RF Sampler Question - Page 1
Forum section: electronics / rf-microwave

author: vk4ffab
author_rank: Frequent Contributor
timestamp: November 25, 2019, 10:12:23 pm
content: Ok there are these rf samplers on ebay, apparently rated to 50w. The voltage divider i can work out the values for, but what would be the value of the in and out series resistors and what would their rating need to be to achieve 50w pass through?

author: TheMG
author_rank: Frequent Contributor
timestamp: November 26, 2019, 01:27:29 am
content: Looks sort of like a T-pad attenuator with a sample port added.Odd arrangement for an RF sampler but I guess it makes sense if the coupling factor is only a few dB in order to maintain the impedance as close as possible to 50 ohms.With most RF samplers such as would be used to connect a spectrum analyzer to the output of a transmitter, the coupling factor would be 30dB or more in which case the sample port has so little effect on the input and output impedance the series resistors are normally omitted entirely.Can you provide a link to these samplers you speak of? What is your intended use and frequency range anyways? Do you require this sampler to have a flat frequency response?

author: vk4ffab
author_rank: Frequent Contributor
timestamp: November 26, 2019, 01:42:54 am
content: https://www.ebay.com.au/itm/NEW-DC-2GHz-40dB-ultra-wideband-RF-sampler-SMA-interface/283235697566?hash=item41f229c39e:g:F-IAAOSw48Vb2BlpI am only really interested in in HF, as long as its flat to 30Mhz im happy, if its flat into the 2m band, winner winner, beyond that my test gear becomes kind of useless. If its just a resistor divider those resistors dissipate bugger power and so i was wondering where they get their 50W rating, as the pass though trace should be large enough to take much more than that.

author: TheMG
author_rank: Frequent Contributor
timestamp: November 26, 2019, 02:08:48 am
content: Being a resistive sampler it should be flat from DC to a fairly high frequency. Its high frequency limit would be dependent on how much attention to detail has been put into controlling the impedance characteristics of the resistors and traces but I would expect 144MHz should be easily achieved even with a relatively crude design.As for the diagram it might just be a standard diagram they use across a range of different samplers, and in this case the values of those two series resistors could very well be "zero". Guess you'd have to take one apart to find out.A very common type of sampler used for LF through to HF frequencies is a current sampler, such as this:http://www.collinsradio.org/wp-content/uploads/2015/05/Build-a-Quality-RF-Power-Sampler-Jackson.pdfI built one a few years ago from scrap parts I had lying around and it works quite well up to quite high power levels.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: November 26, 2019, 04:06:19 am
content: Yeah I am starting to think that the series elements are just transmission line and not actual resistors.I found this online, I think i will just copy it like all good professionals LOL

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: December 06, 2019, 05:15:39 am
content: Quote from: vk4ffab on November 26, 2019, 04:06:19 amThat's mine. I'll send you dimensional drawings if you like. The copper one with Teflon dielectric is better though.

author: Paul Ed
author_rank: Contributor
timestamp: December 24, 2019, 07:38:39 am
content: Nice thread, very helpful for folks like me to 'get back into' such stuff, especially when circuits and builds are linked and great articles:Quote from: CJay on December 06, 2019, 01:22:41 pm...are referenced, thank you!

author: Kartika
author_rank: Regular Contributor
timestamp: January 30, 2023, 12:25:05 am
content: Hi Sir Melt-O-Tronic I m interested for drawing

author: David Hess
author_rank: Super Contributor
timestamp: January 30, 2023, 09:34:58 pm
content: The series resistor values are calculated to keep the input and output impedance at 50 ohms with the high impedance divider in place. In most practical applications, they can just be made zero ohms.For wide bandwidth, the resistive divider needs to be capacitively compensated, but this may be done with the distributed capacitance of the layout. Attenuating oscilloscope probes work in exactly the same way.In RF applications where operation down to DC is not required, the resistive divider might be replaced with a capacitive divider instead.

