Расчёт преобразователя частоты с пьезомеханическим фильтром

                                     ПФ1П-4-3 и отдельным гетеродином.

                                                                                                       + -          

                                                                                                        Е_­п­  

                                                                          R_­ф­                                 

                              R_­1­       С­_­ф­­     ­L_­4 ­ L_­5­   

                                                    С_­2                            ПФ1П-4-3       f_­пр

                   С_­1­               VT1  

           

 

 

                              R_­2­               R_­3

 

                                                      C_­3­       

                                                                                                         

+ -                                                                     Е_­­_­п­

          от гетеродина

 

        Рис.4 Преобразователь частоты с отдельным гетеродином.      

 

В данном преобразователе рассчитаем только смесительную часть.

1.Определяем параметры транзистора в режиме преобразования частоты:

  S_­пр­=0,3S_­max=0,3*30=10 мА/В;

  R_­вх.пр­=2R_­11­=0,2*1500=300 Ом;

  R_{­вых.пр}­=2R_­22­=2*200*10_­­³=400 кОм;

  С_{­вых.пр}­=С_­22­=8 пФ; С_­вх.пр­=С_­11­=70 пФ.

2.Согласование транзистора смесителя с фильтром осуществляем через широкополосный контур. Определим коэффициент шунтирования контура входным сопротивлением фильтра и выходным сопротивлением транзистора, допустимый из условий обеспечения согласования:

 

.

3.Определим конструктивное и эквивалентное затухание широкополосного контура:

, где Q_­эш­=20 –добротность широкополосного контура.

4.Определяем характеристическое сопротивление контура, принимая коэффициент включения в цепи коллектора m_­1­=1:

5.Определим коэффициент включения контура со стороны фильтра:

.

6.Эквивалентная ёмкость схемы: .

7.Ёмкость контура: С_­2­=С_­э­-С_{­вых.пр}­=174-70=104 пФ. Выберем С_­2­=100 пФ.

8.Определим действительную эквивалентную ёмкость схемы:

 C_­э^­’=С_­2­+С_{­вых.пр}­=100+70=170 пФ.

9.Индуктивность контура: .

10.Дествительное характеристическое сопротивление:

.

11.Резонансный коэффициент усиления преобразователя:

.

 

12.Индуктивность катушки связи с фильтром, приняв k_­св­=0,5:

.

13.Рассчитаем С_­2­: .

14.Рассчитаем С_­1­: .

 

 

                                            3.Расчёт амплитудного детектора.

 

                                     L L                                                                C3  

                                                      C4                  VT2                            

                   C5                                      

                                                                                                                C_­н­          

  VT1                    

                   C6                                                                                                       R_­ВХ    

                                                                                         С1                                     УПЧ

                                              R2

                                      R4      R1                                         R3                                                    

 

 

                                          +

                                            E_­П­                                                                                              

                                                -

                           

    

В детекторе используем тот же транзистор, что и в УПЧ.

1.Определим R_­н­ из условия малого шунтирования следующего каскада:

 R_­н­=(5..10) R_­вхн­=(5..10) кОм.Þ G_­н­=0,2мСм.

2.Рассчитаем крутизну детектора: .

3.Найдём эквивалентное сопротивление нагрузки по переменному току:

, где DU_­k­=4В, DI_­k­=1мА при U_­бэ­=const.

­

ÞR_­н~­=1,1кОм.

4.Определим коэффициент передачи: .

5.Определяем ёмкость нагрузки: .

6.Определим ёмкость С3:

7.Рассчитаем элементы цепи питания. Для этого найдём I_­дел­­=(50..100)I_­к0­=(50..100)*20*10^-6=2 мА.

 и .

 

 

8.Вычислим ёмкость С1: .

9.Найдём входное сопротивление: R_­вх­=10r_­11­=10*300=3 кОм и С_­вх­=С_­11­/10=5/10=0,5пФ.

10. Рассчитаем коэффициент включения: . Примем m_­2­=1.

11.Вычислим напряжение на входе УНЧ: U_{­вх унч}­=U_­mд­К_­д­m_­2­=0.4*9*1=3.6В.

На этом расчёт амплитудного детектора закончим.

                              

                                                                                                                              -Е_­0         

.

 

U_­вых                                                                                                                                                                    

                                                                            R3             C3  

                                             R1                                                  

        R4                                                R2                    VT2                                C_­cв

             

                                      R4  

                                                              C2

     C1

                          R5                              R6                            R7                       R8          

                                                                                 +E_­0

           Рис. 7. Принципиальная схема АРУ транзисторного приёмника.

