Fast Tuning Synthesizer

Transcription

1 Project Member: Nathan Roth Project Advisors: Dr. Brian Huggins Dr. Prasad Shastry Mr. James Jensen Date: November 18, 2003 Fast Tuning Synthesizer System Level Block Diagram Overview A frequency synthesizer is a device that is capable of precisely generating many frequencies from one or more reference frequencies. Typically, the output frequency varies by a given step size over a predetermined bandwidth. These devices are used frequently in communications and electronic warfare applications. The purpose of this project is to develop a frequency synthesizer whose output can change in less than 500 ns. Top Level System Figure 1 shows a top level system block diagram. An external sinusoidal reference frequency of 100 MHz and a 3 bit digital input command are the system inputs. The system output is the desired sinusoidal frequency. The digital input command selects the desired frequency. 100 MHz Reference Fast Tuning Synthesizer Desired Output Digital Input Command D2 D1 D GHz Figure 1- Fast Tuning Synthesizer Top Level Block Diagram Figure 2 shows the detailed block diagram for a direct synthesis frequency synthesizer, meaning the output signals are created through direct manipulation of the input signal. Here, the 100 MHz signal enters the system. The signal is then multiplied, mixed, filtered, and divided multiple times to obtain the desired output frequency. Each switch is controlled by the input selection word via the switch logic. This selects the proper filter and reference signals that will be mixed with the 100 MHz signal. Under this configuration, the possible output frequencies are as follows: 3650 MHz, 3700 MHz, 3850 MHz, 3900 MHz, 4450 MHz, 4500 MHz, 4650 MHz, and 4700 MHz. Page 1 of 6

2 100 MHz Reference Signal Input Module Resolution Modules 1, 2, and 3 RF RF RF ¼ ¼ 1 S0 S1 S2 S3 S4 S5 S6 S9 S12 S8 S11 S14 S7 S10 S13 S0 Switch Logic S14 Basis Module Output D2 D1 D0 Digital Input Command Output Module Switch Selection Module Figure 2- Fast Tuning Synthesizer Detailed Block Diagram Page 2 of 6

3 Subsystem 1- Input Module 100 MHz Reference Signal 200 MHz Figure 3: Fast Tuning Synthesizer Input Module Figure 1 shows the input module. For this synthesizer to work correctly, a 200 MHz signal must be utilized. However, only a 100 MHz source is available. Thus, the external signal must be doubled via a frequency doubler to obtain the necessary frequency of 200 MHz. Subsystem 2- Resolution Module 1 Fc = F3db = 120 MHz 200 MHz RF ¼ MHz Fc = 1000 MHz F3db = 120 MHz Figure 4 Fast Tuning Synthesizer Resolution Module 1 Figure 4 above shows the first resolution module of the system. In each resolution module, the incoming frequency will be mixed with a selected basis frequency. The resulting frequency will then pass through a filter. In order for the correct frequency to occur, the signal must be divided by four before leaving the module. In a mixer, the radio frequency (RF) is mixed with the local oscillator () and the intermediate frequency () is the result. Ideally, the + RF and the - RF frequencies are the results of the mixing. However, other harmonic frequencies are also produced which must be filtered out. Here, a 200 MHz signal enters and is mixed with the of either or 1200 MHz. This synthesizer will use the - RF term. Depending upon the desired output, the appropriate band pass filter is selected to eliminate the unwanted mixing products. Finally, the signal frequency is divided by four via a frequency divider before leaving the first resolution module. In this module, the output signals are either 250 MHz or 300 MHz. Page 3 of 6

4 Subsystem 3- Resolution Module 2 Figure 5 shows the second resolution module. The principle of this module is the same as the first resolution module. The variation is the resulting mixed frequencies and the different filter specifications. The output signals are 225 MHz, MHz, 275 MHz, or MHz. Fc = 1183 MHz F3db = 195 MHz 4 th Order MHz RF ¼ Fc = 982 MHz F3db = 195 MHz 4 th Order MHz Figure 5- Fast Tuning Synthesizer Resolution Module 2 Subsystem 4- Resolution Module MHz Fc = 1183 MHz F3db = 108 MHz RF MHz Fc = 982 MHz F3db = 207 MHz Figure 6- Fast Tuning Synthesizer Resolution Module 3 Figure 6 shows the third resolution module. Again, the same principle applies to this module as the first two. The main difference is that the final frequency output is not Page 4 of 6

5 divided by 4. This module s outputs will be MHz, 925 MHz, MHz, 975 MHz, MHz, 1125 MHz, MHz, or 1175 MHz. Subsystem 5- Basis Module Figure 7- Fast Tuning Synthesizer Basis Module Figure 7 shows the basis frequency module. As seen from Figure 2, it is necessary to have external reference signals. In this case, and were chosen to yield the proper output. These frequencies will be generated with voltage controlled oscillators or phase-locked loops. The signal that is transmitted to the resolution modules is determined by the switch configuration. If the signal is not being used, it is grounded. If each switch provides 45 db of isolation, this method theoretically gives 90 db of isolation between the two paths. Subsystem 6- Switch Selection Module S0 S14 Switch Logic D2 D1 D0 Digital Input Command Figure 8- Fast Tuning Synthesizer Switch Selection Module Figure 8 shows the switch selection module. This is used to control the switches. There are eight possible frequency outputs. In order for each output to have a unique input, a 3 bit word must be used. This three bit word will be used by switch logic determine the setting of each of the 15 switches for the proper output. Page 5 of 6

6 Subsystem 7- Output Module Figure 9 shows the output module. For the proper output, the incoming signals must be multiplied by a factor of 4. Due to component limitations, this will be performed by using two frequency doubling blocks. To eliminate unwanted frequencies, the signal will be sent through a band pass filter. This filter must have a pass-band which will allow signals with frequencies of 3300 MHz to 5100 MHz. Depending upon the performance of the multiplier blocks, it is possible that a switched bank of filters will be necessary. This would be caused by low frequency harmonics that are fall within the pass band. However, this will be determined after further study. Finally, the remaining signal is the desired frequency output. Band Pass Filter Fc = 4200 MHz F3dB = 1800 MHz MHz Output Figure 9: Fast Tuning Synthesizer Output Module Page 6 of 6