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That's not nearly enough capacitance.mstechca said:The perfect varactor for me is one that can produce an output capacitance of 0 to 10 pF based on an input voltage from 0 to 9.
It won't tune the entire FM band.
The good news is that I made an outsmarting feature.The voltage of your 9V battery will quickly drop to 7,2V then slowly drop to 6V over its life. The tuning will automatically scan up in frequency as the battery runs down.
WRONG again. The LED shows the output. I can have it autoscan without the LED.audioguru said:1) Why use an LED as a detector diode instead of an ordinary detector diode?
thats ok, because when it gets to the point that it stops, I'll rip out the connection to the clock that restarts the scanning, and it wont rescan any more :wink:2) You're making a scanning radio in reverse. It stops scanning when it detects nothing. Then your transmitter starts modulating and the scanning resumes. Good-bye communication.
why should the obsolete factor of a technology have to do with its availability or productivity?3) Don't you think that AM is outdated?
Cmos ICs have a higher output resistance at lower supply voltages. Using a 5V regulator at its minimum of 4.5V, a typical 74HCxx output can provide 35mA at 2.25V which calculates to be a 64 ohm resistor. Its minimum current is 12mA so its max resistance is 188 ohms.
All the 74HC4017 ICs I've used in my 6V Ultra-bright Chaser project measure typical or better, that's why they're so darn bright!
If you use 10k resistors as the minimum value in your resistor ladder at the outputs of the counter then the circuit's max error would be only 188/10k= 1.88%. If you use 100k resistors then the error is 0.2%.
If you operate the counter with a 6V supply (its absolute max is 7V) then the error is halved.
If you use 5% resistors in your ladder then you don't need an extremely low error with very high resistor values. You also get error with extremely high resistor values due to leakage current of the varactor diode and you won't be able to measure the tuning voltage with a multimeter without a high error.mstechca said:ok, now that I know the minimum resistors to use at the output of my counter, I can't remember the absolute maximum resistances I can use,and why are there maximums when the error level is lower?
Of course not! A 74HCxx IC on a 5V or 6V supply can provide much more than its absolute max continuous output current rating of only 25mA to a short and will go "poof". It can momentarily provide an output current of 60mA to quickly charge stray capacitance for its high speed. An ordinary CD4xxx Cmos IC can provide an output current of only a couple of mA to a short with a 5V or 6V supply.does your error level stated above determine the chances of a short circuit?
I always use 5% tolerance resistors.If you use 5% resistors in your ladder...
If you remember, I'm not using a varactor diode.You also get error with extremely high resistor values due to leakage current of the varactor diode and you won't be able to measure the tuning voltage with a multimeter without a high error.
mstechca said:So do I have to make the resistances low enough to provide current greater than the diode's leakage current to override it or am I off here?
DAC resistance affects 2 things in your circuit:mstechca said:I always use 5% tolerance resistors.If you use 5% resistors in your ladder...
If you remember, I'm not using a varactor diode.You also get error with extremely high resistor values due to leakage current of the varactor diode and you won't be able to measure the tuning voltage with a multimeter without a high error.
I have been playing, and so far, I am using a transistor as a varactor diode. I connected emitter and base together and grounded them, and I connected collector to the coupling series capacitor and the resistors.
So do I have to make the resistances low enough to provide current greater than the diode's leakage current to override it or am I off here?
I would hope that the impedance of the resonating tank would be much higher than just the XL of its inductor. Therefore DAC resistors should be in the hundreds of k ohms. :lol:Ron H said:DAC resistance affects 2 things in your circuit:
1. The Q of your tank.
The reactance XL of your 100nH inductor at 100MHz is 63 ohms. The resistance of your DAC is about half the resistance of the MSB (lowest value) resistor (0.533 for a 4-bit DAC, to be more precise). Q=R*XL. To avoid killing the Q, you should probably keep the resistance at least 100*XL, or greater than about 6k. Nigel's advice is spot on, as usual.
audioguru said:I would hope that the impedance of the resonating tank would be much higher than just the XL of its inductor. Therefore DAC resistors should be in the hundreds of k ohms. :lol:
Leaving the series resistor out would save a whopping 10 cents. :lol:Nigel Goodwin said:This is all ignoring the series resistor feeding from the D2A to the varactor, which Audioguru has rightly included on various diagrams he's posted.