A preamplifier (preamp) is a low-noise, high-gain amplifying device that plays an important role in performing electromagnetic compatibility (EMC) radiated and conducted emissions testing. Preamps significantly boost the signal-to-noise ratio obtainable over that of just a spectrum analyzer or EMI receiver alone. This allows accurate measurements to be made on very low-level signals. As good and useful as preamps are, they are not foolproof. There are certain limitations that must be understood by anyone who uses them for EMC emissions measurements. The following will briefly cover uses of preamps and what issues users should be aware of when performing testing with them.
Minimum noise equals maximum sensitivity in an EMI receiver and when not employing a low-noise preamp it is often the receiver’s own noise floor that sets the lower constraint on the voltage that can be accurately measured with the receiver. Sometimes the receiver’s unaided noise floor (no preamp utilized) has enough sensitivity on its own to enable accurate measurements of the RF energy emanating from the equipment under test (EUT). This is especially the case where the emission limits specified are the highest possible, like those specified as part of FCC or CISPR 32 Class A limits. However, if the test limits are lower than usual, like those specified as part of FCC or CISPR 32 Class B limits, MIL-STD-461G RE102 for Army Ground, or DEF STAN DRE01 limits, then utilizing a preamp to obtain the necessary sensitivity is most likely required.
In order to capture accurate measurements, the noise floor of the system should be at least 6 dB below the established limits. If this requirement is not met with the receiver alone then a preamp must be utilized. The preamp is installed in the first stage of the measurement chain and the overall system noise figure (NF) is dominated by this first stage’s noise NF and gain. This is evident in the formula used to determine total NF of the measurement system. Assuming a two-stage measurement system, total NF = NF1 + (NF2 -1)/G1, where NF1 is the preamp NF, G1 is the preamp gain and NF2 is the NF of the EMI receiver. Typical preamps available have gains of 20-32 dB and NFs from 0.5 to 5 dB. A typical broadband preamp will have a 10 kHz to 1 GHz frequency range, with a gain flatness of approximately 1 dB. These are just some of the basic specifications. There are preamps available with other specifications and with too many to list here.
During emissions testing, the strength of the signal obtained and reported by the measurement system is determined by adding up all of the gains and losses found within the system. These gains and losses include antenna factor, cable loss, connector loss, and any other gain or attenuation (including those of the preamp) that are found within the measurement chain spanning from the measured voltage obtained from the equipment under test (EUT) using an antenna, all the way up to the input port of the EMI receiver/spectrum analyzer. The important point here is to make sure the gain of the preamp is removed in the overall calculation to determine the voltage measurement from the EUT. If it is not, the voltage readings reported by the measurement system will be off by the same amount as the preamp gain.
If used correctly, the gain of the preamp helps us “see” signals that would otherwise be hidden below the noise floor of the measurement system. If used incorrectly, measurement data collected with a preamp becomes invalid. If a measurement error occurs when using a preamp, and if it is not immediately rectified, it is highly likely that test personnel will mistakenly report that a product is compliant with emission limits when in fact it is not. The error may go unnoticed until someone else retests the product, perhaps as part of a customer acceptance plan or by the original designer/ manufacturer after a design change has been made and where retesting is required to confirm continued compliance. Imagine the surprised look on the face of a manufacturer when they first discover their product was tested incorrectly because of an issue with a preamp, and the product is truly not compliant and must be removed from the market until it can be corrected! In contrast, the opposite can also occur. Erroneously failing a product, when it is in fact compliant, is also a possibility when utilizing preamps for emissions testing.
The first error noted in the previous paragraph often occurs when the gain of the preamp is in compression. This means the output of the amplifier does not increase and stays at the same output level, no matter what the input level is. The gain is reduced at all frequencies when compression is present. This gain level does not match the level which the preamp is calibrated at, and this value does not match what the measurement system software has recorded. Since the measurement system performance is compromised, any data obtained from it is in error. The product could actually have emissions over the limit line and the test operator would never know it.
In contrast, erroneously failing a product also occurs because of bad data obtained from the measurement system, mainly due to distortion in the preamp. This distortion can be caused by an input signal (can be the signal of interest or one that is out of band) that is too high resulting in coherently related spectral emissions covering a wide range of frequencies appearing at the output of the preamp / input to the EMI receiver. In this situation, the emissions look like real emissions from the EUT and get reported as failures. This situation is also not good because the manufacturer will have wasted time and resources trying to fix a problem with their design that does not actually exist.
Low-noise preamps are necessary devices for performing EMC emissions measurements involving low-level signals and low limit lines. Test operators should be aware of their limitations and know when they are operating correctly or not. Determining if a preamp is outputting correctly is a complex topic too lengthy to cover is this brief article. Consult reference 2 for a description of the methods that can be used to help determine if gain compression and distortion are occurring in the preamp you are using.