Wednesday, July 22, 2020

Experiments with RF cables

Introduction


Over the years I've grown to have quite the collection of RF cables in my lab, some with better datasheets than others. But how good were they really? I decided to fire up the VNA and collect some data to see how a few of them really performed.

Experimental setup:
  • Data was collected on a Pico Technology PicoVNA 106 with current traceable calibration, at a controlled 21C ambient temperature with 45% RH.
  • SOLT user calibration was performed immediately before data collection.
  • All SMA connections were torqued to 5 lbf-in.
  • SMA female-female couplers were used between the VNA cables and the DUT cables. These were not de-embedded as I don't have a VNA cal kit with SMA male terminations.
All cables were SMA male at both ends. If available, I tested two identical cables of each type to see how well matched they were.

4001 S-parameter points were collected at even intervals from 10 MHz to 6 GHz. I used Sonnet's S-parameter viewer for analysis of the collected data,

Cables tested (all SMA male-male)

Insertion loss

Results were mostly as expected - longer cables had higher loss, and higher quality cables had lower. I was a little disappointed to find that the Crystek semi-rigid cable didn't outperform the Mini-Circuits, though.

S21 of cables under test

After normalizing to cable length, results were:
  • Mini-Circuits .086: 0.71 dB/foot
  • Crystek Microwave .086: 0.76 dB/foot
  • CD International RG-188: 0.78 dB/foot
  • CD International RG-174: 1 dB/foot

Return Loss

Better quality cable assemblies definitely won here.

S11 of cables under test
It's a bit hard to see since the graph is so busy, but the CD International cables (red, blue, black) had consistently higher return loss than the Crystek (pink, cyan) and Mini-circuits (green).

The CD International RG188 cable got as high as -17 dB S11 at 4.77 GHz. The worst Crystek cable hit -25.1 dB at 2.63 GHz, and the worst Mini-Circuits cable hit -25.4 dB at 1.51 GHz.

Propagation Velocity

Nothing too surprising here.

Group delay of cables under test
Everything was pretty flat, and longer cables had longer delay. Calculated propagation velocities (assuming all cables were exactly the nominal length):
  • Crystek .086: 1.68 ns/foot (0.61 C)
  • Mini-Circuits .086: 1.53 ns/foot (0.66 C)
  • CD International RG-188: 1.49 ns/foot (0.68 C)
  • CD International RG-174: 1.60 ns/foot (0.64 C)

Phase Matching

For the cables I tested two of, I zoomed in to compare how tightly the propagation delays were matched.

Most of these measurements are a bit noisy because I'm pushing limits of phase resolution on my VNA.

CD International, 3 foot RG-174. Looks like one is about 50ps shorter?
CD International, 3-foot RG188. Can't see any skew at all.
Crystek Microwave 1-foot .086. Maybe 50ps of skew?
Mini-Circuits 2-foot flex .086. No observable skew.

Conclusions

There's a few takeaways from this little experiment.

First, at the speeds I currently work at, there appears to be no need to worry about buying expensive phase-matched cables. While different types of coax had significant skew between equal-length cable assemblies, skew between two units of the same SKU ranged from "at the edge of my ability to measure" to entirely undetectable.

Given that my oscilloscopes sample at 20 Gsps when not doing channel interleaving, the maximum skew between any of the identical cables tested would result in a single sample of phase error. This shouldn't be enough to cause problems with my measurements.

Second, all of my cables have non-negligible loss. Even the high-quality Mini-Circuits cables have 0.8 dB of loss at the 2 GHz bandwidth limit of my current flagship scope, and 1.1 dB at the 4 GHz bandwidth limit of the new one I have on the way. I'm definitely going to start thinking more seriously about de-embedding cables from my measurements moving forward.

6 comments:

  1. In your section on return loss I think you mixed up the sentences when you were writing it. As it stands you have higher return loss (~ -20dB) as the better thing and lower return loss (~ -30 dB) as a worse thing. The lower the return loss the better.

    So the cheap cables, assuming you labeled the colors correctly, outperform the more expensive ones. But since they're all below -20 dB across the board it's a minor difference.

    ReplyDelete
  2. Pretty sure I had it right:
    * The CD cables are definitely worse (red/blue/black)
    * Crystek (cyan/pink) and and Mini-Circuits (green) cables seem pretty tied, which is better depends on frequency

    ReplyDelete
  3. One of the things I really would consider testing is phase stability with respect to bending. You really want to check this when looking at the phase and not group delay.

    It's really what sets the cheap general-purpose cables apart from the high-end measurement stuff, and is vital when looking at calibration of VNAs (because shift in phase response will also rotate your load around the smith chart, so if you have any mismatch in your cal network, it will reflect on accuracy)

    ReplyDelete
    Replies
    1. Most of these cables are not intended to be phase stable so I expect they'd perform badly. The only cables in my lab marked as phase stable are the N-to-SMA cables that I bought for my VNA.

      I was mostly interested in evaluating these for connecting test fixtures to oscilloscopes or signal generators, in which case absolute phase doesn't matter a ton for single-ended lines. For differential lines skew matters, but so far these cables seem fairly well matched at single digit GHz frequencies.

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