Simulink Product模块混频输出出现5-10dB衰减的原因咨询

Hey there! Let's break down why your carrier-wideband signal mixing via the Simulink Product block is causing that 5-10dB attenuation—even though the waveform looks right. I’ve tackled similar issues before, so here are the top things to check:

1. Output Data Type Mismatch or Truncation #

This is one of the most common culprits. If the Product block’s output data type is set to a restrictive format (like fixed-point or an integer type) instead of inheriting from your input signals, it can silently compress the signal amplitude.

• Right-click the Product block → Block Parameters
• Look at the Output data type dropdown. If it’s set to something like uint16, int32, or a custom fixed-point type, switch it to Inherit: Inherit via internal rule or double (if your inputs are double-precision) and re-run your simulation.

2. Signal Amplitude Imbalance #

Mixing two signals mathematically results in an output amplitude equal to the product of the input amplitudes. If either your carrier or wideband signal isn’t at full scale (e.g., carrier amplitude is 0.5 instead of 1), the output will be scaled down—exactly the 6dB attenuation you might be seeing (20 log₁₀(0.5) ≈ -6dB).

• Check the amplitude settings of your Carrier (e.g., Sine Wave block’s Amplitude parameter) and Wideband signal sources. Ensure they’re set to the levels you expect, then re-measure the output amplitude.

3. Spectrum/Scope Display Normalization #

Sometimes the attenuation is just a display artifact, not an actual signal reduction. Tools like the Spectrum Analyzer or Scope might apply automatic normalization that makes the output look weaker than it is.

• For the Spectrum Analyzer: Open its settings, go to the Amplitude tab, and set Amplitude scaling to Peak instead of Power or Power Density. This will show the true peak amplitude of your signal.
• For the Scope: Check if the Y-axis is set to autoscale with normalization disabled. You can also manually set the Y-axis range to match your input signal’s range to compare levels directly.

4. Accidental Multiplication Mode Setting #

While less likely, double-check the Product block’s Multiplication mode. If it’s set to Matrix multiplication instead of the default Element-wise multiplication, and your inputs are vectors, you might get unexpected amplitude scaling (especially if the vector dimensions line up in a way that reduces the output magnitude).

• In the Product block parameters, confirm Multiplication mode is set to Element-wise.

5. Hidden Gain/Scaling in the Signal Path #

Take a quick walk through your entire signal chain from source to Product output. Look for:

• Accidentally placed Gain modules with a value < 1
• Data Type Conversion modules where Output minimum/maximum is set too low, causing signal clamping or scaling
• Any other blocks that might be modifying the signal amplitude before or after mixing

Quick Test to Isolate the Issue #

To rule out block-specific problems, replace your Carrier and Wideband sources with Constant blocks set to an amplitude of 1. If the Product block outputs 1, the issue is with your original signal sources or data type settings. If it outputs something less than 1, double-check the Product block’s parameters again.

内容的提问来源于stack exchange，提问作者tompak