Noise Considerations

in Broadband Microwave Power Amplifiers

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The Spanawave/Giga-tronics microwave power amplifiers offer linear high-power amplification over a very broad frequency range. RF/microwave power amplifiers used in test and measurement applications are typically characterized by their broadband frequency range, high gain with relatively flat frequency response, low noise and wide linear range.  RF/Microwave amplifiers are characterized by their noise figure. Spanawave/Giga-tronics microwave power amplifiers have relatively low noise figure, ideal for applications where a combination of broad frequency range, high power and low noise are required. This application note discusses noise considerations in broadband microwave amplifiers.

Microwave Amplifier Noise Floor

The Spanawave/Giga-tronics microwave power amplifiers offer linear high-power amplification over a very broad frequency range. They are used to boost signal power levels to the range of +20 to +40 dBm. However, all amplifiers amplify noise as well as the signal, raising the noise floor. And amplifiers add some amount of their own noise as well, as represented by the noise figure of the amplifier.

The noise floor power level is usually negligible at audio frequencies and in many narrowband RF applications. But at broadband microwave frequencies, the noise floor can become significant and may be the limited factor in measurement dynamic range.

The theoretical minimum noise floor is thermal noise, sometimes referred to as “white noise”. Thermal noise is broadband and flat with frequency, and is expressed by the formula:

N_T = kTB 

Where k = Boltzman’s constant = 1.38 x 10^-23 Joules/°K (Watt-seconds/°K), T = temperature in Kelvin and B = the frequency bandwidth (Hz).

For T = 290 °K  (~ 17 °C), then N_T = -174 dBm/Hz 

It is clear that for small bandwidth B, the thermal noise floor is quite low. But with wider bandwidths at microwave frequencies, the noise floor can become significant. The noise floor can be calculated by multiplying -174 dBm/Hz by the bandwidth, or adding the bandwidth in dB. For example, with a 1 MHz bandwidth (10⁶ = 60 dB), N_T = -114 dBm and with a 1 GHz bandwidth (10⁹ = 90 dB), N_T = -84 dBm.

The noise floor at the output of an amplifier is equal to the thermal noise increased by the amplifier gain and the amplifier’s noise figure. These add when calculated in dB:

N_o = N_T (dBm/Hz) + Gain (dB) + NF (dB)

With a high gain amplifier, even with low noise figure, you can see how the amplifier output noise floor can become a significant limitation in the overall dynamic range. Table 1 shows the noise floor at the output of an amplifier with 10 dB noise figure (NF) for gain values of 25 to 40 dB, and frequency bandwidth of 10, 20, 40 and 50 GHz.

Table 1) Output noise floor (N_o) in dBm for various bandwidths and gains (dB) with NF = 10 dB.

The Spanawave/Giga-tronics microwave power amplifiers were originally developed for the purpose of boosting available power from test and measurement instruments such as microwave signal generators and vector network analyzers. Microwave signal generators can provide significant power at microwave frequencies, typically up to +20 dBm (100 mW). Microwave power amplifiers are most often the best solution for increasing that available power from the signal generators.

The Spanawave/Giga-tronics 2520B Microwave Signal Generator combined with the GT-1000B Microwave Power Amplifier can provide +40 dBm (10 Watts) to 8 GHz and +37 dBm (5 Watts) to 18 GHz, significantly higher than from any signal generator alone.

The Spanawave/Giga-tronics GT-1000B microwave power amplifier is very broadband with nominal gain of 35 dB and noise figure of 10 dB.  

One of the things that Spanawave/Giga-tronics has done to help mitigate the effect of the increased noise floor out of the GT-1000B with its high gain and 20 GHz bandwidth in applications where dynamic range is critical, is to provide remote RF on/off control directly from the 2520B microwave signal generator. Turning RF off on the 2520B microwave signal generator will also turn RF off on the GT-1000B and the noise floor at the amplifier output will drop from approximately -26 dBm to -71 dBm as calculated for kTB with a 20 GHz noise bandwidth (see Table 1).  

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