Tag Archives: Amplifiers
This is a (virtually) math-free introduction to microwave amplifiers from an applications standpoint. There are many references available for the aspiring amplifier designer; this series of posts will attempt to quickly elucidate the relevant factors for the RF system design engineer working to evaluate the appropriate amplifier for her system design.
Types of Microwave Amplifiers
There are many ways to classify microwave amplifiers, but we will group them into four categories based on what role they would play in a generic superheterodyne receiver (shown above). The above system could represent a cellular, backhaul, satellite, or other radio communications link; it could also represent a radar or other imaging system. The transmitter alone could represent a jammer or exciter, and the receiver alone could represent an electronic warfare scanner or a test instrument. The requirements that drive the amplifier selection will be the same for most applications.
When you master phase, you become like a God, capable of performing wonders that mere mortals can only dream of. Wonders like making laser beams (using phase engineered quarter-wave reflectors), communicate tremendous information great distances through thin air (all modern communication formats use both amplitude and phase), and create amazing products (balanced amplifiers, balanced mixers, phased array antennas, Mach Zender modulators, the list never ends).
BUT…phase is the hardest thing to understand in microwaves, RF, and photonics. It is hard to measure, hard to visualize, and makes some very confusing homework problems that kept me in the late night coffeeshops of Champaign-Urbana well past my bedtime.
In this post we will make a dent in the universe of phase understanding by clarifying the difference between phase and group delay, and in the process explain why you can’t match phase with variable line lengths. When you buy a phase shifter, it is sometimes what I would call a real phase shifter, and sometimes what I would refer to as a ‘group delay shifter’. The trombone type variable delay lines (we like the ones from sage) are actually variable time delay elements, and not phase shifters.
A group delay (or time) shift is easy to understand: it is how long the pulse (or wave) takes to arrive at your measuring receiver. Differential delay is therefore the difference in how long it takes for two pulses or waves to arrive. In passive components it is just the distance divided by the speed of light (or whatever your wave is) at your frequency in your material.
Phase is much more difficult. It is the integral of group delay over frequency (plus an offset), or differently the group delay is the derivative of the phase vs. frequency. This is why filters can be used as time delays; the edges of the filter have significant phase variation that leads to significant group delay variations over a narrow bandwidth (this is called Kramers-Kronig relation).
A variable length delay line, therefore, can only change the phase by changing the group delay. But by changing the group delay, you are changing the integral (slope) of the phase vs. frequency. This means that the phase change will be different at different frequencies. This is very different than what you get from a quadrature hybrid coupler, or a balun, where the phase shift is constant across frequencies. The difference is shown below. First is a plot of the phase difference between the two outputs of a BAL-0520 Balun (180°), a QH-0226 quad hybrid (90°), a coupler plus two 37.5° Schiffman phase shifters we developed as a custom (165°), a PD-0220 wilkinson power divider (0°), and a PD-0220 with an extra .570″ adapter on one side (variable).
As you can see, the phase is flat across the bandwidth of the device for everything except the PD-0220 with the extra delay line (adapter). This has a rapidly changing phase across frequencies. If we take the derivative of this we should get the group delay, but instead I measured the differential group delay with the PNA-X.
Here you can see that the differential group delay between outputs for each of the devices is 0, except for the power divider with the adapter, which has a flat constant group delay (ignore the big hump, I think that is from the calculation the PNA is doing with the phase flip).
So what is the lesson? You can phase match two outputs using a variable delay line, but only at a single frequency. Otherwise you have to do it with a coupler, a balun, a Schiffman, or some other true variable phase circuit.
We sell a large product line of packaged amplifiers, primarily for use as LO buffer amplifiers, but that can also be used as general purpose gain blocks. While we don’t design the actual amplifier chips, our expertise is in selecting and packaging these chips to provide easy and guaranteed performance from very low (kHz) to microwave frequencies. Amplifiers can be tricky to deal with, since they generate a large amount of RF power and heat, and use MMIC amplifier chips that can break when not used properly. Here are some guidelines for using Marki Microwave amplifiers:
- Always bias the negative port first: This is because the negative voltage is required to pinch off the gate voltage and ensure that the positive current is limited. If the negative voltage is removed the positive current will flow unimpeded, causing the amplifier to heat up until it burns. This is the easiest way to break a Marki amplifier.
- Only operate the amplifier with a matched load on the output: If the amplifier is run with the output open or shorted, the power has no place to go, and the reflected power may (or may not) burn out the chip. In fact, if you are performing an experiment where the circuit might open or short then measures must be taken to protect the amplifier from the reflection, such as adding an attenuator or circulator.
- Obey the datasheet levels for input power: Providing too much input power will cause the amplifier to clip excessively, and may cause damage.
- Provide some type of heat sinking: Our connectorized amplifiers will operate properly in a room temperature environment with just cabling to remove the heat. If in a heated environment the amplifier will need some way to dissipate heat. For surface mount units make sure that the ground pad has sufficient heat sinking capability to a good chassis or case.
In addition to these precautions to avoid damage, there are some further considerations to optimize performance of the amps:
- For linear operation, input to the amplifier should be below around -15 dBm. This is variable with the gain of the amp, but typically non-linear saturation effects occur around this level. These can be seen with an oscilloscope as clipping of a sine wave, or on a spectrum analyzer as creation of harmonic products (particularly odd harmonics).
- For square wave operation, such as for driving a T3, drive the amplifier at the minimum recommended input power level to ensure clipping and good square wave generation. This can also be seen on an oscilloscope. For T3 optimization the desired goal is to minimize the rise/fall time of the signal.
Phase noise, and amplifier noise in particular, has been a pet project of Ferenc’s for some time. This stems from the fact that, as I will detail in a future post, a passive silicon schottky diode mixer adds very little noise. Normally when customers find excess noise on a conversion it is because of LO noise transmitting to the IF/RF output. We found a recent article called “Phase Noise in RF and Microwave Amplifiers”, by Rodolphe Boudot and Enrico Rubiola (IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control, vol. 59, no. 12, Dec. 2012) where they show plots of phase noise of an oscillator after amplification with different output powers. We decided to perform the same measurement. In our experiment we take a reference 80 MHz oscillator (a Wenzel low phase noise crystal) and measure the phase noise. Then we attenuate the signal by 50 dB and amplify it back up by 50 dB. The input noise floor is at -159.4 dBm/Hz, and the output is at 106.5 dBm/Hz. The noise floor increase is as a result of the amplification and noise figure of the amplifier. The phase noise plots are show below, along with an extra line showing the phase noise of the oscillator with a constant noise level equal to the noise floor of the amplifier added to it.
As you can see, there is very little difference between the phase noise of the amplified signal and the phase noise of the oscillator with white noise added to it. Hence the question, do amplifiers add flicker noise? In the frequency range near the carrier we see hardly any noise addition. This suggest that at a minimum, the flicker noise addition for the amplifier we used (a Centellax wideband amplifier similar to our T3 driver amps) is very low. So low that it’s hard for us to believe that it adds flicker noise at all.