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LM1863 Datenblatt(PDF) 11 Page - National Semiconductor (TI) |
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LM1863 Datenblatt(HTML) 11 Page - National Semiconductor (TI) |
11 / 14 page Applications Information (Continued) teristics of the radio are determined by the design of this stage Generally speaking it is very difficult to design an integrated RF stage in bipolar as bipolar transistors do not have good overload characteristics Thus the RF stage is usually designed using discrete components Because of this there is a great deal of concern with minimizing the number of discrete components without severely sacrificing performance The applications circuit RF stage does just this The circuit consists of only two active devices an N-chan- nel JFET Q1 which is connected in a cascode type of con- figuration with an NPN BJT Q2 Both Q1 and Q2 are varac- tor tuned gain stages Q2 also serves to gain reduce Q1 when Q2’s base is pulled low by the RF AGC circuit on the LM1863 The gain reduction occurs because Q1 is driven into a low gain resistive region as its drain voltage is re- duced R10 and C15 set the gain of the 1’st RF stage which is kept high (about 19 dB) for good low signal signalnoise performance The gain of the front end to the mixer input referenced to the generator output is about a10 dB T2 in conjunction with D1 C21 and C26 form the 1’st tuned circuit C26 does not completely de-couple the RF signal at the cathode of the varactor In fact the combination of C26 and C19 act to keep the gain of the whole RF stage con- stant over the entire AM band Without special care in this regard the gain variation could be as high as 14 dB This gain variation would result from the increase in impedance at the secondary’s of T2 and T1 as the tuned frequency is in- creased The increased impedance results from a constant QeRp(wL) of the tanks over the AM band With C26 and C19 the gain is held constant to within 6 dB (including the tracking error) over the entire AM band C27 de-couples RF signal from the top of T2’s primary and allows Q2 to operate properly C18 is a coupling capacitor which in conjunction with C19 couples the signal from the 1’st RF stage to the 2’nd RF stage R20 acts to isolate this signal from AC ground at C11 R19 acts in conjunction with C12 to set a high frequency (ie non-dominant) RF AGC pole which is important for low distortion when the RF AGC is active The dominant RF AGC pole is set by R8 and C11 Q2 is a high beta transistor allowing for little voltage drop across R20 and R8 due to base current This keeps the emitter of Q2 sufficiently high (in the absence of RF AGC) to bias Q1 in its square law region R13 acts to reduce the 2’nd stage gain and increase Q2’s signal handling R13 must not get too large however (ie R13l100 X) or low level signalnoise will be degraded T3 in conjunction with C20 C27 and D2 form the 2’nd RF tuned circuit The output of Q2 is capacitively coupled through C28 to the mixer input The output of Q2 is loaded not only by the reflected secondary impedance but also by R22 R22 is carefully chosen to load the 2’nd stage tuned circuit and broaden its bandwidth The increased bandwidth of the 2’nd stage greatly improves the cross modulation performance of the front end In the absence of this increased bandwidth the relatively large AC signals across varactor D2 result in cross modulation R22 also reduces the total gain of the 2’nd stage R22 does slightly degrade (by about 6 dB) the image rejection especially at the high end of the AM band However the image rejection of this front end is still excel- lent and 6 dB is a small price to pay for the greatly increased immunity to cross modulation R16 and C29 decouple unwanted signals on Va from being coupled into the RF stage This front end also offers superi- or performance with respect to varactor overload by strong adjacent channels This results because of the way that gain has been distributed between the 1’st and 2’nd stages In summary this front end offers two stages of RF gain with the 2’nd stage acting to gain reduce the 1’st stage when RF AGC is active Furthermore a unique coupling scheme is employed from the output of the 1’st stage to the input of the 2’nd stage This coupling scheme equalizes the gain from one end of the AM band to the other Additional care has been taken to insure that excellent cross modulation performance image rejection signal to noise performance overload performance and low distortion are achieved Per- formance characteristics for this front end in conjunction with the LM1863 are shown in the data sheet Also informa- tion with regard to the bandwidth of the front end versus tuned frequency are given below TUNED FREQUENCY b 3 dB BANDWIDTH 530 kHz 66 kHz 600 kHz 72 kHz 1200 kHz 206 kHz 1500 kHz 264 kHz 1630 kHz 36 kHz VARACTOR ALIGNMENT PROCEDURE The following is a procedure which will allow you to properly align the RF and local oscillator trim capacitors and coils to insure proper tracking across the AM band 1 Set the voltage across the varactorse1 volt 2 Set the trimmers to 50% 3 Adjust the oscillator coil until the local oscillator is at 980 kHz 4 Increase the varactor voltage until the local oscillator (L0) is at 2060 kHz and check to see if this voltage is less than 95 volts but greater than 75 volts If it is then the L0 is aligned If it is not then adjust the L0 coiltrimmer until the varactor voltage falls in this range 5 Set the RF in to 600 kHz and adjust the tuning voltage until the L0 is at 1050 kHz Peak all RF coils for maxi- mum recovered audio at low input levels 6 Set RF in to 1500 kHz and adjust the tuning voltage until the L0 is at 1950 kHz Peak all RF trim capacitors for maximum recovered audio at low input levels 7 Go back to step 5 and iterate for best adjustment 8 Check the radio gain at 530 kHz and 750 kHz to make sure that the gain is about the same at these two fre- quencys If it is not then slightly adjust the RF coils until it is The above procedure will insure perfect tracking at 600 kHz 950 kHz and 1500 kHz The amount of gain variation across the AM band using the above procedure should not exceed 6 dB ADDITIONAL INFORMATION R5 and C7 act as a low pass filter to remove most of the residual 450 kHz IF signal from the audio output Some re- sidual 450 kHz signal is still present however and may need to be further removed prior to audio amplification This need becomes more important when the LM1863 is used in conjunction with a loopstick antenna which might pick up an amplified 450 kHz signal An additional pole can be added to the audio output after R5 and C7 prior to audio amplifica- tion if further reduction of the 450 kHz component is re- quired 11 |
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Ähnliche Beschreibung - LM1863 |
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