EMRFD Message Archive 12177

Message Date From Subject
12177 2016-01-07 04:56:01 k1rf_digital_stev... RF Interconnects
Hi group! I am slowly working on my high performance general coverage receiver and have a basic question on RF connections between functional building blocks.  Each building block is internally well shielded to prevent rf feedthrough, primarily around front end filters, and externally well shielded to prevent undesired pickup of strong oscillator signals.  My question is: What are your thoughts on low cost but reliable RF connectors for interconnects between the functional building blocks.  BNC's are inexpensive but rather large.  SMAs are nice but not inexpensive.  Even the old RCAs might be a possibility.  The highest frequency I need to worry about is about a 100 MHz square wave (which means I need to accommodate about 500 MHz if I want to pass the first 5 harmonics or so of the fundamental. I envision some connectors being mounted directly to PCBs while others would be mounted to the enclosure with a short run of coax to the PCB. Or should I forget connectors altogether and just solder RG-174U coax directly between building blocks?  This is low cost but does not allow easy swap-in of different modules if I want to do that in the future. Most of the building blocks will be housed in shielded boxes made from from PCB material with feed-through capacitors for power supplies.

"Digital Steve", K1RF

12178 2016-01-07 05:52:40 ve3ghm Re: RF Interconnects

If it were me, I think I would use the SMA connectors.

yes, some can get very expensive but the typical "cheap and cheerful"
variety that are very common from the numerous Asian eBay stores are
quite reasonable in price and quite useful through at least several
hundred MHz. I use a lot of SMA's just for this purpose and only go to
the much more expensive types when approaching the GHz range; in other
words I use what is appropriate for the task at hand and the expensive
types are not always needed. Available for PCB edge mount, PCB vertical
mounting, chassis mount, (and so on).

Generally, I find the inexpensive import SMA's to be less expensive all
round than BNC types.

There are also MCX connectors which are a smaller than SMA but generally
not a whole lot less expensive and generally less common.

There is also the U.fl type connectors you will find on many GPS modules.
These are really tiny and use RG188 (I think that's the number) coax.
Not so tiny that they can't be easily handled but like SMD parts do
require care in use.

cheers, Graham ve3gtc

12180 2016-01-07 12:21:07 rodwall1234 Re: RF Interconnects

Hi Steve,

Mechanical devices are not as reliable as normal low signal circuits. Connectors are a mechanical device. How often are you going to be changing your system? It dosen't take long to change solder connections. For reliability i would solder your connections.

This is the same with IC sockets. If I must use a IC socket. Then I always use gasless machined pin sockets. Gasless because there is no air between the IC pin and the socket pin. To allow the pin to oxide and open circuit. The pressed tin connection type of IC socket are not gasless and are not reliable. The most reliable IC connections is to solder them directly into the PCB.

That's my two bobs worth,

Roderick Wall, vk3yc.

12182 2016-01-07 20:05:09 Bill Carver Re: RF Interconnects
I make modular things, Stever. Used to use RCA, made some RCA-to-BNC
patch cables to connect them to signal generators, external amps, etc.
I've switched to SMA. They were available at swapmeets, now on eBay.
I've lifted a lot with small hard line coax on them from CATV
breadboards I got at a swapmeet. With a ferrite bead slipped over the OD
that makes a pretty RF clean connection to the next module.

I'm not sure of the reliablity of the Chinese-made connectors on eBay,
but the ex-CATV, and ex-HP instrument connectors and cables I've been
using haven't given me any trouble yet. But the huge advantage is being
able to test individual assemblies........if pull/retest if things stop
working, or you're backtracking trying to find where a problem is coming
from. SMA are good for a few hundred uses which is enough for most projects.

Bill - W7AAZ
12183 2016-01-08 08:42:51 winston376 Re: RF Interconnects
I built something similar several years ago.  Started with type F double shield 75 ohm coax.  Then BNC's.  And now settled on SMA's.  Although more pricey,  SMA's offer excellent frequency response and ease of use and flexability for the interconnecting between individual shielded modules.  Recommend  SMA's for your application.

12184 2016-01-08 09:27:53 k1rf_digital_stev... Re: RF Interconnects
Thanks all your responses all.  I will use SMAs for my application.  I was pleasantly surprised at their reasonable cost and types on ebay.  The "low end" ones are fine for my needs. I may, in some cases, use 0.1 inch center 3-pin female headers on some PCBs with male headers terminated in RG-174 and then to an SMA on the enclosure wall. The headers would use two outer pins connected to RG-174 shield and the center lead of the RG-174 to the center header pin.

"Digital Steve", K1RF.
12185 2016-01-08 15:59:39 kerrypwr Re: RF Interconnects
This topic has drawn my attention to how cheap SMAs are on ebay; thanks Steve.

I was very attracted to the cheap ready-made RG-316 cables with a plug on each end; cut in half, add a plug and you have two cheap interconnects.

They may not be space-station quality but should be fine for our purposes.

I think I will order a few.

Kerry VK2TIL.
12186 2016-01-08 16:44:01 Gene W5DOR Re: RF Interconnects

I bought half a dozen  6" cable assemblies on ebay with SMA on the ends and a couple more with sma on one end and UHF on the other.  They use teflon RG-316 (teflon version of RG-174) and they've worked out great.  if ya want to make your own,   one suggestion would be to buy RG-316 and SMA connectors off eBay. 


I use the teflon coax cuz at my old age my shaky hands with the soldering iron tend to burn the plastic insulation on the RG.174.


Just some suggestions.


Gene, W5DOR





12187 2016-01-08 19:16:47 David J Nushardt Re: RF Interconnects
I'm building a W7ZOI Spectrum analyzer with one hand, because of a stroke, I did the same 6" right angle SMA's Had 3 made in China the rest I am planning to to my self, but havent been able to find a RG 316 stripping tool , just ordered the crimping tool. Chinese SMA'S are about a buck each in small quantity 10 or so. How can you go wrong.


Sent from my Kindle fire HDX 8.9
12188 2016-01-09 11:02:12 winston376 Re: RF Interconnects
Agree....use the teflon version of coax.  Just built up several SMA cables using the RG-174 coax and merely soldering the inner pins to the center conductor makes the insulation swell up so that you can't push the pin portion into the connector body.  Work around is to use an xacto to trim the swelled insulation which is a damage risk.

Also, try to get a high specification level of  "braided shield coverage"... 90 % is the minimum. If you can get 95%, that's about as good as you can get.

12189 2016-01-09 18:47:35 kx4om Re: RF Interconnects
I bought 6 meters of the RG-316 from China a couple of months ago. Recently I bought a 6 foot piece with one male and one female SMA already attached. Pretty convenient that way. Cut what you need, and attach whatever connector to the other end for adapter/patch cables like SMA to BNC. Half the work involved that way.

Ted, KX4OM
12194 2016-01-17 09:23:35 peter_dl8ov Re: RF Interconnects
"Also, try to get a high specification level of  "braided shield coverage"... 90 % is the minimum. If you can get 95%, that's about as good as you can get."

Then wrap your completed lead in copper foil and ground it.

Peter DL8OV