This is one of the most confusing measurement issues I’ve seen in the lab —
not because it’s rare, but because it looks so reasonable when it happens.
You may have run into something like this:
- Same spectrum analyzer
- Same signal source
- Same settings
- The only change: moving the cable to a different port
Suddenly, the measured power is off by 5–6 dB.
What makes it worse is that the number doesn’t jump around.
It’s stable. Repeatable. Hard to ignore.
The first instinct is usually to blame the instrument
When this happens, most engineers start asking:
- Is this port defective?
- Is the front-end attenuation path different?
- Does the instrument need recalibration?
Those are reasonable questions.
But in many real cases I’ve seen, nothing is actually broken.
What changed is much easier to overlook:
the physical path the signal takes into the measurement system.
Ports are not truly equivalent — even on the same instrument
On multi-port instruments, it’s tempting to assume:
“Same model, same settings — the result should be the same.”
In reality:
- Internal routing can differ between ports
- Protection and switching structures may not be identical
- Attenuators or DC blocks may be arranged differently
These differences are often abstracted away in datasheets,
but power measurements are sensitive enough to expose them.
Cable condition matters more than we like to admit
During troubleshooting, I often hear:
“It’s the same cable.”
But what actually changed could be:
- Cable routing or posture
- Bend radius
- Mechanical stress near the connector
- Contact condition at the mating surface
At GHz frequencies, none of this has to trigger a VSWR alarm
to still shift power readings by several dB.
Adapters quietly amplify path uncertainty
Temporary adapters are common during bring-up and debugging.
The real issue isn’t that an adapter was added —
it’s that you’ve started treating a temporary path as a stable reference.
Once an extra interface is introduced, repeatability has already changed,
even if the effect isn’t immediately obvious.
Why this problem is especially misleading
Because it has three dangerous characteristics:
- The numbers are stable
- The settings are identical
- The issue only appears when the port or connection changes
That combination makes it easy to conclude:
“If it’s stable, it must be correct.”
But in measurements, stability only means repeatability — not correctness.
How I approach this in the lab
When I see power shift just by moving to another port,
I don’t dive into hardware right away.
I start with a few basic but effective steps:
Lock down a reference path
Same port, same cable, same adapters.Change only one variable at a time
Today: port. Tomorrow: cable.Track deltas, not absolute numbers
Focus on how much A differs from B.Avoid judging DUT performance until the path is verified
Once this is done, most “mysterious” power discrepancies
become much easier to explain.
A commonly overlooked mindset
People often ask:
“Shouldn’t we calibrate each port separately?”
In real R&D environments, the bigger win usually isn’t more calibration —
it’s awareness.
When you start treating the signal path itself as part of the measurement system,
many of these problems stop being mysterious.
Final thought
If you’ve ever seen this:
- Same instrument
- Same signal
- Only the port or connection changed
- Power suddenly off by several dB
You’re not alone.
The real question isn’t which port is “right.”
It’s whether you truly understand
which path your signal is taking when you measure it.
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