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my varactor extension

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mstechca

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I made some changes to my circuit, and I have upped the resistance values, and I can't seem to get it to change stations. In order for me to change stations, I connect the TRIG input to ground, which creates a keypress.

The pull-up resistor is 27K.

The only diode that seems to work the best is 1N914B.

I think before I continue, I need to know:

1. what is the absolute MAXIMUM current the 1N914B can handle?
2. what is the absolute MINIMUM current the 1N914B can handle?
3. what is the absolute MAXIMUM voltage the 1N914B can handle?
4. what is the absolute MINIMUM voltage the 1N914B can handle?

I think that once I get the above 4 answers, I can adapt my circuit to any diode. what do you guys think?

and yes, a high coupling capacitor seems to make sense, so I'm going with 300pF to 1000pF.

btw, the rest of my radio circuit (including the shown tank circuit) works great.

I can easily tune by adjusting the trimmer, but I want to tune with the varicap as well.

someone please give me some insight.
 

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mstechca said:
I can't seem to get it to change stations. The only diode that seems to work the best is 1N914B.
Of course it doesn't work. You don't have a varactor diode. You also don't have a power-up reset circuit for the counter so who knows what its outputs are doing? You also don't have a debounce circuit to clock your counter, so who knows how many times it gets clocked with each press of the pushbutton.

Look at the miniscule change of capacitance of a 1N914B on its datasheet.
**broken link removed**
It goes from 0.826pF at 6V to 0.865pF at 0V. That's nothing. You need a varactor to go from about 5pF to about 35pF.

I need to know:

1. what is the absolute MAXIMUM current the 1N914B can handle?
2. what is the absolute MINIMUM current the 1N914B can handle?
3. what is the absolute MAXIMUM voltage the 1N914B can handle?
4. what is the absolute MINIMUM voltage the 1N914B can handle?

I think that once I get the above 4 answers, I can adapt my circuit to any diode. what do you guys think?
Who cares about its forward-biased max ratings? They are on its datasheet. It is used reverse-biased in your circuit and its voltage rating is 75V and its leakage current is much less than 25nA.

The resistors at the output of your counter have values much too high so your meter will load down the voltage too much. Use about from 33k to 220k.
 

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Your resistor does not do binary weithing. Frequency change is not progressive with trigger. Frequency jumps forward backward.
First of all you should use so called R/2R network to perform D/A conversion.
Your idea of changing frequency will give you about 800kHz shift in 16 steps. FM is 20.5MHz wide.
 
Didn't I post a circuit on one of your many separate posts where a circuit works with three 2N4401 transistors in parallel, used as varactor diodes?????
 

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A station on the FM broadcast band is 150kHz wide. They are spaced 200kHz apart.
 
audioguru said:
You don't have a varactor diode.
They are too expensive compared to normal diodes.

The resistors at the output of your counter have values much too high so your meter will load down the voltage too much. Use about from 33k to 220k.
How did you calculate 33K to 220K? what parameter did you take fromthe datasheet to calculate the resistance values?
 
Using varicap diode is a must. Othervise you would not be able to cover entire FM band. Capacitance change factor from min. voltage to max. voltage must be greater than 1.7. Ordinary diodes have this factor much smaller.
If varicap diode is too expensive why did you spend so much money so far for a such project at all.
 
mstechca said:
audioguru said:
You don't have a varactor diode.
They are too expensive compared to normal diodes.
Then your tuning control won't work. There is a varactor diode in the "scanning" FM radio at the dollar store. Its retail cost in the radio must be about 10 cents. You can buy a varactor diode that is made to do the job correctly for a cost that is less than half the cost of a single cup of coffee.

The resistors at the output of your counter have values much too high so your meter will load down the voltage too much. Use about from 33k to 220k.
How did you calculate 33K to 220K? what parameter did you take fromthe datasheet to calculate the resistance values?
The diode is reverse-biased and doesn't draw any current (much less than 25nA!). If you want to monitor the voltage with a 10M multimeter without inaccuracy caused by loading then the resistors must be much lower than megohms. In your other thread I showed what happens if the resistors are too low in value like you had before. Didn't you try to measure the voltage steps?
 
As I mentioned the resistor network is wrong. Here is a pisture how the diode voltage will change with number of trigger pulses.
 

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The diode is reverse-biased and doesn't draw any current (much less than 25nA!). If you want to monitor the voltage with a 10M multimeter without inaccuracy caused by loading then the resistors must be much lower than megohms. In your other thread I showed what happens if the resistors are too low in value like you had before. Didn't you try to measure the voltage steps?
I figured that the voltages will be in 6% increments per bit (about 0.36 volts per bit on a 6V supply), because I was told that the counter will produce a low or high output. 14 of the 16 output states should have resistors connected to VCC and resistors connected to ground. This creates a voltage divider network.

Do you understand what I am talking about, or should I show diagrams?

It would be much better if I can complete the circuit with a 1N4007 or a 1N914, because I don't want to go 20km again for a single varicap diode.
Transportation costs money too.
 
You changed threads so many times that I am not going to search for the diodes' spec's again. I recall that ordinary diodes don't have enough of a capacitance change to tune your radio. Isn't that why varactor diodes are made and used?
 
