AM AND PM NOISE CALIBRATION AND MEASUREMENT USING A GAUSSIAN NOISE ARTIFACT AND CARRIER SIGNATURE SOURCE

by Perry C. Bates

Measurement accuracy of AM and PM noise can sometimes be very elusive. Proven devices and methods are available to allow these measurements to be accomplished to an accuracy of ±.75 dB under controlled conditions, and ±1.5 dB under general conditions, such as engineering and production line environments. In addition, calibration and metrology traceability can be maintained for each AM and PM noise measurement which incorporates the entire noise measurement system, not just each component of the system individually. The device used to provide AM and PM noise traceability is a Noise Artifact, Techtrol Cyclonetics, Inc. model NAL2000 series.

GAUSSIAN NOISE ARTIFACT CALIBRATION

The NAL2000 series instruments are designed to be used as traceable metrology, AM and PM noise standards and can be incorporated into a noise measurement system to facilitate traceable AM and PM noise measurement to a known accuracy. The NAL2000 provides a reliable and eloquent but simple method of achieving noise traceability for both the calibration of a noise measurement system or the resulting noise measurement data. The architecture of the NAL2000 series contains only one active device and is therefore very reliable. The Noise Artifacts can easily maintain accuracy over a 12 to 18 month traceable calibration cycle.

The method used is derived from a NIST patent for which Techtrol Cyclonetics, Inc. has a license. The method combines a low noise carrier frequency and a Gaussian noise source which has been measured to have very flat AM and PM noise components. The Gaussian noise is combined with a carrier to allow measurement. A device such as the Techtrol Cyclonetics, Inc. LNC414, generates a suitable carrier and will be described later in this paper. The noise level needed for calibration is produced through amplification and filtration to achieve a bandwidth of slightly greater than 10% and a power level of approximately -110 dBm/Hz. The carrier frequency power level is accurately measured and combined with the noise to achieve a very accurate carrier to noise level dBc per Hz.

The carrier which is applied to the NAL2000 is equally split into two carriers, with one carrier being used as a reference. See the product data sheet for a block diagram. The other carrier from the power splitter is used to produce the noise calibration signal, after being combined with amplified and band restricted noise. The reference channel incorporates a phase shifter in its signal path which allows setting the two carrier outputs to quadrature during the PM noise calibration process. This feature provides for measurement of the noise measurement system noise floor.

The noise from the noise channel can be turned on and off as desired. The result is that two coherent and correlated carriers are produced, one having a higher but accurate noise level associated with it. The use of a phase shifter allows the two carriers to be canceled when the phase discriminator is placed into quadrature during a PM noise measurement. The calibrated noise channel provides a flat calibration data line when the noise is turned "on" and when measured by a noise measurement system. The noise calibration line is approximately -130 dBc if a + 20 dBm carrier is used. The noise calibration line provides calibration over a 100 Hz to 40 MHz bandwidth at X-band. With the Gaussian noise turned "off", cancellation of the carrier and its associated noise persists.

The phase discriminator is biased by the carrier with the noise measurement system's noise floor being measured. The AM calibration process is similar using a detector and only the noise channel. Unlike tone calibration methods which only provide insight to measurement accuracy at one offset frequency and therefore an undefinable error interval, the Noise Artifact method provides a known window of accuracy and a very accurate error interval over the entire frequency offset range. As the offset frequency from the carrier becomes less, de-correlation within the frequency discriminator is decreased. This tends to modify the calibration noise towards the actual carrier noise component. A very low noise carrier frequency source is therefore required. See the LNC414 product brochure. The amplified and band restricted Gaussian noise used for the calibration is very accurately measured by Techtrol or NIST using specific methodology to achieve high accuracy. When used with appropriate noise measurement calibration methods, a traceable AM and PM metrology standard or Noise Signature Artifact is achieved. The unique aspect of the NAL2000 noise artifact is that very accurate noise error interval can be established in the realm of ±.75 dB or better for AM and PM noise measurements. This error interval can be established with traceability from a primary standards lab and therefore allows complete systems, instruments or components to be measured for AM and PM noise with traceability for the first time. In addition, most noise measurement systems are only accurate to ±2 dB for PM and typically worse for AM. By incorporating the NAL2000 into a noise measurement system, a minimum of a 100% improvement in noise measurement accuracy in achieved. In addition, anomalies in the noise measurement can instantly be spotted, which greatly improves confidence in the measurement and avoids erroneous or misleading data. The Noise Artifact can also be incorporated into a system which allows rapid traceable calibration of phase discriminators and diode detectors. This greatly enhances the confidence of AM and PM noise measurement and the data accuracy of a noise measurement system. Tone modulation is incorporated with the Low Noise Signature and combined with the Gaussian Noise Artifact to allow sensitivity calibration of phase discriminators used for PM measurements and diode detectors used for AM measurements.

