PRECISE OFFSET
Many interference problems are due to erroneous planning of the distribution of frequencies in the RF spectrum.  Because of this, all operating transmitters do not have an accurate means of frequency stabilization.  Even the latest generation do not have a standard reference. In the specific sector of AM/FM television transmissions presently in use in the world, frequency stability is very important because in some service areas, channel sharing occurs. For the condition of minimum visual tolerance between two iso-channel transmitters to exist, or “peaceful cohabitation”, the difference between two transmitter fields must be at least –45dB.  But if the frequency difference between the transmitters is +/- 1 Hz, still maintaining the same image quality, it can be lowered to –25dB.  In practical terms, after having synchronized the interference of a 1000W transmitter it will be the same as a 10W transmitter which has a frequency synthesizer. In digital transmission, a frequency synchronizer is obligatory.

ADVANTAGES
The application of high precision frequency synchronization using our “Star Look” on all the transmitters presently in use would solve 80% of the interference problems among television transmitters.  In Italy’s RF environment, surely the most chaotic in the world, it will have the effect of a true “air purifier”.

THE SOLUTION THAT “FELL FROM THE SKY”
The generation of a highly precise reference frequency with high stability has until recently been very costly and difficult to do. Furthermore, such systems were difficult to manage and maintain calibrated, even without considering the inconvenience and weight of conventional atomic generators. With our equipment, appropriately named “Star Look”, we have solved this persistent problem by being able to offer the market an ultra economic frequency generator with 1Hz resolution and 0.002Hz stability guaranteed anywhere in the world when correctly installed.

THE STANDARD OF THE “STARS”
The sample reference of our synthesizer is based on a GPS receiver produced exclusively by Hewlett Packard which receives from the GPS system’s constellation of satellites the same frequency information that is used by physicists, laboratories, space research institutes, aircraft, ships, missiles (including Cruise and Tomahawk). After one hour of functioning, the receiver is able to generate a sample frequency with a stability of 1x10E-11 or better.

THE SYNTHESIZER
Instead of the usual PLL with relative dividers, our generator “constructs” the reference signal (for the PLL reference) with digital technology (DDS).  The resulting system potentially is able to generate any frequency with a resolution of 1Hz. The setting of the output frequency is accomplished via software without any manual adjustment.  The range of the output signal is between 50MHz and 5GHz (for now).  In the near future other frequencies will be available up to 15GHz. The power supply section has plenty of head room and is designed in a modular way to ensure quick and easy upgrading.  The power supply section includes a rechargeable battery that guarantees an autonomy of 20 minutes (extendable by request) in case of black outs or macro interruptions of the power network.

“NO SIDE EFFECTS”
Since the generator is going to be integrated in pre-existing systems (often in limited spaces), it has been engineered to be small and light weight (2 standard rack units, 3Kg, 7 lbs) with a power consumption of 20W in its standard version.  The output section of the generator is configured as a local oscillator plus IF section (intermediate frequency i.e. 38.9MHz) plus 5 outputs of 5MHz or 10MHz for the synchronization of other equipment that needs an external reference with high stability (other synthesizers, test instruments, etc.).
 

HOW DOES THE GPS HP GENERATOR WORK?
In the early 1980’s the U.S. Department of Defense decided to launch a project costing 10 billion dollars to create a system capable of providing the exact geographic position (1 meter tolerance) of a receiver connected to the system in any part of the world.  The system was called GPS (Global Positioning System).  The system is composed of a constellation of 24 satellites (Fig. 1) whose position is constantly controlled and verified from the earth via a master station site in Falcon, Colorado and four control stations around the earth (Fig. 2, 3).

Figure 1

 

Figure 2

Figure 3

The 24 satellites of the GPS system have been placed into orbit following the order in the chart below and they have a guaranteed life of at least 20 years (Chart 1). The GPS satellites send with extreme precision countless bits of data (4MHz bandwidth) in pre-determined intervals on the frequency of 1,575.42MHz using PRN modulation (Pseudo Random Noise) which distributes these bits in a seemingly random way.  This signal’s field strength on the earth is extremely weak (about –145dBW), which is below the noise level.  For this reason, they are not receivable with the standard spectrum analyzer.  But with the special antenna HP designed for this application, these signals can be received. The reception of bit data in the receiver happens through the process of “collation”.
 
