RF output - Xformer or capacitor coupled - Page 1
Forum section: electronics / projects

author: HalFoster
author_rank: Frequent Contributor
timestamp: December 07, 2023, 02:21:03 pm
content: As part of a frequency distribution system: 10 MHz, 50R, all output grounds in common with system ground, output to be used by local equipment - is there any advantage or disadvantage to a transformer coupled output vs a capacitive coupled one?Hal

author: paul@yahrprobert.com
author_rank: Regular Contributor
timestamp: December 07, 2023, 03:32:10 pm
content: DC wise one is floating and the other is ground. Capacitor coupling allows easy biasing of other stages but you don't want a cable that is capacitively coupled at both ends as charge can build up. On the other hand the transformer coupling can draw high currents if a DC voltage is connected.Lastly you could consider the impedances at out of band frequencies. The transformer might look more reflective at harmonics, and you might watch for amplifier stability problems.

author: MarkT
author_rank: Frequent Contributor
timestamp: December 08, 2023, 11:02:01 pm
content: Capacitor is obviously smaller and cheaper. Add 10M resistor in parallel to prevent charge build-up perhaps?

author: TurboTom
author_rank: Super Contributor
timestamp: December 09, 2023, 02:35:57 pm
content: I opted for the transformer coupling when I made adistribution unit for my rubidium oscillatorin order to get rid of any risk for ground loops or other ground-related couplings. It's easy to use ethernet magnetics for this purpose, they are small, cheap and easily available. It may be even possible to use them for external isolating transformers on existing distribution amplifiers or to place them individually at the inputs of instruments to be supplied with a common 10MHz signal. You may just need to design a small interface PCB with BNC/SMA connectors for easy interconnection.

author: jonpaul
author_rank: Super Contributor
timestamp: December 10, 2023, 10:14:46 am
content: We built many such links for master/house clock, digital audio, broadcast.Every one has a transformer for many reasons.CMRRBreak mains ground loopsEMI conducted/radiatedWe use our shielded wideband transformers with BW 20 kHz..150 MHz and ~ 0.3 to 3.0 pF guarded shielded cap.Finally we used MiniCircuits splitters to take a 10.ooo master GPO clock to distribute to 4...10 devices.Enjoy,JonSee our AES and SMPTE papers on digital audio transmission.Jon

author: HalFoster
author_rank: Frequent Contributor
timestamp: December 12, 2023, 07:51:16 pm
content: Thanks all - I went ahead and used Coilcraft 1:1 RF transformers since I already had bought a stick in anticipation. The reasons Jon gave matched my thinking originally but when Googling around I saw that there were a lot of designs that did not use them and I wondered if I was missing something. In truth as it's really only for a local setup under known conditions a capacitor feed would probably have done just fine - but it is something I may pass on to others at some point so erroring on the side of caution seems best. For more details, I am doing all custom boards for a HP 5087A to replace the Brandywine unit I am currently using - a 10MHz input amplifier with a sharp BP filter and two configurable 10 to either 5 or 1 MHz (one of each, also with sharp BP filters) to round out the inputs. Then lightly or unfiltered wideband amplifiers in all the output slots. I currently have 5 of the 5087A units and, of course, not enough cards to fully populate them all. So, besides the fun of making them, the replacement cards will free up enough of the originals to be able to sell the units with a full complement of cards. The card designs and Gerbers themselves I'll probably post here and elsewhere for anyone to use if desired.Hal