Mixer Goal. Translate the carrier frequency of a RF signal. Conceptually this implies a multiplication: cos. V V t t t.

Transcription

1 MICROWAVE MIXERS

2 Mixer Goal Translate the carrier frequency of a RF signal. Conceptually this implies a multiplication: cos ( ) V V cos t V V t t t RF M RF cos ( ) cos o V V V V t V t t t U RF M 0 RF V0 V M t RF t t V M t RF t t cos ( ) cos ( ) Filter RF RF RF +

3 Mixer in communication systems Frequency conversion receiver: Segnale Immagine Segnale RF im RF Mixer RF Filtro RF Filtro A Demodulatore Oscillatore Locale ( )

4 Practical implementation of a microwave mixer At microwave frequency is more easy to realize the frequency conversion exploiting port the non linear devices: V in =V RF +V Non linear V out F() non linear characteristic: port: 3 V out =F{V in } Vout av 1 in av in av 3 in... Substituting V RF and V and using the first non linear terms of the expansion: a Vout V0 VM a1v0cos0tvm cosrft a V t V t a V V t t 0 0 M RF 0 M 0 RF cos cos cos cos...

5 Classes of microwave mixers The most used non linear device is a Schotcky diode. There are two main classes of microwave mixers: Mixers with a single diode (single ended) Balanced Mixers (with o 4 diodes) Parameters of a microwave mixer: Conversion Loss(dB) (5 15 db) Linearity (referred to the translated RF signal) Noise Suppression of spurious products Input output match Isolation(RF, )

6 Analysis of single-ended mixers Equivalent circuit: Z RF Z LO Rd Z V RF V LO Pumped diode The diode (R d ) is represented by the exponential curve: VD VT ID IS e 1 It can be assumed memoryless (in a first as approximation)

7 Distortion in a mixer Effects of distortion: The spectrum of the frequency translated signal around the new carrier frequency is different from the original one New replicas of the original RF signal, LO and combination of them are generated at different carrier frequencies The local linearity is described by the same parameters used for amplifiers (P 1dB, IP); in the mixers case the input output frequencies are different. Moreover the mentioned parameters depend also on the power level of the local oscillator

8 Single-balanced mixers They allow an intrinsic suppression of some spurious products. An higher power from the LO is requested. A better linearity is obtained, together with higher losses (compared to single ended mixers)

9 VRF Vr cosrft V V cos t P P V1 VRF V V V V RF Balance mixer with a hybrid 3 ID av 1 D av D av 3 D I av 1 av av 3... I/V characteristic of diodes: Then: V RF V HYB 3 db I av av av The overall current is given by: Filtro r RF r P RF P RF P I a V V a V V a V V... av 4aV V a V 3 V V RF RF 3 RF RF av cos( t) a cos t cos t 3 a 3 Vr cos RFt3 r P cos Ptcos RFt... I 1 V 1 V I I I I I 1 Z

10 Characteristics of hybrid mixer All harmonics of LO are suppressed at Also the even harmonics of RF are suppressed AM Noise of LO is eliminated (NOT the phase noise) RF LO ports are intrinsically isolated V RF V + V HYB 3 db V V 1 d d V V V 1 V V V V d 1 d d d dv d RF V V, V dv1 dv dv dv 0 SRF 0

11 Double-balanced Mixer Z 0 RF HYB 3 db V RF V RF I 3 I 4 I 1 I V V HYB 3 db VA kia k I I k av a V av a V k a V V a V V k av 1 4aV VRF VB kib kii4kav 1 av av 1 4aV 4ka1V V4 av V4 k av 1 4aV VRF VA VB k V 8aV VRF I A I B Z V V I av av D 1 D D V V V V, V V V V V, V RF RF 1 RF RF 3 4

12 Characteristics of double-balanced mixers ALL harmonics of LO and RF are suppressed ALL even intermodulation products are suppressed Perfect isolation LO RF High LO power required Worsens conversion loss Linearity improvement

13 Image-suppression mixers Z 0 RF V RF HYB (3 db) A V R B VR Splitter V A V I B V I HYB (3 db) LSB USB This configuration separates the translated and image signals. The image filter is then no more necessary (or its requirements are strongly reduced)

14 RF V RF Z HYB 3 db A V R V B R Div. Per V V cos t, V V cos t V cos t 0 0 RF U 0 L 0 V A VU VL B VU VL VR sin 0 t sin 0 t, VR cos0 t cos0 t A U 0 L 0 VI k sin 0 tcos 0t k sin 0 tcos 0t U 0 L 0 k sin 0 tsin tk 4 sin 0 sin 4 t t B U 0 L 0 VI k cos 0 cos 0 cos 0 cos 0 t t k t t U 0 L 0 k cos0 t cost k cos0 t cost 4 4 A B I I L 0 VLSB j k cost A V I 9 B V I 9 B A V I VI U 0 L 0 VUSB j k sin 0 tsin tk sin 0 t V V HYB 3 db LSB USB