Unkown RF adapter  - Page 1
Forum section: electronics / rf-microwave

author: Scrachi
author_rank: Contributor
timestamp: July 09, 2022, 09:55:13 am
content: Hi everyone,Some weeks ago I've bought on a ham convention a frequency meter Sabtronic 8610B, the seller in addition gave me as a gift an unknown RF adapter as you can see attachedYou can also find attached the deducted schematic :It seems to be an impedance adapter from 1M ohm to 50 ohm, but I don't see any inductor in it so, I'm not sure about it.Anyone has an idea about what adapter it is for?Thanks for your help.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: July 09, 2022, 09:58:54 am
content: It's an RF detector diode. Outputs a DC voltage proportional to the peak-to-peak input voltage.

author: Scrachi
author_rank: Contributor
timestamp: July 09, 2022, 10:08:50 am
content: Hi,Thanks for your help, seems clear. I will now investigate on how I can use it.Cheers

author: RoV
author_rank: Regular Contributor
timestamp: July 09, 2022, 01:43:37 pm
content: I would expect the two resistors swapped: 50 ohm on the RF side, 1 Meg on the rectified DC side.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 09, 2022, 05:48:09 pm
content: Quote from: RoV on July 09, 2022, 01:43:37 pmI thought that too, but the electrolytic capacitor has to be on the DC side, i.e. the output side.

author: Scrachi
author_rank: Contributor
timestamp: July 09, 2022, 06:03:27 pm
content: Interesting, I think the device is an homemade one, I will try to swap the resistors. The electrolytic capacitor is "after" the diode so I guess it can be considered on the DC side?Thanks

author: edpalmer42
author_rank: Super Contributor
timestamp: July 09, 2022, 06:59:18 pm
content: Quote from: Scrachi on July 09, 2022, 06:03:27 pmThere's no need to swap the resistors.Thinking about it some more, the 1M side is the input and the 50 ohm side is the output. The adapter is intended to bridge across the measurement point rather than terminate it. You'll have to use some form of splitter to make the connection. For lower frequencies, i.e. tens of MegaHertz, you can use a BNC 'T' connector. For higher frequencies, use a proper RF splitter followed by a 50 ohm feedthru terminator, followed by this unit.Remember that the capacitor will charge to the peak voltage, so you have to scale that back to get RMS to calculate the power. Of course, if you're using this to tune for the peak value, that's unnecessary.Ed

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 10, 2022, 12:46:23 am
content: It looks like the 1M is on the same side as the capacitors while the 51 ohms is on the side with the diodes.I would suggest removing the electrolytic capacitor altogether, 1M and 1uF gives 0.16Hz of demodulation bandwidth which is really slow.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 10, 2022, 01:09:18 am
content: We can't even be sure that this thing works as intended. It could be nothing more than a failed experiment.There isn't any seperation between the 1M side and the 51 ohm resistor. It will significantly load down the input. It would make more sense to have a resistor in series with the input to reduce the loading.NiHaoMike is correct that the electrolytic capacitor looks odd. Unless this unit was intended for use at very low frequencies.Ed

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: July 10, 2022, 01:35:20 am
content: The resistor placement is correct, the 1M resistor is to offer a discharge path for static buildup. The 51 ohm load resistor is a bit low, typical load resistors are more like 200 ohm or so. HP AN923 has info:http://www.hp.woodshot.com/hprfhelp/4_downld/lit/diodelit/an923.pdfhttps://www.infineon.com/dgdl/Infineon-AN_1807_PL32_1808_132434_RF%20and%20microwave%20power%20detection%20-AN-v01_00-EN.pdf?fileId=5546d46265f064ff0166440727be1055The use of such a large capacitor on the output does seem odd, though if it's to monitor the output power of a CW source, it's probably just fine.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 10, 2022, 05:10:00 am
content: Quote from: radar_macgyver on July 10, 2022, 01:35:20 amOr any source, where all that is require is an indication of whether there is RF at the monitoring point.Changes with modulation would be unwanted for such service.

author: Scrachi
author_rank: Contributor
timestamp: July 10, 2022, 10:13:06 am
content: Thanks all for these information, I will experiment and observe the comportment of the device between an signal generator and a oscilloscope to check what's append varying the frequency and/or the peak to peak voltage.Thanks