Many amplifiers in a typical RF/microwave system are used for single frequency applications: for local oscillator and clock generation. However, amplifiers are typically designed and characterized only for signal applications. In this short (30 minute) webinar, we will discuss:
• Do gain flatness, IP3, and noise figure really matter for LO drivers?
• Phase noise: the impact of amplifier selection
• Save time with more usable amps: positive gain slope, positive only bias, and SWAP optimized drivers
Presentation (pdf): Amplifiers for Synthesizer and Local Oscillator Generation – The Unsung Application
Recommended Amplifiers for Marki Mixers
Tech Note: How Do Amplifiers Affect Signal Phase Noise
Tech Note: New UC5 Single Supply Voltage Sequencer Package
Enrico Rubiola’s paper: Phase noise in RF and Microwave Amplifiers
Q: Can a Marki Microwave driver amplifier be used as a signal path PA?
The short answer is yes, they can.
The longer answer is that in a lot of RF signal path applications, you may find that the wide bandwidth of the “driver” amplifier offers some versatility for your frequency plan which is really beneficial. The tradeoff is that wideband amps are usually going to have a step down in power efficiency and/or IP3 when compared to a narrowband alternative. Really it depends on your particular application and what you need from the amplifier in that spot. You’ll have to look at the frequency band, the gain, the power output, the noise figure, and the IP3 to see how the overall performance effects your dynamic range when using it. In short, any amp can be used for any reason, it’s just a question of which performance benchmarks has the designer optimized for, and what are the critical parameters that you need for your system.
Q: Does the DC supply contribute significant phase noise relative to the amplifier it’s supplying?
A: This is a great question! Yes, the DC power supply can absolutely contribute a large amount of phase noise into the amplifier that we are measuring in additive phase noise measurements. When we make additive phase noise measurements and show you guys the measurements publicly, we are hiding a lot of the work and trial and error and optimization of the measurement setup that went into the measurement. We’ve seen that by adding a lot of external bypass capacitance on the power supply pins of the amplifier or even using a voltage regulator on the DC source, we can clean up the measurement quite a bit and get rid of nonphysical humps and spurs that will appear on the data.
Q: How do you remove the oscillator phase noise from the additive phase noise measurement?
Here’s a look at the measurement setup we use:
We have an oscillator that is powered within the phase noise analyzer that is fed through a lot of filtering elements to clean up the signal as much as possible, then it’s amplified further then it’s filtered again. The oscillator and the driver amplifier add a lot of phase noise into the input signal that is almost always much larger than the phase noise contribution of the DUT we are measuring, especially if it’s an APM low phase noise amplifier! We are able to suppress this maybe 30 dB or so by splitting out this input tone into 3 paths and then carefully tuning variable phase shifters and attenuators so that the same phase noise in each period is fed in sync to mixers that are in the phase noise analyzer. This method does have limits, so when measuring very low phase noise amps, it’s very helpful to use an exceptionally clean oscillator to generate that input tone to make it easier to avoid falling into a noise floor in the measurement. This is something that can difficult to do, as you need to be very careful not just about the phase matching, but also about the power levels that are presented to the DUT and to the phase noise analyzer itself. It took us a lot of time and effort to be able to start measuring additive phase noise with success and consistency.
Q: Why does phase noise go down when you use parallel amplifiers?
A: The math that goes into this is pretty complicated, but at least to my understanding, this has a lot to do with the amount of current that is flowing into the device, and I believe this is also very related to why the phase noise drops when you drive the amplifier into gain compression – as amplifiers in gain compression will usually pull more DC current. I’m going to cite Enrico Rubiola on this, as he has a much more complete understanding of this than I do: See Rubiola’s work here.
At the risk of being slightly inaccurate to try and simplify it, my understanding is as follows: When it comes to amplifiers, the phase noise comes from the uncertainty in the time it takes for each carrier to make it across the active region of the device – usually because of trap states and the statistical likelihood of a carrier falling into a trap state or escaping from a trap state at any given moment. This is why FET-based devices have higher phase noise, because FET channels are more or less 2-dimensional channels and surfaces/junctions of materials are riddled with traps, whereas HBTs conduct current through the bulk of the semiconductor.
So if you have more current, you have more carriers crossing the channel at a time. So I would compare it to if you ran an experiment with 10 coin flips, 100 coin flips and 1000 coin flips and so on. The larger the sample size becomes, the less overall variation you expect in the total outcome in terms of a percentage. So more carriers crossing a channel at once causes the probability density function of the randomness in the overall transient uncertainty in each period to collapse towards the expected amount of time per period, which is the period of the carrier frequency.
Q: What do you mean by amplifiers in parallel when talking about phase noise measurements?
We used an RF splitter and then used attenuators to make sure that the DUTs always saw the same amount of input power regardless of the test condition.
Q: Why is there abrupt changes in phase noise in measurements very far away from the carrier in your phase noise measurements/why are there so many spurs in the phase noise data?
A: The short answer here is – phase noise measurements are really hard! There are a lot of signals involved in the measurement – frequencies within the DC power supply, frequencies in the phase noise analyzer itself, the frequency from the wall power supply, etc. And we are measuring power levels at extremely low levels (sometimes over 165 dB down from the carrier!). Extremely minute contributions from all of the circuitry involved in the measurement can result in some pretty dramatic spurs at that level, and it’s quite difficult to truly clean up the data entirely. We also end up seeing some pretty bizarre non-physical behavior when you get far away from the carrier or very close to the carrier. This is some of the reason why phase noise is usually quantified at 10 kHz offset from the carrier – you’ll notice that this is the region where the measurement is the most clear and clean. When you see phase noise data with no spurs, the reason is usually because the spurs were manually removed from the data. We’ve observed this effect on multiple types of measurement systems.
Q: Why does positive gain/Psat slope offset losses from interconnects?
A: Since the interconnect losses are higher at higher frequencies, a lot of system designers will put some equalizers into their chain to add a little more loss at the low end than at the high end to level out the overall spectrum. In the case of a positive gain slope (or a positive Psat slope if in gain compression), you are basically building an equalizer into your amplifier curve, so it accomplishes the same thing.
Q: Is it necessary to use a linear regulator after the charge pump in your UC5 board?
A: We don’t but this was something we considered when we were designing it. The inverting charge pump does use an internal oscillator to generate the negative voltage, however, the magnitude of this oscillator tone that comes out is proportional to the current that you are drawing out from it. Since this is a FET based device with nominally zero gate current, the tone contribution was miniscule. We measured this and put a section for it on our website, but even without a linear regulator on this negative voltage, the power of this spur was suppressed by about 100 dB from the carrier, so it’s pretty hard to imagine that actually impacting anyone’s system performance.
Q: Can even harmonic distortion presented to the LO of a mixer cause a DC offset that would require DC blocking?
A: Usually the LO port of a mixer is either an open or a short to ground, so most of the time it won’t matter much in this regard, but even harmonic distortion in the LO can manifest itself as a duty cycle distortion in the mixer and it degrades the multi-tone suppression.