Signal Generator Source Impedance

Nobody is gentle on Usenet.

There actually is such a generator:

"1-1.25 V into 60 ohms (IF) with attenuation to 100 db in 1 or 10 db steps."

Such generators appeared when the manufacturer wanted to produce a generator that worked reasonably well with both 50 and 75 ohms. Hint: (50 + 75) / 2 = 62.5 ohms My guess(tm) is that users were losing (or stealing) the minimum loss pads required to go from 50 to 75 ohms with one front panel connector:

The mismatch loss from 50 to 75 ohms is only 0.4dB, so you're not likely to see much of an effect from using the "wrong" impedance unless you're working with microwave frequencies.

A much bigger problem is that 50 and 75 ohm BNC connectors use different center pin sizes.

I wrecked the 75 ohm receptacle on a CATV sweep generator by force fitting a 50 ohm BNC plug. Color coding my cables have largely prevent a recurrence. Green tape or nail polish for 50 ohms and violet for 75 ohms.

Can you not just use a bit of 50 ohm microstrip PCB? sever the trace in one place and insert a 10 ohm SMD resistor into the gap. Then place it in series with the output from your 50 ohm generator?

Aha. Saw what you did there, Jeff: resistor colour code; green = 5 But Violet Gives Willingly. ;-)

I thought about it and decided I didn't need to color code the cables beyond the green (5) and violet (7). For 50 ohms, a second stripe would have been black (0), which is invisible when applied to coax cable with a black outer jacket. For 75 ohms, the use of a green second stripe to would guarantee confusion with the 50 ohm cables. I decided that a 2nd color coded stripe was a bad idea.

Mostly, I use vinyl electrical tape, but had some difficulties obtaining violet tape. Shrink tube was better, but more difficult to attach with different diameter coax cables. I finally settled on embalming the tape with transparent packing tape, which also allowed me to attach text to the cable.

I also have small round sticky dots in green or violet for identifying the impedance of the BNC receptacles. Noticing a color mismatch before inserting the BNC plug has prevented a few disasters when I'm working late and half asleep.

In message snipped-for-privacy@4ax.com, Jeff Liebermann snipped-for-privacy@cruzio.com writes

Despite over 40 years in the CATV industry, I've never seen any 50 or 75 ohm BNCs that weren't mechanically mateable. The mating parts of the metalwork are usually pretty-well identical.

OK, with some 50 versions the point of the male pin might be just a little bit blunter, but there's little chance of it splaying the female receptacle. Otherwise, the diameter of the pin is the same. But note that with the 75 ohm the hole at the rear end of the pin and receptacle might be a rather tight fit for the inner of 50 ohm coax.

What makes one connector 50 ohms and the other 75 is that the amount of the PTFE dielectric is considerably cut back in the 75 (making it as air-spaced as possible). IIRC, the 50 is the more constant-impedance, which makes it more-usable to higher frequencies).

On the other hand, the N-connector is a very different beast. The pin of the 50 ohm has a much greater diameter than the 75 (which may be what the photo shows). If you insert a 50 ohm male into a 75 ohm female, you will almost certainly cause irreparable damage to the female (in some companies I believe it's still a hanging offence), while a 75 male into a 50 ohm female won't male contact.

Finally, the inner parts of a female BNC and an N are very similar, and you can usually mate either with a 75 ohm N (but not with a 50, as the pin diameter is too large).

In message snipped-for-privacy@4ax.com, Cursitor Doom snipped-for-privacy@notformail.com writes

Some German equipment had impedances of 60 ohm .

Same danger with SMA and 3.5mm connectors. At first glance they appear the same, but they're not - and mating them can cause *very* expensive damage if it's a high-end RF cable.

That is the way I have seen it in the US. Ok to mate the 50 and 75 ohm BNCs but not the 50 and 75 ohm N connectors.

I do not recall which way it is if you stick a BNC to a N connector. Just that it can be done if the pin size matches up.