Compression Marker
Understanding the Compression Marker
When designing an RF Power Amplifier, the most critical specification is the 1dB Compression Point (P1dB). This is the exact power level where the transistors run out of voltage headroom and the amplifier's gain physically collapses by 1 decibel. Finding this exact point manually is incredibly tedious: the engineer has to sweep the input power, watch the S21 (Gain) line on the VNA, set a marker at the low-power flat region, drag a second marker up the curve, and do mental math to find where the difference is exactly -1 dB. To automate this, modern VNAs feature a Compression Marker.
The Compression Marker is an algorithmic search function built into the VNA's UI. When activated during a Power Sweep (where the X-axis is Input Power in dBm, and the Y-axis is Gain in dB), the VNA supercomputer takes over. It automatically identifies the flat, linear region of the amplifier at the far left of the screen. It memorizes that baseline gain. It then mathematically scans the curve to the right until it finds the exact pixel where the gain has dropped by 1.000 dB, and aggressively snaps the marker to that exact coordinate.
Dynamic Tracking
The true power of the Compression Marker is that it tracks in real-time. If the engineer takes a screwdriver and adjusts the bias voltage on the physical amplifier on the test bench, the P1dB point will instantly change. The Compression Marker will dynamically slide back and forth across the screen, constantly hunting down the new 1dB drop point at 10 frames per second, allowing the engineer to instantly see the results of their tuning.
Gain( Pin ) = Gainlinear_baseline - X dB
Where 'X' is the target compression level. While 1 dB is the industry standard (P1dB) for amplifiers, engineers designing heavy limiters or mixers might tell the marker to search for a 3 dB compression point (P3dB). The software instantly adjusts the search criteria.
Comparison
| Marker Function | How it Works | Primary Use Case |
|---|---|---|
| Peak Search Marker | Snaps to the absolute highest Y-value on the screen. | Finding resonant frequencies or maximum gain. |
| Delta Marker | Calculates the difference between Marker 1 and Marker 2. | Manual measurement of bandwidth or ripple. |
| Compression Marker | Algorithmically finds the specific -1 dB gain collapse point. | Automated PA testing and P1dB extraction. |
Frequently Asked Questions
Does the Compression Marker work on a Frequency Sweep?
It can, but it requires a much more complex setup. In a standard Power Sweep (CW frequency), finding compression is easy because the X-axis is Power. If the X-axis is Frequency, the VNA must perform a '2D Sweep' (sweeping frequency, and then sweeping power at EVERY frequency point). Advanced VNAs have a dedicated 'Gain Compression Application' that runs this massive 2D matrix sweep and plots a single line showing exactly what the P1dB power is across the entire frequency band.
What is 'Gain Expansion' and does it break the marker?
Certain transistor technologies, particularly Gallium Nitride (GaN), suffer from an anomaly called Gain Expansion. As you increase the power, the gain actually goes UP by 0.5 dB before violently crashing into compression. A poorly coded Compression Marker will lock onto the linear baseline, see the gain go UP, get confused, and fail to find the -1 dB point. Modern VNAs allow you to set the reference point to 'Peak Gain' instead of 'Linear Baseline' to properly handle GaN expansion.
Can I use the compression marker to find P_out?
Yes. The marker itself usually sits on the X-axis of Input Power (Pin). However, the VNA software simultaneously displays the Y-axis value (the compressed Gain). By simply adding the Input Power (Pin) and the Compressed Gain together, the software outputs the exact Output Power at the 1dB compression point (Pout_1dB), which is the number listed on the amplifier's datasheet.