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: February 08, 2023, 04:11:26 am
content: Kartika, please accept my apology for taking so long to reply.Attached is a PDF file with the dimensions for versions 2, 3 and 4 of my power tap.I couldn't get a PDF to fit within the upload limit, so I posted .jpg instead. Sorry to do that to you.I'll also have to apologize in advance that the dimensions are in inches and that the drawings don't follow any engineering standard. They're laid out this way to make it easy for me when I do the machine work (manual mill).A couple of important notes about each version:Version 2(the one pictured in the post above) was made of brass. The interior is sized for air dielectric. It's performance is OK up to maybe 450 MHz. There are six 1.65K SMD resistors in 2 layers from the thru line down to the sample port. They are a pain to assemble.Version 3was made of copper. Brass would be fine and easier to machine. Its interior cavity is sized for Teflon/PTFE dielectric. I buy the thinnest PTFE sheet I can and very carefully cut layers that stack up to fill the space. It also uses the same 2 layer SMD resistor chain. Its performance is quite a bit better, but I can't objectively say by how much. I believe it's good up to around 1 GHz, more or less.Version 4is made of brass, but I have not finished building it yet due to some machine trouble and other priorities. It should be the best of the three. In this version, the thru line becomes an air-dielectric 50 ohm coaxial transmission line and the sample chain cavity is a Teflon/PTFE dielectric (effectively a buried microstrip). It also uses a much simpler 1-layer resistor chain made of four 619 ohm 2512 SMD resistors. The bottom- left-hand corner of page 3 shows how to machine a piece of flat aluminum to use as a jig for soldering the resistors so they're almost perfectly straight and flat. I used 16 ga magnet wire in the side channels as fingers to hold the resistors down during soldering. I also used a ball end mill to open up the area under the junctions so the aluminum doesn't wick away the solder heat.More specifics on v4:Notice that the thru line is a 50 ohm coaxial transmission line with an air dielectric. The tap intersects it in the form of a buried microstrip transmission line through a rectangular aperture. Ignore the color difference. I was experimenting with nickel plating:The center conductor between the two N connectors has 2 little apertures; one to accept the very tip of the resistor chain and the other for the 22 ga PVC insulated wire that acts as a gimmic capacitor. The important part here is that its outside diameter is about 0.156 inches.Here's a general idea of how it's constructed. The two halves are unequal thickness, so you don't have to machine a matching channel in the top half:And here are pictures of the resistor soldering fixture since it's not well described in the PDF:I would like to believe that v4 is good to above 2.5 GHz, but the truth is that once I finish it, I still won't have the ability to measure it that high.

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: February 08, 2023, 04:42:14 am
content: Here is the v4 drawing. The board wouldn't let me upload it to the previous post.

author: Kartika
author_rank: Regular Contributor
timestamp: February 16, 2023, 12:35:02 pm
content: Hi Thank youwhy you not use SMA instead BNC22ptfe?https://www.audiophonics.fr/en/mono-conductor/neotech-sost-22-wiring-cable-up-occ-ptfe-22awg-p-10649.htmlbrass is all two or only one part'

author: coppercone2
author_rank: Super Contributor
timestamp: February 16, 2023, 12:55:09 pm
content: because you turn it 1/4 by hand rather then worry about a wrench and multiple turns!

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 16, 2023, 02:37:01 pm
content: This is interesting. Here is my story about something similar.I bought a big military type 50 ohm load for a few bucks at a hamfest, It had a N connector and then an unusual connector, possibly a Navy Type Gs 491859, looks like a short PL 259. This is in a "T" type connector although the ends of the "T" are the copper tube welded to the input and load.I thought I would take it apart and see what was inside. A bunch of black wire around a slug came out and did not look good at all. OOPS!It was in contact with a copper tube inside another copper tube similar to the construction of the power take off that was nicely documented here.I kludged together a coil on a form with a slug, approximating the length of the blackened wire that I removed. I connected this to a SMA connector secured to the outer tube.I am able to get a flat neg 40 db response to 300 MHz. The reading climbs up after this.I think I would like to try the resistor network described here, since I already have the physical copper tubes and threaded connectors. Any comments?Wally KC9INK

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: February 16, 2023, 03:44:08 pm
content: Kartika, feel free to adapt the design for any connector of your choice. I do like the scalloped center pin on this type of BNC which receives the end of the resistor chain nicely.Personally, I'm not crazy about SMA's because they take much longer to connect/disconnect (and they're ubiquitous, so that multiplies time wasted) and also the original SMA design is only rated to 500 mating cycles. I wish that all the SMA's in my lab were SMB instead, just for the time factor.Wallace Gasiewicz, building things is fun! You should definitely do it and post your adventure!

author: AndyKeir
author_rank: Newbie
timestamp: February 21, 2023, 03:53:34 am
content: When it comes to RF samplers for HF for use in things like looking at IMD on a spectrum analyser or the envelope on an oscilloscope, it's worth noting that the sampler/sniffer does NOT need to have a flat frequency response. You will only be looking at a spot frequency and relative values such as the level of IMD in relation to PEP or one or other of the two tones in a two tone test.Indeed, many samplers such as that built into the Bird 4431 Wattmeter are adjustable. You simply adjust the tap to give you sufficient level to drive your oscilloscope or spectrum analyser.The only time you need a sampler with a flat frequency response is if you're looking at harmonics.With regard to the input and output resistances you refer to in the question, these would be impedance, not resistance.It would normally be something like 50 Ohm stripline.

author: TimFox
author_rank: Super Contributor
timestamp: February 21, 2023, 04:02:27 am
content: One thing I found handy when using kilowatt-level RF power (below 100 MHz in my case) was a high-power-rating attenuator (from Bird).For use as a dummy load, one only needed to put a suitable termination on the low-power end of the attenuator, but if you wanted to look at the signal the low-power end could go to your oscilloscope or spectrum analyzer (possibly with another lower-power attenuator depending on attenuation and power ratings).

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 21, 2023, 08:26:49 pm
content: A basic quick and dirty way of observing an output on a transmitter is to just put about 6 turns of wire around the OUTSIDE of the coax feedline that goes to antenna or load.Apply some tape to keep it in place and connect to your scope, SA or freq counter. Works with all sorts of transmitters from a few watts up.Not calibrated but give you freq and spurs and freq.