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LMX2240M bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI) |
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LMX2240M bảng dữ liệu(HTML) 10 Page - National Semiconductor (TI) |
10 / 12 page Applications Information THE INTERMEDIATE FREQUENCY LIMITER The IF limiter has a large amount of gain at high enough frequency to cause concern about oscillation To ensure that the limiter does not oscillate a few precautions should be taken The compensation capacitors that are used should be chosen to roll off any unwanted frequencies be- low the band of interest The capacitor should be a high Q RF type ceramic chip capacitor For DECT the capacitor value should be 100 pF and the capacitors should be sol- dered as close to the LMX2240 as possible This will create a pass band from 40 MHz to 150 MHz The AC coupling capacitor at the input to the limiter (from the SAW filter) should be the same value as the compensation capacitors THE DISCRIMINATOR There are two types of discriminator that can be used to demodulate FM signals The first is a delay line discrimina- tor which uses a delay in one path of the received signal to introduce a phase difference between it and the received signal The operation of the delay line discriminator is de- rived in the inset box The other type of discriminator relies on a quadrature tank to directly introduce a phase shift in the received signal This is the type of implementation that is commonly used in mobile communications because of its relative ease of construction and low cost The discriminator operates best when the inputs to it are hard-limited (ie square edges) If the input signal is small enough such that the IF amplifier cannot limit it the output voltage swing of the limiter will suffer Typically the mini- mum voltage swing the discriminator can see and still fully switch is about 100 mVPP The two inputs to the discrimina- tor can be of different peak-to-peak voltage swings as long as both are over the lower limit This allows the quadrature tank circuit to have some insertion loss In fact up to 8 dB insertion loss can be tolerated while still ensuring that the discriminator output won’t suffer The quadrature circuit can also affect the discriminator out- put voltage swing The discriminator output voltage swing specified assumes perfect quadrature at the frequency of interest (mixer operation) With available analog compo- nents perfect quadrature is not possible This is due in part to the high frequency of the IF and the proportionally very narrow bandwidth of the desired signal For example a DECT signal is about 1 MHz wide which is k 1% of the IF at which the demodulation occurs This makes the quadra- ture circuit difficult to achieve With moderately high Q com- ponents however a reasonable phase shift can be achieved with a single pole tank This is illustrated by the following equation the output of the discriminator is given by (1) DISCout e cos(0c t ) cos(0c t aw) which results in (2) DISCout e cos(0c t a0c t aw) a cos(0c t b0c t bw) When the double frequency component is filtered out with a low pass filter the cosine of the phase remains (3) DISCout e cos(bw) e cos(w) It can be seen that at 90 phase shift the output will be zero At 0 the output will be 05 and at 180 it will be b05 The output swing is then set by the multiplication of the cosine term with the discriminator output amplifier’s gain With a circuit that gives an output peak-to-peak voltage of 10 VPP (min) with ideal quadrature the slope is seen to be 55 mVdegree With a practical quadrature tank circuit at 1106 MHz the phase shift over a 1 MHz bandwidth is about 45 –50 which translates to an output peak-to-peak voltage of about 250 mVPP Assume the FM modulated signal is denoted as s(t) e cos (0ct a m(t)) (4) where m(t) e m t b % b(t) dt and b(t) is the modulating baseband signal The con- stant m is defined as m e 2DfTb The signal s(t) must be delayed by some u so that l(t) e s(tau) e cos (0c(tau) a m(tau)) (5) If the delay u is such that 0ct e 2nq a q 2 n e 0 1 2 3 (6) then s(tau) e sin(0ct a m(tau)) (7) and multiplying (4) and (7) yields s(t) l(t) e cos (0ct a m(t)) sin (0ct a m(tau)) e 1 2 sin (20ct a m(t) a m(tau)) (8) a 1 2 sin (m(tau) b m(t)) The double frequency component can be filtered off with a lowpass filter If u is kept small 1 2 sin (m(tau) b m(t)) 1 2 m(tau) b m(t) e m 2 tau b % b(t) dt b m 2 t b % b(t) dt (9) e m 2 tau t b(t) dt u m 2 b(t) The object for a delay line then is to maximize the de- lay while retaining the approximations necessary to sat- isfy (9) u k 01 Tb 10 |
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