Audioguru,

We understand varicaps change capacitance based on the controlling voltage.

and we understand that resistors can provide the proper voltage.

and we understand that each diode has a different capacitance range.

so here is my question:

Why is it that I get bad results when I use resistors in the 1K - 10K range, I get fair results when I used resistors in the 100K to the 470K range, and I get even worse results when using resistors in the megaohm range?

according to logic, the answer must be that the diode can accept only current within a certain range. Am I right?

when you think about it, all the values from 1 to 15 have pull-up and pull-down resistors, and the voltages for each binary output is the same.
 
mstechca said:
Audioguru,

We understand varicaps change capacitance based on the controlling voltage.

Yes.

and we understand that resistors can provide the proper voltage.

Yes, IF wired correctly!.

and we understand that each diode has a different capacitance range.

Yes, and you've been repeatedly told that a small signal diode is completely useless as a varactor - best is a proper varicap diode, but power rectifiers work far better than small signal ones.

so here is my question:

Why is it that I get bad results when I use resistors in the 1K - 10K range, I get fair results when I used resistors in the 100K to the 470K range, and I get even worse results when using resistors in the megaohm range?

Because you've ignored almost everything you've been told, and ignored all the circuits that have been posted. You should have a single resistor (as shown in the circuit above) feeding the voltage to the varicap from the voltage source - this resistor should be VERY close to the varicap, and isolates capacitance changes in the resistor switching from the RF circuit.

according to logic, the answer must be that the diode can accept only current within a certain range. Am I right?

NO! - completely wrong, as you've also been told repeatedly, there's NO current involved, varactors work by applying a reverse bias voltage - and no current will flow, or there's something seriously wrong!.

when you think about it, all the values from 1 to 15 have pull-up and pull-down resistors, and the voltages for each binary output is the same.

No, as you've (yet again!) been told repeatedly, you need an R2R ladder to do that - your scheme just provides 'confusing' voltage changes, up and down - as bloki kindly drew.
 
A picture is worth a thousand words:
 

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Diode can be 1N4007. Coupling capacitor 1nF has to be smaller. How much depends on wanted frequency deviation and value of trimmer capacitor in LC tank.
Using only 4 bit counter means you will have only 16 steps or frequencies. This will not allow you to tune receiver to a station because frequency step is about 1.3MHz for full FM band. Stations are spaced 200kHz appart in USA. So you need minimum 8 bit counter to get about 100kHz steps. Even this resolution does not guarantee accurate tunning because voltage steps does not produce equal frequency steps. More bits are needed.
If you want to tune receiver UP/DOWN counter is neccessary.
R/2R ladder network will give you too low voltage to tune frequency in full band. Amplifier is needed.
 
You do not need an R-2R ladder to make a simple D/A converter. The original circuit will work, but you have the bits reversed. You need the smallest valued resistor (R)on the MSB, 2R on the next bit, 4R on the next, etc. See the revised schematic below. The color coding follows your original notes. You can make a perfectly useable DAC in this manner, so long as you don't want to go too fast or have too much resolution.
Having said that, you will have tuning resolution problems if you want to make a receiver. You might get by in a simple transmitter, but your channel spacing will be non-uniform because of the voltage-vs-capacitance curve of the varicap.
Also, at zero volts bias, the sine wave on the diode will forward-bias it on a portion of each cycle, which will probably cause you all kinds of grief.
You would probably be better off to use 4000 series CMOS so you can build in a reverse bias at zero code to prevent this problem, and also get a wider tuning voltage range.
 

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Hee, hee. :lol: Is the tuning for the 15uW toy transmitter, or is it for the super-regen toy radio with its tank in its emitter instead of in its collector which eliminates its "super" self-quenching part????????????

It's for both? Maybe it is a single tuned circuit for both, then it doesn't need a range more than 15 feet.
:lol: :lol:

It doesn't use a varactor diode, but here is the familiar-looking schematic of a super-regen receiver in a Chinese toy car:
 

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audioguru said:
Hee, hee. :lol: Is the tuning for the 15uW toy transmitter, or is it for the super-regen toy radio with its tank in its emitter instead of in its collector which eliminates its "super" self-quenching part????????????

It's for both? Maybe it is a single tuned circuit for both, then it doesn't need a range more than 15 feet.
:lol: :lol:
Laugh, I don't care, but at least I got great progress, considering no professor taught me how to make a single radio. In fact, a book taught me.

did I say progress? :shock: maybe I have to stop using that P word. (progress *gulp*) :lol: :lol: :lol:

and check this attachment. Am I thinking right with the circuits below, or am I off?
 

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Horray! Your yellow is an ADC!
Good going, MStechca! :lol: :lol:

When I was in college my professors were imports and could hardly speak English. I don't think they ever saw a radio nor understood how one works. I taught them lotsa stuff about electronics, English and life over here. Their daughters from over there were ....! (wild!?) :lol: :lol:
They were (are?) weird people (animals?) but lots of fun. :lol:

Nobody taught me how to make a radio, certainly not my professors, they didn't know how.

With my diploma in my hand, I got a good job working for Philips, who had real engineers who taught me lotsa good stuff! Most of them spoke pretty good English too. And their daughters were .....!!!!!!!! Outa sight, man!!
 
don't forget to read the text in my attachment.

and you yaying at my yellow schematic is not yayable because the one before is the one I need improvements on.

I guess if you were yaying, then it means the other two schematics are correct.
 
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