# Capacitor value

Discussion in 'Homework Help' started by mihobbi, Apr 6, 2008.

1. ### mihobbi Thread Starter New Member

Apr 6, 2008
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I need to determine the vaule of a coupling capacitor between the collector of a receiver IF mixer transistor at a frequency of 47.21 MHZ and the input of a bandscope.

2. ### beenthere Retired Moderator

Apr 20, 2004
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What is a "bandscope" and what is it's input impedance? Got the circuit? What signal bandwidth are we dealing with? Can't be broadcast band, because those IF's are 10.7 MHz or 45 MHz, or AM at 455 KHz (I know, old automotive AM at 267 KHz).

Lots of IF's take signals off the emitter, with the collector tied to a voltage rail. More information would be very helpful.

3. ### mihobbi Thread Starter New Member

Apr 6, 2008
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OOPS! I forgot to state the input impedance. It's 50 ohms. Sorry, "bandscope" is used interchangeably with "spectrum analyzer". The desired bandwith is 100KHZ to 500KHZ. Yes, of course, the signal is taken off the emitter. The receiver has 3 IFs, 455 KHZ, 10.7 MHZ and 47.21 MHZ and unfortunatly the variable frequency oscillator mixes with the 47.21 which is the stage before the filtering circuits. Taking the signal sample after this stage would make the signal to the input of the spectrum analyzer too narrow. I believe the value for a 455 KHZ IF would be about 12pf.

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
12 pF would attenuate the signal. The reactance is about 3000 ohms @ 455KHz. 6.8 nF gets you more like 50 ohms.

5. ### aamash New Member

Apr 13, 2008
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What Is The Relatio Between Magnitic Field And Magnetic Momentum/

6. ### aamash New Member

Apr 13, 2008
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Thanks For Reply The Problem Is That I M Chemistry Student So My Physics R Not Very Good