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Questions to Ask When Purchasing Preamplifiers for EMC Use

Preamplifiers or otherwise called Low-Noise Amplifiers (LNA), are commonly used for Radiated emissions (RE) and sometimes Conducted Emissions (CE). These devices help boost the measured RF signal. Boosting the signal or increasing it also increases the signal above the noise floor of the measurement system (receiver or spectrum analyzer), allowing smaller signals to be seen. In this article, we will go over what to look for when selecting a preamplifier.

What is the gain of the preamplifier?

Gain is the number of dBs it will increase the input signal to the output. Gain changes across the frequency range but may be relatively flat. A higher gain output is often seen as better; however, this is not always true. Too much gain can overload the input of your receiver/spectrum analyzer. Many receivers will detect this and give a warning. If this is seen, additional attenuation must be used. Adding attenuation after the amplification is counterproductive. You would save on the cost by purchasing a unit with less gain. To determine how much gain you may need, one can look at adding together antenna factor and cable losses to identify the needed adjustment. This is a good starting point for how much gain you need in a preamplifier.

If your receiver has a preamplifier built in, be sure to take this into account.

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What is the Noise Figure (NF) of the preamplifier?

The NF is a measurement of how much noise you will add to your receiver’s noise floor. A lower value for NF is always better. You can expect to see values of about 3 to 5 dB. This is a relative number that can be calculated to a noise floor increase by using your receiver’s bandwidth and gain of the preamplifier. The value varies based on gain, frequency range, and quality of the preamplifier.

What frequency range do you need?

Amplifiers come in different frequency ranges. In most cases, a wide band preamplifier is required for EMC testing as testing normally covers a wide frequency range. Using multiple preamplifiers to cover the range that a single preamplifier does, in some cases, costs you more in test time with setup changes than it may be worth. A preamplifier that covers the whole band of one antenna makes the test setup easy (for example, a 30MHz – 7GHz preamplifier to cover a CE testing with a Biconical/Log-Periodic (TriLOG, combination antenna) or a 1-18 GHz preamplifier to cover a double ridge horn antenna). The cost will increase as the frequency band increases and the highest frequency increases.

What is the P1dB compression point of the preamplifier?

A higher value is better. ~10dBm is a good level for a preamplifier. The P1dB compression point is a common specification to denote where an amplifier starts to saturate. This value is normally given in power as dBm. It is considered that output signals lower than this will be linear. A linear signal will not be distorted and, therefore, will not produce harmonics and inner harmonics. When in saturation, harmonics and inner harmonics increase and produce false signals seen on the receiver. This must be avoided.

How is the preamplifier packaged?

Preamplifiers can come in different packages.

  1. A benchtop package with a power supply and amplifier in the same enclosure can be placed near your receiver or at the base of the antenna/mast in the chamber.
  2. An antenna-mounted package where the unit is mounted onto or near the antenna.
    • AC Power supply is separated from the preamp to reduce the size/weight on the mast/antenna and make it easier to power on the chamber floor.
    • Battery-powered to remove the need to have AC power
    • Use Bias T to send DC power down the measurement coax cable to power the preamp.
  3. A preamplifier without any power supply, requiring the user to select or build their own power supply. This is not a preamplifier prepackaged for use in EMC and should be avoided except for very experienced RF engineers.

There are arguments for and against all of the above packaging solutions. It is best to understand your setup and what may fit best. Here we can discuss some of the reasoning for different setups. Placing a preamplifier near the antenna is argued as a better solution. The reasoning is the preamplifier is only amplifying the signal from the antenna and not the noise that may be picked up from the long length of the coax cable if positioned at the receiver. The counter-argument is that not much noise is picked up through a high-quality well shielded coax cable in a shielded chamber. There is more chance of noise being picked up outside of the chamber. So placing the preamplifier inside the chamber does have validity, just maybe not at or near the antenna if it is not convenient for the setup.

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Powering the preamplifier is up to the user’s preference. Using batteries does have the benefit of not having to use a power supply in the chamber, making setup less complicated. Batteries require charging and wear out over time, adding the possibility of downtime, especially when emissions testing is in high demand and can run for long periods. Powering the preamplifiers with a power source removes this negative aspect of batteries.

Using a bias T is nice as it can power the preamplifier indefinitely from a DC power source outside the chamber through the coax cable. However, this setup is more complicated and leaves some room for user error. Outside the chamber, a Bias T injects DC into the coax cable used for measurements. It then feeds into a preamplifier with a bias T that pulls the DC off the coax cable to power itself. If the coax cable is plugged into anything other than a preamplifier, it could damage this other device since the DC is present. The bias T adds items to the measurement path and puts DC on the measurement path, which all must be checked so there is no effect on the measurement. This added complexity makes a bias T only a good choice for experienced users. Using an AC power source is the best option as it is easy to set up, can be well protected from damage and regulated from damaging the preamplifier, and can be powered indefinitely.

Does the preamplifier offer input or power protections to protect against damage?

As mentioned above, any power feeding the preamplifier, whether AC or DC, must be well-regulated and protected. In the EMC lab or other environments, power can fluctuate and have faulty grounds. A power source that has protection allows for a much more rugged setup to have better longevity. Therefore a preamplifier specifically made for EMC testing with these safeguards is more desirable.

Does your receiver have a preamplifier? Should it? If yes, do you need an external preamplifier?

Having the preamplifier built into the measurement device is positive. It allows the device to have a lower noise floor when the preamp is turned on, giving you more versatility when testing. The receiver is, in most cases, used for multiple tasks, not just an RE test, so having more versatility within the device extends its usefulness. Having this built into your receiver does not eliminate that an external preamplifier is still required. It comes down to your possible losses from coax cable and antenna factors.

In conclusion, a preamplifier is required in many radiated emissions setups to compensate for the high losses in the antenna and coax cables. A good quality preamplifier with a low noise floor with high P1dB. A wide frequency range to match your antenna and a gain that compensates for your calculated losses.

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