First, a fixed bandpass filter from 3MHz to 30MHz isn't going to do you much good unless you have a stupendously crappy radio. If you wish to build one none the less you'd be best to just cascade a 30MHz lowpass with a 3MHz highpass.
An adjustable bandpass filter that lets you peak your receiver's response at one frequency and avoid strong signals that are pounding your receiver's front end may be useful, but making one that goes continuously from 3 to 30MHz would create difficulties, but it could be done.
I'd suggest you get a copy of the ARRL Handbook and start perusing it -- it has lots of useful information for this sort of thing and who knows? Maybe you'll want to get a license and get on the air.
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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
I am getting back into electronics, reading as much as I can and experimenting. I got to a part in a book that discussed filters and became very interested. I am also a Shortwave radio user and decided to create a bandpass filter for the SW radio, 3MHZ-30MHZ.
Now I start researching and there are way tooo many types for a beginner to start with. I realize that this filter may not be the best but want to build my own. I would like to create a good filter for that range? Recommendations on where to start, which filter type is better? Is one better than another for a beginner to tweak and build? Is one better in the freq range?
With this sort of oscillator, you have to analyze several completely separate phases.
One phase starts the moment the output transistor begins to turn off, and collector voltage heads toward ground by the pull down action of the 470 ohm resistor. during this phase, D1 first forward biases and clamps the base voltage for Q1 to no more than a diode drop and C1 charges through R2.
Eventually (this is where a bit of RC analysis comes in) the current through D1 goes through zero and C2 changes charge rate because the current through R1 in series with R2 sets the charge rate for C2. During this phase, the base voltage at Q1 swings between negative one diode drop toward positive one diode drop, where Q1 begins to conduct as the current through R1 detours from C2 to the base emitter junction.
Once Q1 conducts enough to start to turn Q2 on and the voltage drop across R3 starts to increase, a positive feedback loop forms that very quickly drives Q1 and Q2 into saturation because the positive voltage change across R3 gets coupled back to the base of Q1 through C1 and R2, making the current from R1 insignificant. This phase lasts as long as C1 R2 can supply enough current to Q1 to keep both transistors well saturated.
Once C1 charges enough that this current is no longer available, Q1 and Q2, while still conducting somewhat, fall out of saturation enough that the voltage drop across R3 starts to sag. At that moment, the current through C1 R2 reverses direction and Q1 shuts down and you are at the starting point of this description.
Each of these phases has to be analyzed to predict both the on and off time.
Please disregard my last post. I not only jumped between threads, I jumped between newsgroups. This was an answer to an entirely different post. My apologies.
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