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.

Amplifiers

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:

Amplifier

MSA1105

UTO1005

UTO1023

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.

Filtered Amplifiers

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.

Frequency Multipliers

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.

Mixers

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

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.

Misc.

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

Our
In-house Systems

Vibration Induced
Phase Noise