If you do RF measurements long enough, you’ve probably seen this:
Yesterday the power looked fine. Today—same board, same setup (supposedly), same instrument—the reading is off by 1 dB… sometimes 2 dB.
And the most annoying part? Hand the job to another engineer and you get a different number again.
At that point it’s very easy to blame the DUT.
You start thinking the PA is unstable, the bias moved, the layout is sensitive, the module “changed state”… and you begin tweaking the circuit late into the night.
Then the next day you realize the painful truth:
it wasn’t the DUT. The measurement chain was quietly changing.
I’ve been working around RF / optics / high-speed test setups and instrument supply for years, and I’ve seen this story repeat way too often. Over time I’ve learned to treat “drifting results” as a system problem:
Most of the time it’s not that your instrument isn’t good enough.
It’s that repeatability was never built into the workflow.
This post isn’t a textbook. No brand wars, no pricing talk, no model-number shopping list.
Just a practical framework I use in R&D lab/debug situations to make RF measurements feel stable and trustworthy again—especially when the issue is power/amplitude drifting by a couple dB.
First: be honest about the goal. You’re not chasing “absolute truth” — you’re chasing repeatability.
In R&D debugging, you usually don’t need lab-grade “metrology perfection” on day one.
What you actually need is this:
- If you see a change today, can you reproduce it tomorrow?
- If your colleague repeats the test, do they land in the same ballpark?
- When you change one component, is the difference real—or just the test chain playing tricks?
So I like to frame the goal in plain language:
Define what “close enough” means for your project, then control the variables that make your readings wander.
The worst situation isn’t a small error.
The worst situation is not knowing where the error came from.
For 1–2 dB power drift, the “usual suspects” are surprisingly consistent
When a team tells me “our power readings drift”, I don’t start with fancy theories. I go through a short list of variables that show up again and again.
1) Connector contact (the classic invisible killer)
This sounds obvious, but it’s responsible for more “mystery dB drift” than people want to admit:
- not fully seated / not tightened properly
- dirty contact surfaces
- worn connectors
- slight looseness after repeated mating
In RF, “slight” is enough to waste your afternoon.
2) Cable posture and mechanical stress (not a price argument—an engineering argument)
I’m not here to say “cheap cable bad”. That conversation gets emotional fast and doesn’t help.
What I am saying: cable bend, twist, tension, being pressed against a table edge—these can change results.
And the higher you go in frequency, the more obvious it becomes.
A practical reality:
At higher frequencies, your cable is not a passive accessory. It participates in the measurement.
3) Warm-up and temperature drift (instrument + DUT + environment)
One common trap: you power on and measure immediately, then treat that as the baseline.
Thirty minutes later, the instrument and DUT have warmed up, conditions shift, and suddenly “the DUT changed”.
PAs, oscillators, front-end chains—temperature sensitivity is real.
Sometimes what looks like “power drift” is simply “temperature drift”.
4) Calibration/verification isn’t consistent (done today, skipped tomorrow)
Many teams aren’t “anti-calibration”. They’re inconsistent.
- today you verify the chain
- tomorrow you’re in a rush and skip it
- next week another engineer uses a different “quick check”
If the verification step isn’t consistent, your data won’t be consistent.
5) Key settings quietly change (you think they’re the same—they’re not)
On a spectrum analyzer alone, small changes can affect amplitude:
RBW/VBW, detector mode, averaging method, reference level, input attenuation…
If any of those move between measurements, you can absolutely get different power readings.
A lot of drift begins with:
“Yeah I just adjusted it a bit.”
6) DUT state isn’t locked (power supply / cooling / shielding details)
In R&D labs, the circuit might be unchanged, but the conditions aren’t.
- different supply, different ripple, different current limit
- fan angle or airflow changed
- shield can on/off
- board placement changed
All of these can show up as power differences.
The “minimum repeatability loop” I use (simple, not bureaucratic)
I don’t like heavy SOPs for R&D debugging. Teams won’t follow them, and they slow iteration.
What works better is a small repeatability loop—simple enough that people actually do it.
Step 1: Make the conditions comparable
Don’t compare numbers taken under completely different conditions.
At least ensure:
- you’re not comparing a “just powered on” reading with a “fully warmed up” reading
- the DUT is roughly at the same operating point (and ideally similar thermal state)
- you’re not changing cable routing and cooling while trying to compare results
Step 2: Freeze the connections (boring—but effective)
If I had to pick one high-impact action, it’s this:
Reduce reconnects and keep cable routing consistent.
- seat and tighten connectors properly
- keep the cable path consistent (don’t let it change shape every day)
- if it’s critical, do basic strain relief or simple physical fixing
This alone often cuts “mystery drift” dramatically.
Step 3: Do a 30-second sanity check before you start
The goal isn’t “perfect calibration”. The goal is to answer:
Is my chain basically normal today?
That requires a reference point—which leads to the next section.
Step 4: Stop relying on memory—capture key settings
Teams think they “used the same setup”. In reality, each person tweaks something slightly.
Capture at least:
- key instrument settings (RBW/VBW, detector, averaging, ref level, attenuation)
- measurement path details (ports, external attenuation, any couplers/amps)
- DUT conditions (bias, supply, cooling, shielding)
It doesn’t have to be fancy. Screenshots, a simple template, consistent naming—anything that lets you reproduce the measurement next week.
A blunt rule I use:
If you can’t reproduce it later, you didn’t really “measure” it—you just looked at it once.
One move that saves a lot of arguments: build a “reference object” mechanism
When results drift, the expensive part is not the drift itself—it’s the time wasted debating whether the DUT changed or the chain changed.
So I strongly recommend building a reference mechanism inside the team:
- a reference source or stable output state
- a “known-good” reference device (a golden sample)
- a baseline dataset captured under stable conditions
Then when someone says “it’s drifting today”, you run the reference first:
- if the reference drifts too → suspect chain/connection/settings/thermal/verification
- if the reference is stable but the DUT drifts → suspect the DUT (bias, supply, thermal, state)
This simple habit saves days of circular debate.
Five mistakes I still see all the time
1) Changing the DUT before locking down the chain
2) Letting cable routing vary day to day
3) Recording only final numbers, not the setup/conditions
4) Treating verification as optional or “when I have time”
5) Assuming a more expensive instrument will automatically fix repeatability
Very often, the instrument is fine. The workflow isn’t.
Quick self-check if you’re dealing with dB-level power drift
Before you redesign a circuit, ask:
- Did you control connector contact, cable posture, warm-up/thermal state, settings consistency, and verification step?
- Do you have a reference object that can tell you “chain drift” vs “DUT drift” quickly?
If you want, you can DM me two short lines:
- what you’re measuring (rough band / DUT type)
- how it drifts (how many dB, and under what conditions it’s worse)
I can usually help you rank the most likely variables, so you can troubleshoot faster and avoid chasing ghosts.
About me & contact
I work on RF / optical / high-speed test setups and instrument supply chain. I help teams balance performance, budget, and risk—whether you’re buying new gear, used gear, renting, or using lab resources.
Website: https://maronlabs.com
Email: contact@maronlabs.com
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