Application Note – 36
Design of Distribution for RF Signals
Overview
Many applications in digital video broadcasting, terrestrial and satellite communications, require a precision frequency reference, often an RF sine wave in the 1MHz to 30MHz range, in order to insure precise, on frequency transmission. Many of these systems also require a precision timing signal (e.g. one pulse per second or 1PPS) to synchronize in the time domain the many items of equipment used in final signal generation.
Once these precision signals are generated, it is often required to distribute them to a number of items of equipment (up converters, down converters, satellite modems etc.). The quality of the distribution equipment used is of paramount importance in maintaining the quality of the originally generated signals.
Background
The vast majority of time and frequency applications today utilize the GPS or other satellite system to provide a highly accurate reference. Due to the nature of the GPS (or other satellite) signals, the raw reference provided is rarely suitable directly. This is due to the “noise” introduced due to the nature of the signal, which is very weak by the time it reaches the earth, travelling through unpredictable atmospheric conditions.
The received signal requires significant post-processing before it is suitable for use as a precision reference. This post processing is achieved by utilizing high quality oscillators (sometimes atomic oscillators such as rubidium) and “disciplining” (locking) them to the incoming received signal.
The output from this reference now has a very low phase noise, determined by the quality of the oscillator, is highly accurate (locked to the satellite reference), and can be divided down to produce a short (typically 20 to 25 microseconds) pulse, once per second that has a much lower noise (jitter) than the original received signal. These signals are used as the reference and synchronization for the other equipment.
Signal Distribution Important Parameters
On the face of it, distributing these signals is simple, however, the devil is in the details!
The key parameters of importance in the generated reference signals are usually;
- Accuracy
The accuracy of the generated frequency, expressed as an average value over a period of time, usually 24 hours.
- Stability
This is a slightly more difficult concept, but typically describes the variability of the signals over a specified time period (1 second, ten seconds etc.) in the time domain. This is a very critical parameter in determining the “real time” (instantaneous) accuracy of the signals, that can never be better than the worst stability. Often stability is expressed in terms of jitter (short term stability), and wander (long term stability)
- Phase Noise
Phase noise is another parameter for determining stability, usually in the short term (.1Hz, 10Hz, 100Hz) and expressed in the frequency domain (i.e. inversely proportional to stability). This a measure of signal quality typically applied to RF sine wave.
- Jitter
Jitter is yet another way of describing instability (noise), usually in the time domain, expressed in pico/nano/micro seconds and usually as an rms (root of the mean of the squares) number, effectively the integral of phase noise numbers in several frequency bands. Jitter is typically used as a measure of signal quality of digital signals (e.g. our generated one pulse per second).
- Isolation
Isolation is a measure of the extent that individual outputs are independent of (not influenced by) the other outputs or input.
- Noise (harmonic/non-harmonic)
Noise is also a measure of signal quality, and is a measure of the additional external noise added to the reference signals by the distribution system itself. Usually noise is divided into two areas, harmonic noise is directly related to the reference signal, and is noise at harmonic frequencies of the reference. Non-harmonic noise is completely unrelated to the reference frequencies, e.g. mains hum from the power supply.
Signal Distribution Design Considerations
In terms of the above identified parameters, the design of a high quality distribution amplifier primarily focuses on stability, phase noise, harmonic/non-harmonic noise, and isolation.
Isolation is not a characteristic generated by the reference, as it is an expression of the impact an output perturbation of one of the distributed outputs, has on the input, or on other outputs of the distribution.
Stability and phase noise on the other hand, are characteristics that define the quality of the reference signal, and therefore the additive effects of these parameters generated by the distribution amplifier itself are required to be minimal.
For all but the most exotic frequency references, typical values for stability and phase noise parameters are shown below:
Stability expressed in terms of Allan Deviation
Tau (seconds) Allan Deviation
1 3 x 10-12
10 5 x 10-12
100 1 x 10-12
1000 5 x 10-13
Phase Noise
Single Side Band Phase Noise expressed in terms of dBc / Hz at frequency offsets from a 10MHz carrier
Frequency Offset(Hz) Phase Noise(dBc/Hz)
1 -100
10 -125
100 -150
1,000 -155
10,000 -160
100,000 -162 (typical noise floor)
It is the additive effects of the phase noise and (in)stability generated within the distribution amplifier in which we are interested.
The addition of these quantities is not a simple linear equation. For example for phase noise, the equation for calculating the effect of additive phase noise is shown below;