 

Block & NAVSTAR#
SVN
PRN
Launch Date
Operational
Date Status
II-1
14
14
02/14/89
04/14/89
Operating
II-2
13
2
08/10/89
07/12/89
Operating
II-3
16
16
08/17/89
09/13/89
Operating
II-4
19
19
10/21/89
11/14/89
Operating
II-5
17
17
12/11/89
01/11/90
Operating
II-6
18
18
01/24/90
02/14/90
Operating
II-7
20
20
03/25/90
04/19/90
Operating
II-8
21
21
08/02/90
08/21/90
Operating
II-9
15
15
10/01/90
10/20/90
Operating
IIA-10
23
23
11/26/90
12/10/90
Operating
IIA-11
24
24
02/03/91
08/30/91
Operating
IIA-12
25
25
02/23/92
03/24/92
Operating
IIA-13
26
26
04/10/92
04/25/92
Operating
IIA-14
28
28
07/07/92
07/23/92
Operating
IIA-15
26
26
09/09/92
09/30/92
Operating
IIA-16
27
27
11/22/92
12/11/92
Operating
IIA-17
32
1
12/18/92
01/05/93
Operating
IIA-18
29
29
02/03/93
04/04/93
Operating
IIA-19
22
22
03/30/93
04/13/93
Operating
IIA-20
37
7
05/13/93
06/12/93
Operating
IIA-21
39
9
08/28/93
07/20/93
Operating
IIA-22
35
5
08/30/93
09/28/93
Operating
IIA-23
34
4
10/26/93
11/22/93
Operating
IIA-24
38
5
03/10/94
03/28/94
Operating
Chart 1

The microprocessor in the GPS receiver keeps in its firmware the code strings of all the satellites and comparing its own data with that of the received signal, it recognizes and synchronizes with what the first satellite receives.  With the data from this satellite, it automatically excludes all the satellites out of its view and proceeds to find and synchronize with the other satellites in its view. The HP synchronizer used in the “Star Look” contains a special software filter capable of eliminating or drastically reducing the noise deliberately introduced for military security reasons.
The setting of the clock (Fig. 4) is a simple procedure.
 
 


Figure 4

The satellite transmits impulses and the receiver calculates the time between successive impulses.
The elimination of all errors due to propagation delay is done in the receiver comparing the data received from four different satellites (Fig. 5). The end of this complicated operation is the generation of a clock signal locked to the precision, stability and phase of the GPS system.

To obtain maximum precision and best rapidity, the receiver antenna must be able to receive the most distant satellites (those situated near the horizon).  In fact, the greater the distance, the greater the time needed for a signal to arrive, and therefore the average calculation is more accurate.  The parameter that identifies this accuracy is called DOP (dilution of precision). The time necessary for the receiver to “trace” a satellite is 12.5 minutes.
The receiver offers its true performance after 50 minutes of functioning when it is locked with at least four different satellites (Fig. 6).  The microprocessor, once synchronized with the GPS system, goes to control the frequency value of a quartz oscillator (oven) and adjusts the frequency value of the generated signal with the precision and the phase of the GPS system.  This signal reaches the “Star Look” with a frequency accuracy of 1x10E-11. 

 

Figure 6

Other more costly synchronizers are available from HP with greater accuracy, but they are not needed for this application.  Furthermore, our system has intelligent software that studies the behavior of the oven and acquires data to construct its graphic curves.
It then uses this data to correct its behavior.  In this way the system maintains the same standard precision (1x10E-11) for more than 24 hours, guaranteeing operation even in the absence of GPS signals.  After this period the precision declines to a minimum of 1x10E-10 where it remains for several months. The GPS system also uses another frequency at 1127.6MHz which is used only for military purposes.  The precision obtained by using both frequencies would permit us to do miracles (1x10E-14), but unfortunately this service is not accessible for civilian purposes at the moment.

Now you are ready to start with “Star Look”.

 Have a good trip!
 



e-mail: rtmsrl@libero.it

Fax +39 736 390231