The low noise figure, high gain and flat frequency response of the Spanawave/Giga-tronics amplifiers allow them to be used as preamplifiers for high noise figure spectrum and signal analyzers. The Giga-tronic amplifiers used as a preamp can help bring low level signals above the noise floor of the analyzer.

The approximate formula for noise figure of cascaded amplifiers is calculated in linear terms of noise factor (F):

Where F₁ is the noise factor of the first amplifier, G₁ is the gain of the first amplifier, and F₂ is the noise factor of the second or following amplifier. Note that in the exact formula, there may be additional terms for additional amplifiers following the first two, but these terms are negligible if the gains are reasonably high, since the gains multiply in the denominator of additional terms.

For example:

NF_amplifier = 6 dB (F = 3.98)

NF_analyzer= 20 dB (F = 100)

GAIN_amplifier= 40 dB (10,000)

F_system = 3.99

And finally:

NF_system = 6.1 dB

Note that the system noise figure is greatly reduced by the preamplifier and for a high gain preamplifier is essentially the noise figure of the preamplifier.

In this way, adding a preamplifier increases the sensitivity of the analyzer by decreasing the displayed average noise level (DANL) or noise floor of the analyzer.

There are a couple caveats to be aware of when using an amplifier as a preamp for spectrum and signal analyzers. First, it is assumed that the noise figure of the amplifier is lower than that of the analyzer. Second, if the amplifier gain is high, it may be necessary to adjust the analyzers input attenuator to maintain the signal in the analyzer’s “sweet spot”, where the power level at the analyzer’s mixer is at an optimum level for the best analyzer performance. The “sweet spot” mixer power level is a tradeoff between the analyzer’s sensitivity and distortion products.

Note that while manufacturers of spectrum and signal analyzers recommend you use their own high-priced amplifiers as preamps for their analyzers, there is no absolute requirement to do so. Compare specifications for frequency range, gain, gain flatness, noise figure, harmonics and spurious, and the Spanawave/Giga-tronics amplifiers will very likely offer higher performance at a lower cost.

The broad frequency ranges of the Spanawave/Giga-tronics amplifiers, from 100 MHz to 20 GHz, 40 GHz and 50 GHz, well match the frequency range of the majority of spectrum and signal analyzer applications. The Spanawave/Giga-tronics parallel-MMIC design provides wide linear range and a sharp saturation knee. The result is outstanding performance when used with signals having high peak to average ratios. The broad frequency range provides excellent pulse performance preserving the fast rise and fall times of high speed signals and narrow pulses.

Noise Considerations in Broadband Microwave Power Amplifiers

Using Preamplifiers to Reduce System Noise Figure