GaAs mesfet karıştırıcılarda af geribeslemenin gürültü sayısı üzerine etkisi
GaAs mesfet karıştırıcılarda af geribeslemenin gürültü sayısı üzerine etkisi
Dosyalar
Tarih
1991
Yazarlar
Sezer, Rıza
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Özet
Bu çalışmada, FET ve MESFET kullanılarak gerçekle nen kuvvetlendiricilere göre gürültü sayıları yüksek olan karıştırıcıların gürültü sayılarını düşürmek amacıyla uygun ara-frekans geribesleme yöntemleri ve geri besi emenin gürültü sayısı üzerine etkisi araştırılmıştır. îkinci bölümde. FET'li karıştırıcının gürültü sayısını asal tan iki ara-frekans geribesleme devresi verilmiştir. Bunlar kapasitif ve endüktif bölmeli ara-frekans geribesleme devreleridir. Analis sonuçlarına göre kapa sitif bölmeli ara-frekans geribeslemeli FET karıştırıcı nın gürültü sayısının, kaynak iç direnci 100 Ohm alındığında, yaklaşık 3,5 dB düşürülebileceği gösterilmiştir. Üçüncü bölümde, MESFET'li karıştırıcının gürültü sayısının kapasitif bölmeli ara-frekans- geribeslemesi uygulanarak düşürülebileceği gösterilmiştir. Analis sonuçlarına göre, iletim açısının 70 ve kaynak iç direncinin 10 Ohm olması durumunda, gürültü sayısında yaklaşık 5 dB lik bir asalma olacağı bulunmuştur.
In this work, the main purpose was the reduction of noise figures of JFET and MESFET mixers together with an appreciable amount of conversion gain. To accomplish this aim, the idea proposed by Strut t £93 was used. His idea was the reduction of noise figure by the application of feedback from output signal to input at intermediate-fre quency CIF3 in the triode mixer stages, but he did not made an attempt to clarify his idea. By the same purpose, van der Ziel T 33 applied RF feedback to FET mixer and he experimentally proved that it can be obtainable an appre ciable amount of noise reduction. But, theoretically or experimentally in puplished literature there is no work of applying IF feedback to the FET mixer circuits. To see the results of IF feedback in JFET and MESFET mixer stage, it is applied this kind of feedback to the aforementioned mixer stages by extending the the ory proved by Ziel to our circuits under some assumptions. In chapter 3, it is applied IF feedback to the JFET mixer. It is found two feedback circuit giving same re sults; the first one is the capacitive feedback of the IF output signal current CFigure 13 and the second is the inductive feedback of the IF output signal current. Noise figure analysis for capacitive feedback mix er circuit is performed as follows. It is assumed that IF noise produced by signal source and FET* s input con ductivity g can be neglected for the sake of simplicity. Induced gate noise and the correlation between the drain and the gate noise is assumed to be very small. At the output RF signal is short-circuited. In Figure 2 it is .o>2C2E mi t 2 2 2 to the input conductance. Where g is the conversion transconductance of the device anc? u>. intermediate-frequ ency. According to van der Ziel 'Ell 5, i,the noise for short-circuited output without feedback, fs i* =£.4kT.g. Af do mo (2) where g is the RF transfer conductance and ^=2/3 for a Junction°gate FET. Voltage across the capacitor C4 is found as ; v =- i 1oiCR(-i )Eg -wCCK-ju, «C+C +CRg +g EC>3 v i do n ». 1 v 1 t* fnl £q -ii>ZCC E32+ 22 ~n t i ı 1 "n rn± C3> VI It- is easily seer» that vis real when ; 9n C = Ü. <4> 4 6>2CR v Using eq. C43 and CI 5 in eq. C3Ü> gives R gf Vi (l+g R>. (q +g > ( ido> " (5> ml ~ r> 1 At the instant t, the IF noise at the output can be rep resented as i, =a. cosiij. (t-r> (6) de i After mixing, the output noise current due to voltage v is i -g gf£ q v =, "° pw t - ra.cos[w.(t-T>+WT] <7> -mo i ı p rol n f where uyp is local oscillator frequency. This noise must be added to a. cosa. Ct-iO. Here after, by following the mixer theory proved by van der Ziel [203, the IF noise with feedback is found as - - g i* =i*. (1-3F - ÎÎÜ+F2> C8> d do eg c where F is given by gmo9rR c
In this work, the main purpose was the reduction of noise figures of JFET and MESFET mixers together with an appreciable amount of conversion gain. To accomplish this aim, the idea proposed by Strut t £93 was used. His idea was the reduction of noise figure by the application of feedback from output signal to input at intermediate-fre quency CIF3 in the triode mixer stages, but he did not made an attempt to clarify his idea. By the same purpose, van der Ziel T 33 applied RF feedback to FET mixer and he experimentally proved that it can be obtainable an appre ciable amount of noise reduction. But, theoretically or experimentally in puplished literature there is no work of applying IF feedback to the FET mixer circuits. To see the results of IF feedback in JFET and MESFET mixer stage, it is applied this kind of feedback to the aforementioned mixer stages by extending the the ory proved by Ziel to our circuits under some assumptions. In chapter 3, it is applied IF feedback to the JFET mixer. It is found two feedback circuit giving same re sults; the first one is the capacitive feedback of the IF output signal current CFigure 13 and the second is the inductive feedback of the IF output signal current. Noise figure analysis for capacitive feedback mix er circuit is performed as follows. It is assumed that IF noise produced by signal source and FET* s input con ductivity g can be neglected for the sake of simplicity. Induced gate noise and the correlation between the drain and the gate noise is assumed to be very small. At the output RF signal is short-circuited. In Figure 2 it is .o>2C2E mi t 2 2 2 to the input conductance. Where g is the conversion transconductance of the device anc? u>. intermediate-frequ ency. According to van der Ziel 'Ell 5, i,the noise for short-circuited output without feedback, fs i* =£.4kT.g. Af do mo (2) where g is the RF transfer conductance and ^=2/3 for a Junction°gate FET. Voltage across the capacitor C4 is found as ; v =- i 1oiCR(-i )Eg -wCCK-ju, «C+C +CRg +g EC>3 v i do n ». 1 v 1 t* fnl £q -ii>ZCC E32+ 22 ~n t i ı 1 "n rn± C3> VI It- is easily seer» that vis real when ; 9n C = Ü. <4> 4 6>2CR v Using eq. C43 and CI 5 in eq. C3Ü> gives R gf Vi (l+g R>. (q +g > ( ido> " (5> ml ~ r> 1 At the instant t, the IF noise at the output can be rep resented as i, =a. cosiij. (t-r> (6) de i After mixing, the output noise current due to voltage v is i -g gf£ q v =, "° pw t - ra.cos[w.(t-T>+WT] <7> -mo i ı p rol n f where uyp is local oscillator frequency. This noise must be added to a. cosa. Ct-iO. Here after, by following the mixer theory proved by van der Ziel [203, the IF noise with feedback is found as - - g i* =i*. (1-3F - ÎÎÜ+F2> C8> d do eg c where F is given by gmo9rR c
Açıklama
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1991
Anahtar kelimeler
Geri besleme yöntemi,
Gürültü,
Karıştırıcılar,
Feedback method,
Noise,
Mixers