RF Attenuator for Oscilloscope - Page 1
Forum section: electronics / rf-microwave

author: brims
author_rank: Contributor
timestamp: December 19, 2018, 10:58:04 pm
content: I recently tested for my Extra license and received it but I want to know more than the ARRL book even tries to cover. I have a cheap 40m CW transceiver kit coming today, I have a small 15w dummy load kit that I soldered together and tested with my HT, a simple RF probe kit I put together, and I own a Fluke 87V, a Rigol DS1052E, and other basic tools. I understand the very basics of electronics but I want to learn more with what I have. My plan for the transceiver is to use the RF probe with my meter and oscilloscope to understand what the different parts of the radio are doing.The transceiver is rated for 5 to 8 watts depending on the power supply I use. Do I need an attenuator in order to hook my Rigol to the output of this transceiver? I had my HT hooked to the dummy load and using the RF probe, I saw about 6VDC on the center pin (the BNC is board mounted, so I can access the pins in circuit). My understanding of a T sampling attenuator is that it's a voltage divider and at 6VDC on the transmitter output that wouldn't harm the oscilloscope. Or is it the act of pumping RF power into the oscilloscope that would be the problem?

author: DaJMasta
author_rank: Super Contributor
timestamp: December 20, 2018, 02:17:32 am
content: If your load is connected, it will terminate the line at 50 Ohms, and then the 1M Ohm input impedance of the scope means only a tiny fraction of the power dumped in the load actually makes it into the scope. Using a 10x probe instead of a 1x gives you more impedance into the scope frontend and likely a fair bit less parasitic capacitance when probing it too, so less likely to detune things. So long as you have the dummy load connected, the relatively low impedance of it means the voltage the scope sees is mostly defined by the load.8W into a 50 ohm dummy load is 39dBm or so, a little under 60Vpp, so your scope should be alright even with a 1x probe.

author: brims
author_rank: Contributor
timestamp: December 20, 2018, 06:49:15 pm
content: Great, thank you for your help.

author: Derbigdog
author_rank: Newbie
timestamp: December 20, 2018, 07:05:44 pm
content: Congratulations on the Extra Class License. I just got mine as well but have been doing radio stuff for 50 years. Rigol sells a 50 ohm feedthrough terminator that is made for hooking your Oscilloscope to a 50 ohm line. You can hook your oscilloscope without it but you will see a signal that is about 2 times the value a 50 ohm load would see.

author: MF-jockey
author_rank: Regular Contributor
timestamp: December 24, 2018, 09:21:26 pm
content: The Rigol feed-through terminator has only about 1 W.https://www.rigol-uk.co.uk/Rigol-ADP0150BNC-Impedance-Adapter-p/adp0150bnc.htmYou will burn it quickly with 10 W!

author: tkamiya
author_rank: Super Contributor
timestamp: December 28, 2018, 02:03:33 pm
content: I believe a proper way to do this is to feed transmit power into a dummy load, and use 10x probe to measure the voltage across the resister.1x may not cause any damage (I wouldn't do this) but impedance is low. It will appear as something being parallel to the output and skew the result. 10x is high impedance. In lower frequency, you can ignore its impact to the circuit. What are you exactly trying to measure?Another way, perhaps better way, to do this is to tap off with a small capacitor.

author: Wolfgang
author_rank: Super Contributor
timestamp: December 28, 2018, 02:27:21 pm
content: Congrats for your license !The proper way is to use a hefty RF attenuator (20dB, better 40dB) and either 50Ohm scope inputs or a 50Ohm terminator.If you want to save some money you may convert a dummy load into a power attenuator, like here:https://electronicprojectsforfun.wordpress.com/rf-module-gallery/the-rf-splitter-and-combiner-gallery/a-dummy-load-converted-to-a-power-attenuator/

author: xyrtek
author_rank: Regular Contributor
timestamp: December 31, 2018, 11:29:54 am
content: Congratulations on the Extra class.Although is not a bad idea to have a good attenuator set around, the proper way to do it has been explained already, see Wolfgang post.Head to Ebay and steal one of those, and while at it search for TRILITHIC LFP-50, make sure is NOS