Low Phase Noise Systems – Hints and Tips
Designing low phase noise systems that take full advantage of modern high-performance sources can prove quite challenging. Virtually every component in the system can seriously degrade the phase noise or generate unacceptable harmonics, sub-harmonics and spurious signals and the phase noise performance of most RF modules is not specified. In addition to the intrinsic noise of the individual components, spurious signals and phase noise can result from undesired and often unexpected interaction between components – even system power supply noise and signals can modulate RF devices sufficiently to degrade system performance. To combat these various phase noise demons, the designer needs a large bag of the “tricks of the trade” and this paper will review a few favorite techniques to add to the bag. Many of the following techniques are incorporated into the Blue Tops RF Modules.
Packaged MMIC amplifiers are the most common component found in RF systems and several devices offer good phase noise performance. Evaluating MMICs for phase noise performance from data sheet information can be difficult. When a predominant carrier is present, the phase noise far from this carrier may be estimated by adding the amplifier’s noise figure to -174 dBm and subtracting the signal level (all in dB). For example, a +3 dB noise figure amplifier with a -20 dBm input level would exhibit an Sθ of -174 + 3 – (-20) = -151 dBc.
But for complex signals or frequency offsets near a carrier the MMIC will exhibit 1/f or “flicker” phase modulation as the L(f) noise table for popular MMICs shows below:
|Phase Noise 100 Hz, 1 kHz, 10 kHz||-158, -165, -170||-167, -171, -172||-165, -170, -172|
|Max. Output||+17 dBm||+20 dBm||+28 dBm|
These L(f) numbers were measured at 5 MHz using two Wenzel Associates ultra-low noise oscillators. Most MMIC manufacturers do not supply such phase noise data and experimentation is usually required – the MMICs above are among the best. Wenzel Associates offers phase noise tested amplifier modules (LNBA) featuring popular MMICs chosen for their phase noise performance. These amplifier modules have built-in attenuators at both the input and output to optimize the signal levels and to improve reverse isolation and VSWR.
In addition to phase noise, amplifiers can generate undesired harmonics or intermodulation products at both the input and output. Harmonics produced at the input of an amplifier are often overlooked and these undesired signals can work their way “upstream” spoiling the signal quality in other parts of the system. An input attenuator can reduce harmonic generation by preventing amplifier overload and will attenuate distortion generated by the amplifier by the attenuation value. If the input signal frequency range is narrow, another option is to add a bandpass filter at the input of the amplifier to pass only the desired signal and block harmonics – in both directions. The input filter also reduces intermodulation products when amplifying a large signal in the presence of other undesired signals. Filtered amplifiers are also useful with filters exhibiting recurring passbands. Wenzel’s new filtered amplifiers (LNFA) include a bandpass filter, input and output attenuators and the MMIC amplifier in one package. The dual filtered amplifier (LNFDA) from Wenzel Associates has a low-pass filter and attenuator on both the input and output and an additional attenuator between the two MMIC amplifiers. When it is desired to have a filter at the output of the amplifier, choose the amplifier filter (LNAF). Stand-alone filters (LNF) are also available featuring low microphonic and noise components and construction.
Frequency Distribution Amplifiers
Systems that buss a standard frequency to several points within the instrument are well served by a carefully designed distribution amplifier. In many instances, an ordinary power splitter is sufficient but the designer should be aware that the isolation will be modest and may be compromised by the need for additional amplification after splitting. If amplifiers are used on all outputs to increase the isolation, choose an amplifier with high reverse isolation to forward gain ratio and consider the addition of passive attenuators in the signal paths. Low noise standard frequency sources can exhibit close-in noise below the flicker level of available MMICs and a discrete design may be necessary. Bandpass or lowpass filters on the outputs can increase port-to-port isolation at frequencies away from the standard frequency. The LNDA Frequency Distribution Amplifier from Wenzel Associates has a noise floor of -178 dBc and only -150 dBc at 10 Hz offset and provides up to +16 dBm on all eight outputs. The intrinsic port-to-port isolation is 60 dBc and individual attenuators are available for additional isolation. The unit includes an internal voltage regulator with excellent line rejection. The internal amplifiers include bandpass filters to reduce harmonics and increase port-to-port isolation away from the carrier.
