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Preamplifiers: Internal and External, Smart and Not

Blog Post created by benz on Sep 14, 2016

Originally posted Sept 19, 2014

Boost those electrons at your first opportunity

Preamplifiers are a time-tested way to improve measurement sensitivity and accuracy for small signals, especially those near noise. Some new external preamps, used alone or along with those internal to signal analyzers, may give your tiny signals the right boost in the right place to makebetter measurements. In the bargain, they’ll simplify small-signal and noise-figure measurements.

Once you’ve switched attenuation to 0 dB in a signal analyzer, the next step toward better sensitivity is some sort of amplifier. Many signal analyzers offer internal preamplifiers as an option, and it’s generally easier than employing an external preamp. The manufacturer can characterize the internal preamp in terms of gain and frequency response, and this can be reflected in the analyzer’s accuracy specifications.

However, internal preamplifiers may not have quite the gain you want over the frequency range you need, and an internal unit can’t be placed as close as possible to the signal-under–test (SUT). This is important for microwave and millimeter signals because they can’t travel far without significant attenuation, and because they tend to gather unknown and unwanted signals along the way. This is especially troublesome when you’re measuring small signals close to noise.

External preamplifiers are available in a wide range of frequency ranges, gains and noise figures, in both custom and off-the-shelf configurations, and can provide excellent performance. Unfortunately, it can be a challenge to integrate them into an end-to-end measurement. Accurate measurements require correcting for gain versus frequency and, if possible, noise figure, impedance match and temperature coefficients.

That’s where Keysight comes in. It recently introduced several external “smart” preamplifiers that automatically integrate with the measurement system and are compatible with all of the X-Series signal analyzers. They connect directly to the RF input of the signal analyzers, as shown below, and can function as a remote test head, providing amplification closest to the SUT.

An external USB smart preamplifier connected to an X-Series signal analyzer. The preamp can serve as a high-performance remote test head for spectrum and noise-figure measurements. The USB cable connecting the analyzer and preamplifier is not shown.

An external USB smart preamplifier connected to an X-Series signal analyzer. The preamp can serve as a high-performance remote test head for spectrum and noise-figure measurements. The USB cable connecting the analyzer and preamplifier is not shown.

The U7227A/C/F preamplifiers use a single USB connection to identify themselves to the analyzer and download essential information such as gain versus frequency, noise figure and S-parameters.

As described in a previous post about smart external mixers, the combination of downloaded data and analyzer firmware fully integrates the amplifier into the measurement setup and effectively extends the measurement plane to its input. This allows Keysight to provide a complete measurement solution with very high performance and allows you to focus on critical measurements instead of system integration.

The USB preamplifiers have high gain and very low noise figure, and can be used in combination with the optional internal preamplifiers of the X-Series signal analyzers. The result is a very impressive system noise figure, as shown in the example below.

The displayed average noise level of the Keysight PXA signal analyzer is shown without a preamp (top), with the internal preamp (middle) and with the addition of the external USB preamp (bottom). Note the measured 13 GHz noise density at the bottom of the marker table of -171 dBm/Hz.

The displayed average noise level of the Keysight PXA signal analyzer is shown without a preamp (top), with the internal preamp (middle) and with the addition of the external USB preamp (bottom). Note the measured 13 GHz noise density at the bottom of the marker table of -171 dBm/Hz.

The performance and USB connectivity of the external preamps improves and simplifies noise-figure measurements and analyzer sensitivity, giving those few critical electrons a boost just when they need it most.

For more detail please see the USB preamplifier technical overview.

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