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this varicap is 520pFAveragesupernova said:You will have a hard time finding a 500 pF varicap diode. I did a quick search on digikey and found: https://www.digikey.com/products/en?keywords=varactor
Svein said:The varicap is voltage driven, so you must isolate it with a 1nF capacitor at each end. Or - bring one end of the varicap to ground like this:
View attachment 221826
I search a lot in internet and i so your circuits, and i did not find help so i come here to help me, because i have no variable capacitorSvein said:The varicap is voltage driven, so you must isolate it with a 1nF capacitor at each end. Or - bring one end of the varicap to ground like this:
View attachment 221826
Which varicap? The link I posted offers a number of them and nothing comes close to 500 pF.michael1978 said:this varicap is 520pF
A minor difficulty with that circuit is the floating voltage of the tuned circuit secondary.michael1978 said:i found this am receiver, but is using variable capacitor, how can i replace of do with varicap this circuits(i hava varicap bb112)
yes me too, but i connect without antenna i hear noiseMerlin3189 said:Datasheet
min = 440 pF, typ = 470 pF, max = 520 pF
I can't vouch for the datasheet as I've never had a BB112, but Siemens should know.
Edit: PS and BTW, I'm testing some diodes - varicaps, LEDs and simple power diodes - at the moment, and the record is over 1 nF for a Lucas DD710 low voltage power diode
@michael1978. That looks like LTspice. Have you been successful running that simulation ?michael1978 said:No this was orginal circuits so i replace varicap bb112, but i hear only noise
yes i send you, if you can help me... I RENAME am.asc into am.txtBaluncore said:@michael1978. That looks like LTspice. Have you been successful running that simulation ?
There are some obvious problems, it is easier to edit your file than create another.
Can you please attach your file.asc to a post with the txt extension as file.asc.txt
Attach the plot file in the same way if the simulation worked. file.plt.txt
Baluncore said:Here is a modified bias circuit, no model for BB112 or Pot, but circuit should work.
View attachment 221877
C1 and C2 must be low leakage caps. Avoid power supply bypass caps.
thank you very much, but i have just 2 more question, why you pick 47K to potentiometer, normally they say a few megaohms, and last where to connect antenna and i need simple inductor 200u ? and that was only for test trnsformer thanksBaluncore said:Here is a modified bias circuit, no model for BB112 or Pot, but circuit should work.
View attachment 221877
C1 and C2 must be low leakage caps. Avoid power supply bypass caps.
1. The resistor needs to be big to keep the Q of the LC circuit high. Any coupling capacitor or varicap reverse leakage current will flow through the resistor, so I use 47k as a starting value. Any resistor over 100k is likely to leak due to environmental contamination.michael1978 said:but i have just 2 more question, why you pick 47K to potentiometer, normally they say a few megaohms, and last where to connect antenna and i need simple inductor 200u ?
helloBaluncore said:1. The resistor needs to be big to keep the Q of the LC circuit high. Any coupling capacitor or varicap reverse leakage current will flow through the resistor, so I use 47k as a starting value. Any resistor over 100k is likely to leak due to environmental contamination.
2. For MW reception I would wind 75 turns on a ferrite rod and use that for the tuning inductor and the antenna. No external antenna is needed.
You could experiment with 50 turns of wire on a 2" or 50mm plastic pipe or cardboard roll. Wind a temporary primary of 12 turns over the coil to couple an external antenna.
sorry i make you tired, can you answer me please... thnxmichael1978 said:hello
but can i use inductor in form of resistor? in place of ferrit rod they are the same of not?
an inductor ISNT a resistormichael1978 said:hello
but can i use inductor in form of resistor? in place of ferrit rod they are the same of not?
He might be asking if he can use a high-resistance wire-wound resistor as an inductor, but I'm not able to decode his questions very well.davenn said:an inductor ISNT a resistor
berkeman said:He might be asking if he can use a high-resistance wire-wound resistor as an inductor, but I'm not able to decode his questions very well.
sorry you misundestand me, i have a inductor(i don't know how they call? they look like resistor, but they are inductor)davenn said:
yes like that...Merlin3189 said:
sorry you misundestand me, i have a inductor(i don't know how they call? they look like resistor, but they are inductor)davenn said:
I have those inductor, thay work of not with this inductor?Baluncore said:1. The resistor needs to be big to keep the Q of the LC circuit high. Any coupling capacitor or varicap reverse leakage current will flow through the resistor, so I use 47k as a starting value. Any resistor over 100k is likely to leak due to environmental contamination.
2. For MW reception I would wind 75 turns on a ferrite rod and use that for the tuning inductor and the antenna. No external antenna is needed.
You could experiment with 50 turns of wire on a 2" or 50mm plastic pipe or cardboard roll. Wind a temporary primary of 12 turns over the coil to couple an external antenna.
OK, got that cleared upmichael1978 said:I have those inductor, thay work of not with this inductor?
View attachment 221909
If you use a two terminal axial inductor, how will you connect the antenna?michael1978 said:I have those inductor, thay work of not with this inductor?
thanks merlinMerlin3189 said:I don't know whether these inductors would work in this tuned circuit, but since he has them he could try it out. I would expect to be able to couple to it with an overwinding of a few turns, just as in the original circuit.
Baluncore said:If you use a two terminal axial inductor, how will you connect the antenna?
The antenna connects to the primary of the RF transformer, the secondary is tuned to resonance with a capacitor.
sorry i did not know about that, i was think you bay axial inductor and you fixit, so you need a transformer and for every tunned receiver, but for what you use axial inductor?Baluncore said:If you use a two terminal axial inductor, how will you connect the antenna?
The antenna connects to the primary of the RF transformer, the secondary is tuned to resonance with a capacitor.
Axial inductors were used in “through hole” circuit boards, but where still needed are replaced by surface mount components. Axial inductors were used to reduce the size of PCBs in analogue television and video tape recorders.michael1978 said:but for what you use axial inductor?
It is hard to keep the magnetic field inside a small inductor that is wound on an open ferrite spool. Well designed axial inductors will have an outer magnetic sleeve or screen to reduce the external stray fields. That will make it more difficult to magnetically couple by over-winding a primary.Merlin3189 said:I don't know whether these inductors would work in this tuned circuit, but since he has them he could try it out. I would expect to be able to couple to it with an overwinding of a few turns, just as in the original circuit.
thanks for helping me... ;-)Baluncore said:It is hard to keep the magnetic field inside a small inductor that is wound on an open ferrite spool. Well designed axial inductors will have an outer magnetic sleeve or screen to reduce the external stray fields. That will make it more difficult to magnetically couple by over-winding a primary.
It would be good to have a working receiver of conservative design, then to try minor changes in an attempt to improve performance.
It is a truism that, implementing too many new ideas in one prototype will always result in a failure.
Small inductors usually have thin wire, so high resistance. That makes the Q low and so reduces the selectivity if used in a tuned circuit.
Tuned circuit Q will be reduced by the varicap series resistance of a few ohms. That needs to be countered by an inductor with lower series resistance.
Maybe something is different, or they do not know what they are doing or what they are saying.michael1978 said:what doe mean, when they say primary coil 200micro, and what about secondary? they don't say nothing about secondary?