author_rank: Super Contributor
timestamp: February 23, 2019, 05:55:03 pm
content: Sure, but do you have hams in your area that actually do that? And if you do, you know exactly where to put your notch filter... the point is that even if you have a local source broadcasting, it's very unlikely that you're going to find several sources, across the spectrum, that would be even close to that power level and require more than just a notch filter at one or two frequencies. And the same can be done with an antenna - you use anything handheld size and you're unlikely to see 3.6MHz band at that high a power level, even next to the transmitter.My SA is rated to max 30dB at the input, if you have a handheld one, it's probably rated higher. That's a lot of received power if you're using a passive antenna, so there are relatively few situations where you could damage your system that way - but if you find yourself in one, external attenuators are the easy option, and external filters to get rid of the high power source (since even in this unlikely situation it's unlikely there's more than one) preserve most of your dynamic range while protecting your input.

author: ogden
author_rank: Super Contributor
timestamp: February 23, 2019, 06:31:08 pm
content: Quote from: radiolistener on February 23, 2019, 02:38:29 pmThere are many powerful things around. Low frequency CW generator of 7.965 GW (GigaWatts) net power is in operation. So what? When there's risk of damaging SA input using broadband monitoring antenna, you have to be more concerned about your own health.

author: radiolistener
author_rank: Super Contributor
timestamp: February 23, 2019, 07:07:34 pm
content: Quote from: DaJMasta on February 23, 2019, 05:55:03 pmyeah, that's a lot, for some reason I was thought they can handle max 10 dBm.But there is also another issue with antenna. It can produce high voltage pulses from static electricity (several kilovolts or more for example). Sometimes you can even catch random sparks on disconnected connector of antenna. It happens when static electricity is so high, so it can make dischage through several millimeters of air gap in the connector. Is it safe for SA input? I mean the case when antenna produces one short pulse with 10-100 kV

author: ogden
author_rank: Super Contributor
timestamp: February 23, 2019, 07:17:16 pm
content: Quote from: radiolistener on February 23, 2019, 07:07:34 pmSA inputs are very ESD-unsafe. DC voltage is #1 reason of SA input damage, ESD is #2.http://literature.cdn.keysight.com/litweb/pdf/5989-8791EN.pdf