Reading a Front-End Module Data Sheet: Key Numbers First

A public data sheet for a filter, switch or LNA is short, but every line depends on the lines around it. A loss figure means little without its band. An isolation figure means little without the port pair it was measured between. This article gives a fixed scanning order, so you check the same things every time before you compare two parts.

Step 1: Confirm the part type and the band

Start with what the part is. A filter, a switch and an LNA are judged by different numbers.

• A filter or switch is passive. You read insertion loss, isolation and return loss.
• An LNA is active. You read gain and noise figure, plus return loss at its ports.

Next, find the frequency range the numbers apply to. Look for the passband of a filter, the operating range of a switch, or the operating range of an LNA. Every later figure is valid only inside that range. If the table gives one number for a "band", check whether it is a limit across the whole band or a value at one frequency.

Step 2: Read insertion loss

Insertion loss is the loss of signal power between input and output in the passband. It is given in dB, and both ports are terminated in the reference impedance:

A smaller number is better. Because the formula uses 20·log10 of a voltage ratio, it equals 10·log10 of the power ratio, so you can convert back to power:

For example, 1 dB gives |S21| = 10^(−1/20) ≈ 0.891 and a power ratio of about 0.794. About 79% of the power arrives, and about 21% is lost (mostly as heat, plus any reflection).

The block diagram below shows where each of the three main numbers comes from. Insertion loss is the forward path from Port 1 to Port 2. Return loss is the reflection back at Port 1. Isolation is the path to a port that should not receive the signal.

Step 3: Read isolation and return loss

Isolation is the attenuation between two ports that should not couple. It is also −20·log10|S21|, but taken between those two ports, with all ports terminated in the reference impedance. Typical cases are an off-state switch port and the two outputs of a diplexer. A larger value means better isolation. If the data sheet gives one isolation figure, find out which port pair and which switch state it describes.

Return loss tells you how well a port is matched to its reference impedance, usually 50 ohm:

Use |S22| for the output port. A larger value means a better match. Return loss of 10 dB means |S11| ≈ 0.316 and about 10% of the power is reflected. Return loss of 20 dB means |S11| = 0.1 and about 1% is reflected.

The two ideas are easy to mix up. Insertion loss and isolation both use S21, but between different port pairs and in different regimes. In the passband you want a small number. For isolation you want a large number. Return loss wants a large number too.

Step 4: Read the test conditions

Data sheet values hold only under the stated conditions. Before you trust any figure, find these items:

• Reference impedance. Usually 50 ohm. If the part was measured in another system, the numbers do not transfer.
• Temperature range. A value at one temperature differs from a limit over the whole range.
• Value type. Check whether the number is a typical value, a minimum or a maximum. A typical figure is not a guarantee.
• Port state. For a switch, note which path is on and which ports are off.
• Reference plane. Check whether the figure includes the test board and connectors or only the part.

Put these next to each number in your own notes. A table with only values and no conditions is incomplete.

Step 5: Check the same numbers in an S-parameter file

A data sheet gives single points. An S-parameter file lets you check the same numbers across frequency instead. If a part comes with a Touchstone file, open it and read |S21| and |S11| at the band edges, not only at the center. The file also shows ripple and dips that one number hides.

Each data line in an S2P file holds a frequency and the four parameters. The figure below shows how one line is laid out in MA format (magnitude and angle).

To convert a linear magnitude to dB, use 20·log10 of the magnitude. A magnitude of 0.891 gives −1.0 dB, so insertion loss is 1.0 dB. The pages linked at the end of this article cover the file formats and a free viewer.

Worked example: comparing two parts

The numbers below are made up to show the method. They are not from any product.

Suppose Part A lists a maximum insertion loss of 1.0 dB, a return loss of 10 dB and an isolation of 40 dB. Part B lists a maximum insertion loss of 1.5 dB, a return loss of 20 dB and the same isolation.

Convert each to linear terms:

• Part A: |S21| ≈ 0.891, so about 79.4% of power passes. |S11| ≈ 0.316, so 10% is reflected.
• Part B: |S21| = 10^(−1.5/20) ≈ 0.841, so about 70.8% of power passes. |S11| = 0.1, so 1% is reflected.
• Isolation of 40 dB means |S21| = 0.01 between the two ports, so 0.01% of the power couples across.

Part A looks better on loss and Part B on match. Before you pick one, check the conditions. Suppose Part A states its loss at room temperature and Part B states it over the full temperature range. Then Part B's 1.5 dB is a worst case, and the two numbers are not comparable. Ask for the same temperature range, or compare S-parameter files taken under the same conditions.

In plain words · for students and beginners

Data sheet numbers in plain words

An everyday picture

Think of a window. Insertion loss is how much light the glass lets through. A clean pane loses a little. Return loss is how much light bounces back at you instead of going in. A good match is a pane with almost no glare. Isolation is a closed, thick door between two rooms: how little light leaks from one room to the next.

The picture breaks down in one place. Decibels (dB) are a logarithmic scale, so 20 dB is not twice as good as 10 dB. Each 20 dB cuts the signal amplitude by a factor of 10.

The plot above shows S21 and S11 of a low-pass sample filter. It is computed from a circuit model, not measured data. The passband has S21 near 0 dB (little loss), and S11 is small there (a good match).

The same example in simple steps

Take a part with 1 dB insertion loss and 20 dB return loss.

1. Insertion loss of 1 dB means about 79% of the power gets through.
2. Return loss of 20 dB means about 1% bounces back.
3. Isolation of 40 dB means only 0.01% leaks to the other port.
4. Now read the small print: the band, the 50 ohm reference and the temperature range. If these differ from another part, the comparison is not fair.

Mistakes beginners make

• Comparing a typical number on one data sheet with a maximum on another.
• Thinking a bigger return loss is worse. Bigger is better, because less power is reflected.
• Reading a number outside the band it was given for.
• Using insertion loss for an LNA. An LNA is judged by gain and noise figure.
• Ignoring the temperature range and the 50 ohm reference.

Remember

• Read the band and the test conditions before any number.
• Insertion loss: smaller is better. Return loss and isolation: larger is better.
• Compare two parts only under the same conditions, or check the S-parameter files.

Related pages on RF View

• How to Read One Line of an S2P File (MA, DB and RI Formats)
• Online S2P and Touchstone File Viewer