1 dB Gain Compression Measurement (P1dB)

An amplifier maintains a constant gain for low-level input signals. However, at higher input levels, the amplifier goes into saturation and its gain decreases. The 1 dB compression point (P1dB) indicates the power level that causes the gain to drop by 1 dB from its small signal value.

Measurement Setup

Measuring the 1 dB gain compression point of a device requires driving the UUT into compression without driving the RF Vector Signal Analyzer (VSA) into compression. This requires proper attenuation at the RF VSA and a PXI-565x RF Signal Generator to compress the UUT. You may apply attenuation by programming the internal input attenuators or using external attenuation.

The 1 dB compression point is derived from the gain relationship between output power and input power. Using the measurement setup shown in the figure below, source amplitude is slowly increased while the UUT output is monitored.

Output power is plotted against input power as shown in the figure below:
The straight line on this graph is an extrapolation of the small signal gain of the UUT. The input 1 dB compression point is the input power that causes the UUT gain to drop by 1 dB from this small signal value, or approximately –12 dBm in this case.

Understanding RF VSA Compression Limits

Like all signal analysis devices, the RF VSA is not completely linear and will eventually reach compression. However, the RF VSA architecture possesses a high degree of linearity, and its compression point is typically 5 dBm or higher.

Ensure accurate UUT compression measurements by limiting the signal at the RF VSA input mixer to 20 dB below the compression point listed in the NI PXI-5660 RF Vector VSA Specifications document included in your RF VSA kit.

Choosing the Optimal RF VSA Attenuation Setting

Choosing the optimal attenuation settings for a UUT compression measurement requires taking a couple of factors into account:

· The maximum output signal of your UUT must be attenuated to 10–20 dB less than the compression point of the RF VSA.
· The resolution bandwidth setting of the RF VSA must be low enough that small signals used to determine the linear gain of the UUT are not overwhelmed with noise from the RF VSA.

To set the proper RF VSA attenuation level for a compression test on a UUT with known output compression estimate and known approximate gain, complete the following steps:

1. Set the RF VSA mixer level to –20 dBm and its reference level to 10 dB above the estimated UUT compression point. When using the RF VSA Demo Panel, mixer level = reference level – attenuation.
2. Set the RF VSA center frequency to your intended testing frequency, its span to 1 MHz, and its resolution bandwidth to 1 kHz.
3. Inject a signal into the UUT small enough that its output level is at least 20 dB below the estimated UUT compression point. If the UUT output signal level is too close to the noise floor of the RF VSA, decrease the RF VSA resolution bandwidth.
4. Increase the input signal to the UUT. If the output signal has reached 5 dB below the RF VSA reference level and compression of the UUT has not been reached, increase the reference level by 10 dB.
5. Repeat step 4 until compression appears in the UUT.
This setting is the optimal attenuation setting.

Resources

Common RF and Microwave Measurements
This is the main page of a series of tutorials focused on common RF measurements involving signal generators and analyzers.

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