 В этом расчёте применим транзистор типа 1Т322А. В соответствии с требуемой требуемой глубиной регулирования будем иметь: ³.

Учитывая, что U_­рмакс­=0,5В и U_{­mвыхмин}­= U_{­выхУПЧ}­=1В, определим требования к усилению в цепи регулирования:  . Следовательно, поставленные к системе АРУ требования можно удовлетворить без дополнительного усиления, полагая, что коэффициент передачи детектор-АРУ должен быть К_­д­³0,8.

Примем сопротивления R_­4­=680Ом и R_­1­=3.5кОм. Тогда для сохранения исходного смещения на базах регулируемых транзисторов найдём силу тока I_­п1­, протекающего через сопротивление R_­4­:        

      . Приняв U_­R5­=0 и число регулируемых каскадов n=3 найдём R_­2­:

 

. Выберем R_­2­=4.7 кОм.   

Сопротивление нагрузки детектора АРУ R_­3­ вычислим на основании требуемого коэффициента передачи детектора (К_­д­³0,8), предварительно вычислив вспомогательный коэффициент q:  тогда .

Выберем R_­3­=510 Ом, при этом необходимое условие R_­3­£0,2*(R₁+R₂+R₄)=0.2*(3.5+0.68+4.7)=1,78.

 Для определения элементов схемы задержки зададимся R_­8­=2 кОм и найдём:

.

Применим однозвенную структуру фильтра АРУ, при этом сопротивление фильтра будет R_­2­. Постоянную времени фильтра АРУ при помощи следующего выражения, предварительно вычислив параметр М:

При заданной длительности переходного процесса t_­АРУ­=0,5с получим:

. Ёмкость фильтра должна быть:  . Выберем С_­2=1,5 мкФ. Чтобы обеспечить реализацию однозвенной структуры фильтра АРУ, постоянная времени нагрузки детектора должна быть:

R_­3­C_­3­£0.1R_­2­C_­2­, тогда: . Выберем С_­3­=1,5мкФ.

На этом расчёт курсового проекта закончим.

 

                                                                                                                                                                                                                                                                                                                                                                                                                                 &                                                                                                                                                                                                                                                                                                                                                                                                                                                    

                                                                                                                                                                                                                                                +1                                                                                                                                                                                         C1               &                                                                          Q1                                                                                                                                                                                                                  +1      Q1    -1                                         

   R1                            R2                                                                                                                                                                     Q2                                                                                                                                                                                                                                                                          Q3                 Q2                                                                                                                                                                                                                    -1           Q4                                         

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       D1                                                                                                                                                                    &                                         &    D1                                     D2       Q3                                                                                                &                                                                                           D2                              D3                                                                                                                                                                                                                                      D3       P_­+­    D4                                                                                                                                                                                                                                           D4        P_­-­                                  Q4                                              

    R3                           R4  C2                                    R5                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      S                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               S                                                                                                                                                                                                                                                                                                                                                                                                                                                       C3                                                                                                                                                                                             D1         B/U                

                    VT                                                                 L                                                                                     S                                                                                                                                                                                                                                                                                                                                                           D2                                           КГ     С4                 С5                                                                                    S                                                                                                                                                                                                                                                                                                                                                         D3                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     4          D4                                                          R7                                                                                                                                         R                                                                                                 

       R6                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               R                                                                                                                   R8                                     R9                                                                                                                                                                                                                                                                                                                         +E                    S    Q                     R                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             C                                                                                                                                                                                                                                                              1       D                                  R                                                                                                                            З Р                                                                                                     R  Q                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                

Список используемой литературы:

 

1.Бобров Н.В., Максимов Г.В., Мичурин В.И., Николаев Д.П. «Расчёт радиоприёмников». М.1971г

2.Павлов К.М., Екимов В.Д. «Проектирование радиоприёмных устройств». Из-во «Связь»-1970г.

3.Под общей редакцией Горюнова Н.Н. «Справочник по полупроводниковым диодам, транзисторам и интегральным микросхемам». М. «Энергия»-1979г.

4.Криницин В.В., Сергеев В.Г. «Методические указания и задания к курсовому проектированию по дисциплине «Приём о обработка сигналов» для студентов IV курса». МГТУ ГА-1994г.