Various frequency multiplier topologies are available, all of which exhibit some noise in excess of the theoretical noise degradation due to multiplication (20 log n). Here are a few types to consider:
Step-recovery diodes make excellent frequency multipliers but the signal levels must be high to avoid excessive flicker noise. Input levels above 26 dBm are commonly used but the conversion loss can be quite low.
Logic devices may be used for frequency multiplication and the phase noise performance can be fairly good. One approach is detailed in “HCMOS Gates Make Frequency multipliers“. It is best to condition the signal before applying to the logic device to enhance the slope at the logic threshold. (See: “Waveform Conversion“.) Odd-order frequency multipliers may be realized simply by filtering the output of the circuits used to “square up” the sinewave for the logic device. An example using a differential amplifier is titled, ” Frequency Tripler using the CA3028“.
An excellent JFET frequency doubler was developed at the National Institute of Standards and Technology (NIST) for low frequencies. The Wenzel Associates LNHD Model is a unique implementation of this design for input frequencies up to 25 MHz with a companion VHF version (LNVD) for input frequencies to 350 MHz. A quadrupler combining two of the VHF doublers is available.
Schottky diode doublers can provide excellent phase noise performance and provide reasonably low conversion loss. An example, the FD25 from Watkins-Johnson, has an input referred noise near -174 dBc. Doublers may be purchased from several manufacturers or the designer may choose to build his own with special characteristics. A typical design is described in, “Switching Diode Frequency Doublers“. Bandpass or lowpass filters are often necessary to control the multiplication products at both the input and output of diode doublers. The Wenzel Associates model LNDD features the FD25 or FD25H and includes input and output bandpass filters and an output attenuator.
A unique diode odd-order multiplier is described in, ” New Topology Multiplier Generates Odd Harmonics“, reprinted with permission of RF Design Magazine. A special version of this topology is available in the Wenzel model LNOM which also includes three bandpass filters and two MMIC amplifiers. These odd-order multipliers feature very low flicker and excellent noise floor.
Low noise mixing usually involves Schottky diode double-balanced mixers but these devices can create hair-pulling problems! The primary problem is caused by the intermodulation products that appear at all of the mixer ports and the most effective solutions include filtering and buffering. Mixers are indispensable function blocks but mixing should be avoided when simpler approaches are available. For example, if a desired frequency may be achieved by multiplying instead of mixing, the surrounding filters will confront a few bright lines instead of a forest of spurious signals. If mixing is chosen to synthesize a low phase noise carrier, it is usually best to apply the higher of the two frequencies to the IF port at the mixer’s design level and the lower frequency to the RF port at the maximum allowed level. The reason for applying the lower frequency to the RF port is that the allowable RF input level is typically below the IF level so the sidebands from the lower frequency will be lower at the output. They are more difficult to filter since they will be closer to the desired frequency. It is desirable to reduce the RF port level to reduce spurious signals but the phase noise will be degraded at some point. The higher level mixers will give the best phase noise if suitable signal levels are available. Mixing is a low noise way to generate higher frequencies since the noise powers of the two sources simply add but the spurious signals can be more trouble than the improved noise is worth.
Frequency dividers will, in theory, improve the phase noise of a carrier just as a multiplier degrades the noise. In practice, logic devices are usually noisier than the better sources and the faster devices generate harmonics which can spread throughout a system. Bandpass filters may be needed on both the input and output to control the spurious frequencies. (See: “Waveform Conversion“.) The LNFD is a divide-by-two (or four) with bandpass filters on the input and output and an input conditioning amplifier which accepts signal levels from 0 to +20 dBm. (Other reading: “Unusual Frequency Dividers“.) The Bluetops line also includes an unusual regenerative divider that provides 1/2 and 3/2 F with excellent phase noise performance.
Some other trouble spots include:
- The power transformer’s magnetic field – consider a toroid design, preferably with an internal electrostatic shield. Mu-metal shields are often helpful.
- Noisy devices can degrade power supply rails and can generate noise on grounds. Try to control the current paths of noisy devices to keep the current away from sensitive components. Offending devices may include three-terminal regulators, logic, and motors. Even low noise oven oscillators may have noisy oven current. (Other brands, of course!)
- Some devices have high microphonic sensitivity and may require vibration and acoustic isolation.
Phase Noise Measurement