Originally posted May 13, 2013
How far do you want to go? Do you feel lucky?
I guess it’s easy for me (someone who works for a manufacturer of test equipment) to simply suggest that you use only cables and accessories designed for the frequencies you’re dealing with. And I have indeed made that suggestion in articles and app notes and presentations. But we’re engineers after all, and so we’re resourceful and inquisitive and budget-sensitive and we can make do.
Indeed we can, and sometimes it’s a rational choice. In this post I’ll explore the choice through an example. Let’s start with insertion loss measurements of a 2.4 mm cable and an SMA cable with appropriate adapters:
Comparing insertion loss measurements of SMA and 2.4mm over 30-50 GHz. The amplitude scale is 0.2 dB/div.
It’s important to keep in mind that this is just one example and therefore your mileage may (will) vary, but the measurement does reveal a few issues related to pushing the SMA cable beyond its normal frequency range.
First, the average insertion loss over 30-50 GHz is actually a little better with the SMA cable than the 2.4 mm. Perhaps this isn’t surprising, given the larger geometry of the cable.
The behavior of the insertion loss, however, is significantly worse for the SMA cable. There is a significant drop or null just below 36 GHz, and many smaller nulls at higher frequencies, especially between 36 and 40 GHz.
What’s happening in the SMA cable at these high frequencies is called “moding” and it involves excitation of the first circular waveguide propagation mode in the coax. To avoid moding the dimensions of the coax must be much smaller than the signal wavelength, and that is not quite the case for the SMA cable. Since some waveguide propagation is happening along with the normal signal transmission in the coax, the effective signal transmission is not consistent or well behaved. You could say that the SMA cable is starting to behave like very poor waveguide.
On the other hand you might observe that the nulls or ripple aren’t that bad, and things look pretty good for SMA below 35 GHz. That’s true in this case, and it might be true for your specific situation too. Your own needs for accuracy and flatness may be modest and performance such as this may cause no problems.
So yes, you can sometimes make do with gear designed for lower frequencies, as long as you know the limitations. But you should be aware of all the limitations and consequences. For example these measurements haven’t shown the phase behavior of the SMA cable, and it is likely to be ill-behaved too. And all these frequency response disturbances will be rather unpredictable and not repeatable. Their frequency and size will be affected significantly by cable routing and movement, and will vary with the specific cables and connectors/adapters used.
Lastly, note the narrow bandwidth or high Q of these nulls. That means that any signal that encounters them will experience amplitude and phase modulation as its frequency varies, even if the frequency variations are small. Unintended modulation is your bonus and you never know exactly where you’ll find it!
By the way, since moding is a consequence of geometry and the geometry of 3.5 mm is generally the same as SMA, you can expect similar behavior from both. The precision of the 3.5 mm hardware should mitigate the repeatability and predictability problems a little, though I wouldn’t rely on it. Just confine your use of SMA to situations where it won’t cause errors large enough to be a problem for you, and reach for the higher frequency connection gear when performance and confidence are important.