(this is described in more detail in the ptf Frequency and Time handbook available from the downloads page at www.ptfinc.com)
As an example, if the distribution amplifier generated phase noise was equal to the reference phase noise of say -125dBc/Hz at a 10Hz offset from the carrier this would result in;

20 log10 √ ( 10 -125/10 + 10 -125/10 )
= 20 log10 √( 3.162 x 10-13 + 3.162 x 10-13)
= 20 log10 √ ( 6.324 x 10-13 ) = 20 log10 (7.95 x 10-7)
= 20 x -6.099 = -121.989 dBc/Hz ~ -122 dBc
i.e. the output from the distribution amplifier would be approximately 3 dBc worse than the input reference source, not very good!
However, if the distribution amplifier generated phase noise is significantly less than the reference phase noise at 1Hz, for example the distribution amplifier generates a phase noise of -150dBc, that equation now becomes:

20 log10 √ ( 10 -125/10 + 10 -150/10 )
= 20 log10 √( 1 x 10-12.5 + 1 x 10-15 )
= 20 log10 √ ( 3.172 x 10-13 + ) = 20 log10 (5.63 x 10-7)
= 20 x -6.24 = -124.986 dBc/Hz
In other words, the impact of the distribution amplifier additive phase noise is almost negligible, with a degradation of just 0.014dBc.
Phase noise is directly related to stability, and the additive effects of distribution amplifier generated instability can be calculated in a similar way.
The key then is to generate a design, in which these parameters are sufficiently small to have virtually no impact on the signal being distributed, and to insure to the greatest extent possible that the isolation between outputs, and input/outputs is sufficient that unwanted perturbations one output will not affect other outputs.
The main considerations for generating such a design, lay in the selection of suitable components, power supply generated noise, and the physical layout of the printed circuit board and the enclosing chassis.
The design of the ptf 1203C broadband RF distribution amplifier utilizes two stages of buffering, one on the input and one on each output. Key characteristics of the broadband amplifiers used within the design are:
Phase noise: -130 dBc/Hz @ 1Hz
-165 dBc/Hz @ 1kHz (floor)
Bandwidth: 500kHz to 60MHz
Allan deviation: 1e-13 @ 10sec
Harmonic distortion: < -54 dBc
Input to output isolation -80dB (theoretically can give up to -160dBc for two stages)
Additional considerations are that the internal amplifiers must be operated well within their voltage operating range to insure there is no partial saturation that would cause additional distortion, and that the circuit board material must be selected for minimum variation with temperature that could contribute to instability and variations due to temperature.
As an indication, the final ptf 1203C design has channel to channel skew at 10MHz of less than 2 nsec, and a temperature sensitivity of approximately -2 psec/degC (mainly due to the PCB).
Summary
Generating high quality reference signals suitable for use in applications such as satellite communications, HDTV broadcasting, and many others, can be quite challenging and expensive.
Distribution of these signals to multiple pieces of equipment (up-converters, down-converters, satellite modems and many others) is often required, and whilst on the face of it, distribution amplifiers appear to be quite simple, a poor design can significantly distort a high quality reference signal, resulting in degradation of performance of an overall system.
Precise Time and Frequency, Inc. has been designing and manufacturing a range of RF and other distribution amplifiers for well over a decade, and these designs are used in many performance critical applications around the world.
Major companies rely on this uninterrupted performance for industrial, commercial, and military applications, many of a critical nature.
For further information on this or other time and frequency applications, please contact Precise Time and Frequency, Inc. or visit the web site at www.ptfinc.com .