VSS C_HDRM (Cascaded Headroom) Measurements

Problem

• How to compute output referred cascaded headroom?
• How to compute cumulative headroom for the entire RF chain?
• What is gain adjusted C_HDRM measurement?
• How to compute amount of compressed gain for the system?

Solution

C_HDRM measurement falls under the  RF Budget cascaded measurement category and is used in the computation of the difference between individual element input power level and the element's input 1 dB gain compression level. This measurement is expressed as:

C_HDRM = Input P1dBm - Input Power (dBm)

To emphasize the measurement definition, C_HDRM is an element-by-element measurement, not an overall cumulative system measurement.

A typical level diagram vs. RF stage may look like:

The blue trace is the signal power at each node, the red trace is the 1 dB output compression level at each node in the RF chain. The magenta trace is the C_HDRM measurement highlighting the fact that the headroom is for each individual stage, not a cumulative system measurement. Also note that the C_HDRM is specified at the input of the blocks that have gain compression (the two amplifier blocks in this example).

How to Compute Output Referred Headroom

Amplifiers are normally specified with output P1dB, not input P1dB. The relationship between input 1dB gain compression level and output gain compression level is 1 dB. The following graph of swept power illustrates this concept:

The measurement is that of a single amplifier with gain of 10 dB and an output gain compression level of +10 dBm. The blue trace is normalized gain with the marker, m2, set at the power level where the gain drops by 1 dB. The red trace is the amplifier output power and the magenta trace is the amplifier input power.

For power levels far below gain compression the difference between the output and input power traces is the gain of the amplifier, which is 10 dB. But at the 1 dB gain compression power level, the difference between the output and input power traces is reduced be 1 dB.

C_HDRM measurement works accordingly. To compute C_HDRM at the output of the element, subtract 1 dB. A full equation of the C_HDRM measurement is:

C_HDRM = Input P1 dBm – Input Power [dBm] – Margin

where Margin is the measurement parameter highlighted below:

So, for input C_HDRM, leave Margin in dB at its default value of 0 dB. For output C_HDRM, set this parameter to 1 dB.

Gain Adjusted C_HDRM

The C_HDRM has options for Headroom Type: Input Power to IP1dB and Input Power to IP1dB, Gain Adjusted.

The Gain Adjusted type is the default setting. The following system shows an amplifier with +10 dBm output P1 dBm preceded by a 3 dB fixed attenuator. The output headroom is 13 dB as shown in the following figure:

The magenta trace is the gain adjusted C_HDRM measurement showing an input headroom value 1 dB higher than the output headroom value. The loss of the fixed attenuator is not factored into the gain adjusted C_HDRM measurement at the input of the fixed attenuator. However, the green trace which is C_HDRM without gain adjusted does show the loss of the fixed attenuator.

The gain adjusted C_HDRM is the true headroom value of the RF chain. The C_HDRM that is not gain adjusted is supplemental information that could be used in consideration if the loss in front of the gain element was removed. More of a what-if analysis on whether to include the input loss element or not.

Frequency Dependent C_HDRM

It may be beneficial to monitor the headroom for each stage as a function of frequency. The following example will be used to illustrate:

The amplifiers have frequency dependent gain and output gain compression levels.

A test point is placed at the output of each active stage. Measurements for each test point are constructed with the End Test Point parameter set to the test point of the stage of interest:

With the following plot of C_HDRM vs frequency, the headroom of each active stage can be determined.

Cumulative System Headroom

The C_HDRM measurement is not capable of computing cumulative system level headroom. Simple Output Equations can be written for cumulative headroom. Subtract node power from output gain compression as shown:

The result is the magenta trace added to the data from the previous C_HDRM vs. frequency example.

Compressed Gain Measurement

Instead of headroom between actual power and the P1dB level, this example shows the actual compressed gain measurement. Use a SWPVAR block that controls the power of the source. Use two sweep values, one where the source power is far below gain compression and the other at the operating power level. In the following example, these two values are -30 and 0 dBm respectively:

Create an output equation where the transducer gains for the two swept powers are subtracted from each other:

The plotted result shows the actual compressed gain vs. frequency: