The Sound of Silence - Low Noise Amplifiers

The Sound of Silence

In many applications spurious noise of RF signals can present a major problem. In cases where RF energy is used simply to heat or agitate via a transducer, noise may not be so significant. However, in instances where the signal is being used to carry information or provide information after being incident upon some load, noise can be very detrimental to the quality of the system.

Noise on the output signal of an RF Amplifier can come from a variety of sources. Each of these sources needs to be considered separately in order to eliminate or minimize the noise in each case.

There is the noise that is generated in the RF Amplifier itself. This is due to non-linear elements that cause distortion of the signal in the time domain, which translates to harmonically related spurious signals in the frequency domain. In many cases, by virtue of the fact that the harmonics are at a higher frequency than the fundamental, they can be filtered out by an RF low pass filter. However, in broadband applications, this may not be possible. E&I's broadband amplifiers are designed to operate well within the linear range of the RF transistors to ensure minimal distortion from that source, all magnetic cores in the circuitry are operating well below their saturation point and finally, push-pull circuitry is used to minimize the second harmonic. Consequently, most of our amplifiers are rated at -25 dBc mx harmonic distortion at rated output power. This means that the worst case harmonic distortion is 25 dB below the power level of the carrier. In a 100 Watt amplifier this means that the power in teh harmonic is about 0.3 of a Watt or 300 mW.

Then there is noise from the AC input to consider. Power companies, in addition to providing the 50 or 60 Hz that you ask for, tend to throw a few extra bits in free of charge. And other equipment operating in the vicinity can put noise onto the AC line input. We use two line filters operating in series to prevent this distortion from entering the amplifier circuitry. These line filters also prevent any unwanted signals from getting out of the amplifier and onto the mains which could cause other equipment problems.

The AC power is converted to the DC power that the RF Amplifier requires, by a switch mode power supply, SMPS. These types of supplies, working in the KHz region, generate noise that can mix with the RF signal and so cause non-harmonically related spurious noise. The figure below gives an example of how this can manifest itself.

This shows noise at around 20 KHz with a power gain of - 54dBm, see left image. This may not seem significant as in Watts it only represents 0.004 µW. But, many specifications call for this level to be 0.001 µW.

In this case the SMPS that we use has a lot of internal filtering, however, there is still some noise present.

Additional circuitry was designed to reduce this noise even further and yet does not compromise the fidelity of the RF signal. It is important when adding circuitry that the slew rate of the amplifier is not negatively affected and the rise and fall time numbers for pulse applications are not adversely impacted.

As can be seen here, (see right image) the noise was reduced dramatically to less than -77 dBm or 0.00002 µW. We were able to achieve this with no impact on the RF performance.

Similiarly we can look at the result in the time domain, as voltage. Before the filter circuit was implemented we could see low level noise at 2.3 Volts p-p, (see below, left image).

And afterward at less than 160 mV p-p, (see above, right image). This work was done on the E&I 350L Broadband RF Amplifier. It is now being implemented on all of our models.

Tony Harris; President and Jeff Kopalek; Service Manager

July 2011