The tones are also used to determine the orthogonality or rejection of the converse noise, AM/PM and PM/AM. The Gaussian noise is amplified and therefore noise bandwidth is a critical factor. With noise " OFF" the noise floor of the Noise Artifact needs to be at least 15 dB below the noise of the amplified Gaussian noise with the noise turned "ON". This factor restricts the noise bandwidth to approximately ? 10% at a given center frequency. The NAL2000 series Noise Artifact is therefore provided in specific frequency bands, at the customer’s discretion, as to center frequency. Multiple frequencies are also achievable taking into account the Gaussian noise bandwidth verses the noise measurement calibration bandwidth. That is to say, with a 10 GHz NAL2000 Noise Artifact having a useable ±1 GHz bandwidth, several ±10 MHz calibration bandwidths are achievable.

THE NAL2000 PROVIDES:

1. AM noise calibration, error interval ±1 dBm.
2. PM noise calibration, error interval ±1 dBm.
3. PM noise measurement system noise floor determination.
4. Calibration of a phase discriminator sensitivity at a given power level for PM noise measurement.
5. Calibration of a diode detector sensitivity at a given power level for AM noise measurement.
6. Calibration of a phase discriminator AM rejection level at a given power level for PM noise measurement.
7. Calibration of a diode detector PM rejection level at a given power I level for PM noise
measurement.

EXISTING FREQUENCIES:

100 MHz, 110 MHz, 400 MHz, 440 MHz, 640 MHz
800 MHz, 880 MHz, 1240 MHz, 2480 MHz, 4860 MHz
9.92 GHz, 9.6 GHz, 9.7 GHz, 10.0 GHz, 10.36 GHz
Customer specified frequencies are no problem.

CARRIER SIGNATURE CALIBRATION

A carrier source is combined with the Gaussian Noise Artifact to provide the I bias for the AM detector or the PM discriminator. The Carrier Signature Source itself, however, is generally the method used to evaluate the noise measurement accuracy of a noise measurement system by using it as a transfer reference. A Carrier Signature Source, such as the Techtrol Cyclonetics, Inc. LNC414 series very low noise source, can be used as a traceable noise signature by itself, if calibrated properly. The LNC414, one of several sources available from Techtrol, is typically used for AM and PM calibration of RADAR, single carrier, noise measurement instruments. The accuracy achieved is ±1 1.5 dB.

The Noise Artifact needs to have very good close to the carrier noise performance to achieve calibration closer than 1 KHz from the carrier. The LNS, LNC and RM series Carrier Signature Sources have been designed to achieve this performance. Carrier Signature Calibration is not as reliable by itself as when combined with the Noise Artifact. To generate very low noise close to the carrier, a carrier signature is generated through the use of crystal oscillator based technology and therefore a long chain of active devices is involved. Even if close to the carrier noise is not an issue and a fundamental, free running oscillator is used, several factors such as temperature, humidity, extraneous noise and aging of the components within the source will affect the noise signature. The most reliable method to achieve AM and PM noise calibration and traceability is to combine both methods.

The LNC414 for example, is also very useful for measuring the residual AM and PM noise of amplifiers. Because the AM and PM noise signatures of the LNC414 are very low, good carrier cancellation can be achieved during a residual PM noise measurement. Residual noise is defined here as the additive noise added to a carrier during the amplification process. A residual PM noise measurement can be accomplished by feeding a carrier into a power splitter using one branch as a reference signal and the other branch incorporates the amplifier under test. Both signal paths are then feed into a mixer. When the mixer is placed in quadrature, the carrier and its associated noise will be canceled. The remaining amplifier additive noise is then measured. Additive AM noise is somewhat more difficult, because an AM detector typically uses only one carrier. This means that the carrier source AM noise must be lower than the amplifier AM noise. In some cases at higher frequencies, a Gunn oscillator is useful. If extremely low noise performance is not expected, a free running oscillator at the desired frequency can be used. Another method to understand both the additive AM noise level and to allow identification of where the noise source within the amplifier originates, is to measure the amplifier under gain compression. This effectively measures the AM to AM performance of the amplifier and also is useful in evaluating its AM to PM performance.

CONCLUSION

The NAL2000 series Noise Artifacts and the LNC414 series Carrier Signature Sources make up a very powerful tool for AM and PM noise metrology and calibration. For a description of an existing noise calibration system which has been functioning for over three years, see the white paper "Phase Noise Calibration" by William F. McMillan and Perry C. Bates. Also see the white paper "Product Description and Application for Model NAL2010 AM and PM Noise Calibration Reference" by Perry C. Bates.