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Everything posted by walid

  1. I have not oscilloscope or other devices to monitor the change so i ask
  2. Is there any difference between MC3361 & MC3371, and can one of these be used in place of other, thanks
  3. What the meaning of Jr, thanks
  4. hi all, i want to put a picture under my name as you do but i don't know how! Please tell me.
  5. hi Alun Assume that I have a transformer and I want to put its dots. I put it in a circuit, connect its primary with suitable AC volt then monitor the i/p & o/p on an oscilloscope if they are in phase I must put the dots on the primary and secondary points that connected to the +ve (center electrode) of the channel cables. This is ok. But here the dots may be at the ends of the coils rather than the start. If the signals are out of phase, I must put dot at primary connected to the +ve and on secondary terminal connected to the -ve of the channel cable. Is this what you try to tell me 100 years ago……… Thank you Alun.
  6. yes Mr Alun i want to calculate LC but the problem is in C itself. Is Ceq = (15p in series with 3.9p) // (varactor capacitance) OR Ceq = the three elements in parallel. THIS IS MY QUESTION. NOTE: Alun, your answers always talk about basic electronics, may be my questions not clear but please take into your account that I'm finished these topics long time ago, all what i need is the design. When I see any circuit in the web i look to it as it is a monaleeza, asking myself why this resistor is here or this diode. I dream to be a big designer so i ask you and others about this and only about the secrets of the design.
  7. Hi Audioguru You said: "It is difficult to calculate the gain and input impedance of the mic preamp you posted because it has negative feedback applied to its input. The transistor has an input impedance of about 2k ohms and the negative feedback of about 2k ohms is in parallel with it." (1) Are you mean by (The transistor has an input impedance of about 2k ohms) that Beta*re and you suppose that Beta=200 and re =10 ohm so (The transistor has an input impedance of about 2k ohms). (2) Are you mean by (and the negative feedback of about 2k ohms is in parallel with it.) that the 2k //FB resistor, that is 2K // 100k = approx 2k which is the overall i/p impedance. or you mean 2k i/p Z and 2k fb resistor are in parallel = 1k ohm, if this how you convert 100K feedback resistor to only 2k ohm. (3) As you continuing in your discussion you introduce the MIC impedance, why? You say that the 5K MIC // 10K resistance = 3.3k ok but this is not related to the transistor i/p Z if we talk about a high pass filter composed from 100n cap and this 3.3k. (4) You finally add the 1k ohm result from (2k//2K) + 3.3k = 4.3k and said this is the i/p Z of transistor!!! I am never seeing this approach. (5) You benefited me 100% in related the 100n cap to the i/p Z that makes with the cap a HPF and that you design its lower cutoff freq. = 20.6Hz to be better for music and the other FM Tx has a lower cutoff freq. = 372Hz which is the lower limit of human voice. this solves most of my main question about the mentioned coupling cap but introduce a new problem about Z calculations. (6) I read in an electronic book (ELECTRONIC DEVICES AND CIRCUIT THEORY by ROBERT and LOUIS) that for the first stage transistor, Zi looking into the capacitor = re/[(1/Beta)+(Rc/RF)] AND Voltage gain Av = -Rc/re approximately but in some web page i read this important rule for Av in a collector feedback configuration: Av = Rfb/Rc
  8. Hi audioguru and Staigen How you think calculate this value, this what i need to know gesign gesign gesign gesign gesign gesign gesign gesign gesign
  9. To Alun, if they in phase or not where to put the dots
  10. thank you Alun it is good answer
  11. it is very good answer audioguru but still one point : 10uF is not so big value and can be used instead of 100 n is this true? And how you choose this cap in your FM Tx thanks
  12. Alun please reread my question carefully before you trying answer. OK i am weak in ENGLISH but i think my question is obvious. thank you walid
  13. Thank you alun you don't understand me what i mean is if i want 100nh air coil and wound it in different way but the same value, is this ok or not thanks
  14. This is a diode operate on Infrared light emitted from an IR diode, the question is: What happen when IR ray strike this diode? Is it become sort circuit? Are there any differences if we connect it in reverse, that is kathode inplace on anode and vise versa. THANKS
  15. In many FM Txs they use a coil connected from the o/p of a high frequency stage to the power supply, this coil is of fairly larg value (for example 100uH). The purose of using such coils is to chock (prevent) high frequency sigals from going to other stage through the power supply. For example in 100 MHz FM Tx, they use 100 uH coil which has impedance of (2*pi*f*L) about 63 Kohm. My question: Is it better to use 1 mH (1000uH) which has 630 Kohm ot taking the size into account? Or is this true:"the more L the more chock effect"? What are the limits? thanks.
  16. walid


    Any transmitter is connected to the antenna through some length of wire. If the frequency is high say 100MHz i think that the waves will radiat out of this conncter. What we must do to prevent this radiation.
  17. how can i wound 100 uH coil
  18. VCO used frequently in FM transmitter for modulation. In Fig.VCO shown below I cut the VCO section from a FM transmitter (http://hem.passagen.se/communication/bug_rf.html) for discussion. VCO start from BB149A and end at the emitter of T1. BB149A, 3.9 pF, 15 pF and L1 make LC circuit with C varied with the i/p voice signal. My question is if we assume (for discussion) that no voice at all (i.e. capacitance of BB149 is constant value) then the frequency is constant. How we calculate it, explain with math expression. Or can you give me a method to design VCO like that shown in the figure. thanks very much.
  19. The varicap is a special diode that which changes its capacity according to DC voltage applied to it in reverse manner, that is when high DC => low capacitance and vice versa, I think this is OK. Now, looking to Fig.varicap (FROM: http://hem.passagen.se/communication/bug_rf.html), the varicap is BB149A, this varicap (according to its datasheets) has acapacitance of more than 20pF for DC votage less than 1VDC. Lets assume for a moment that its capacitance is 20pF. As seen in the Figure, this BB149A is connected to other capacitors,i.e. 3.9pF & 15pF and the three elements with L1 combine an LC circuit. My question is: What is the equivalent capacitance of the three mentiond above with explination about series or parallel. THANKS.
  20. In Fig.MIC cap (part of FM Tx from http://hem.passagen.se/communication/bug_rf.html) below you can see the MIC connected to 100nF cap. I Know that 100nF value is not critical and I may use say 10uF, but to what level this value is not critical? What happen if i use a 4700uF or 1pF, please explain. Thanks WAKLID.
  21. in many circuits there is an air coil, they told you that it is say 0.1 uH and also told you number of turns you must made and the former diameter and if this close wound or loose. If i try different values to achieve the same 0.1 uH using some formula, is then this coil operate ok as the previous mentioned. Thanks.
  22. i read this frequently, how I know that this antenna is 50 ohm, or how i can made a 50 ohm antenna, what the theory behind this. If it is a transmitter antenna and the transmitter operate at 50 MHz Fm what is the optimum length of this antenna to be 50 ohm. What the length if the freq. = 100 MHz. Thanks.
  23. When I read this statement I understood that if the carrier signal is say 100MHz it will be shifted to 100.01 MHz up and 99.99 MHz down, is this true? Thanks.
  24. i see often dots in transformer symbols, how these dots. How can I determine these dots practically on a real transformer. Thanks Walid.
  25. just plain cordless phones, i have one i need to repair it or at least understand its different circuits.
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