House de Kris said:
Now that I know that my adapters probably have a bandwidth of less than 1GHz, pretty much all network analyzer pictures of digital audio cables terminated in at least one RCA connector done by me don't represent the true cable. Sorry about that folks. I'll search for some wide bandwidth controlled impedance RCA adapters, and redo some of these tests if I can find better adapters.
Shortly after posting this, I received a private email with a link to some Monster RCA-BNC adapters that claimed to be 75ohms. The picture in the link appeared to be different than my cheapie adapters, so it would seem possible that they would be better than what I already have. So, I set out at lunch last Friday to find better adapters. Since the Monsters were sold in sets of three, this got me to thinking I should be able to find this stuff at any video shop.
Three stores later I was able to locate some Audioquest RCA-BNC adapters. The box didn't make any mention of it being 75ohms or not (unlike the F-adapter right next to it), so I asked the salesman. The conversation was comical enough to warrent inclusion here:
HdK: Is this adapter 75ohms?
SalesDroid(SD): Yeah, it'll pass a 75ohm signal.
HdK: Are you saying, then, that this adapter is 75ohms?
SD: We use them here on the demo floor, looks good.
HdK: All that aside, what's the impedance of this adapter?
SD: What are you trying to do?
HdK: I'm trying to determine if this is a true 75ohm path through this adapter.
SD: It'll pass a 75ohm signal.
HdK: Does the manufacterer claim this is a 75ohm adapter?
SD: I dunno, but we use them here and they look good.
I kept pushing him for infomation he didn't have, so finally he offers to consult the AQ website. No additional info there. So he calls up the AQ hot line. There is little need to go into a blow-by-blow of the phone call with AQ, but it became evident that the salesdroid learned most of his witty responses from AQ. The AQ rep is being just as evasive as the salesdroid, finally he offers to make a cable for me and terminate it however I want. I had to reiterate that I didn't want a cable, all I wanted was to know what AQ claimed for the characteristic impedance of this adapter. I finally just bought the thing, I could have measured it in the amount of time I wasted there at that store.
Opened the box and was pretty sure I shouldn't have wasted my time. It looks exactly like a gold colored version of the adapter in the previous post. Measured pretty much the same as well. I've included pictures for fun. In AudioQuest_BNCRCA.GIF, the yellow trace is the Audioquest adapter. The two blue traces are in memory and show the adapter path for measurement (explained later). The middle of the screen is 50ohms, and it is pretty evident that this is NOT a 75ohm path inside that adapter. Just for additional fun, I put a comparison picture (AQ-CheapCompare.GIF) between this and the cheapie adapter I had used. In this case, the AQ is blue and the cheapie is yellow. A little different, but not significantly.
Today I searched more for adapters that claim to be 75ohms. Finally found a set of five Monsters. At least they didn't appear (physically) to be photocopies of my cheapie adapters. The shield ring didn't get as close to the center as the AQ or cheapie. Plus, the BNC end looked more like a 75ohm connector than the AQ. This has to do with the amount of teflon in the area between center and shield. Put them on the TDR too and have included a picture of it here called MonsterBNCRCA.GIF. In spite of the claims printed on the packaging "...featuring precision 75-ohm impedance..." this adapter is nowhere close to right. I also picked up some precision 75ohm Monster male RCA to female BNC and added that to the stack and took another picture, MonsterBNCRCABNC.GIF (which is three adapters in a row).
Here's a quick guide for reading these TDR pictures. The first wave I put into memory (blue trace) is with nothing attached to the front of the scope whatsoever. At two divisions left of center, the transmission line (50ohm) ends and the impedance rises to the impedance of air (377ohm). With the addition of an SMA-BNC adapter, I put the next waveform into memory. The pathway is now about 250ps longer and will rise up to air impedance closer to the middle of the screen. Notice how this adapter is able to maintain a nice 50ohm environment from the front of the scope to the end of the adapter, this is a quality adapter. Finally, the live trace (yellow) dips way down, then finally drunkenly heads towards the impedance of air. At least with the Monster adapter, you can see that the impedance rises slightly before it takes a huge dive. This is because they actually attempted to make a 75ohm environment in the BNC section of the adapter. But, the majority of the signal path is in the RCA section and suffers accordingly.
My quest for quality BNC-RCA adapters continues. Not only that, but I think I'll drop Monster a line and see what they say.