Inductor for RF transmission and a few questions

[bananasiong]

Hi,
For an example, a FM transmitter, why does the coil need to be hand made but not the inductor? Is it because of the inductor consists of a coil and a resistor in series and a capacitor in parallel with them (I found this inductor equivalent circuit from the reference book)? I've made a FM transmitter, by calculation, the inductance of the coil is around 760 nH. It works with a 33 pF trimmer capacitor. In order to get the FM band, the inductance of the tank has to be around 76 nH. Is it because of the stray capacitance and inductance? So for designing such circuit, we have to reduce the value of the L and C for an expected frequency?

Since the frequency is inverse proportional to the product of LC, by increasing the L, is the quality, Q also increased?

I'm still learning RF oscillator from a reference book, but I can't figure out why a RFC is needed as the load of the collector or the drain of bjt or fet for Colpitts and Hartley oscillator.

I've seen a power supply design with additional of RC filter between the main filter capacitor and the load to reduce the ripple. Will the limitation of the load current be reduced?

Thanks

 

[Hero999]

Please post the schematic.

 

[Papabravo]

I thought a coil and and inductor were the same thing. Maybe I missed something. AFIK either one can be manufactured or handmade.

 

[RadioRon]

Hi,
For an example, a FM transmitter, why does the coil need to be hand made but not the inductor? Is it because of the inductor consists of a coil and a resistor in series and a capacitor in parallel with them (I found this inductor equivalent circuit from the reference book)? I've made a FM transmitter, by calculation, the inductance of the coil is around 760 nH. It works with a 33 pF trimmer capacitor. In order to get the FM band, the inductance of the tank has to be around 76 nH. Is it because of the stray capacitance and inductance? So for designing such circuit, we have to reduce the value of the L and C for an expected frequency?

Since the frequency is inverse proportional to the product of LC, by increasing the L, is the quality, Q also increased?

I'm still learning RF oscillator from a reference book, but I can't figure out why a RFC is needed as the load of the collector or the drain of bjt or fet for Colpitts and Hartley oscillator.

I've seen a power supply design with additional of RC filter between the main filter capacitor and the load to reduce the ripple. Will the limitation of the load current be reduced?

Thanks

I'm confused by all the questions. First of all, there is no difference between "coil" and "inductor", these terms mean the same thing. But I suspect the poster is using Coil when they really mean RF Choke or RFC. A component manufactured specifically to be used as an RF choke may have compromised Q because this is not as critical a parameter for an RFC application. But an RFC may simply be an inductor with an appropriately larger value of inductance for the circuit.

Not seeing your schematic, I'll just make a general comment to the other questions. RFCs are used to separate the DC bias circuit from the AC circuit. If an RFC is used, it is probably so that DC current goes through the RFC whereas AC does not.

I calculate that your inductance when using a 33 pF trimmer should be about 100nH in order for the trimmer to tune to 88 MHz at maximum capacitance. Yes, circuit parasitics will add to the inductance and capacitance but if reasonably well built with short wires, the extra amounts may not be more than about 10% or so in such a circuit as this.

 

[bananasiong]

I mean the air core hand made coil versus the inductor with value. There is no schematic, I just want to know their different. Why can't I just use a 100 nH (if available?) instead of fold the enamel coated wire?

 

[kchriste]

Why can't I just use a 100 nH (if available?) instead of fold the enamel coated wire?

You can... Except you will find it hard to find and way cheaper to coil your own.

 

[RadioRon]

The resistance of an inductor and hence its Q is a function of core material amongst other things and this dependence is greater with higher frequencies. When you wind your own inductor as an air coil, you know that it is optimum because an air coil has no core losses. When you use a manufactured inductor you might not know what its Q is, if you don't have the manufacturer's specification for it.

However, when faced with having to guess, I would bet that your 100nH inductor will work as well in your application as a hand made air wound coil of the same inductance. This is because I have found that most inductors that have lossy cores are much higher value, like 5 uH and higher. An inductor that is labelled as 100nH is probably the type with pretty good Q.

 

[bananasiong]

No wonder last time before I start doing the transmitter, the manufactured inductor is not encouraged.
You said that not more than 10% different, but from calculation, the inductance of the coil is 760 nH... almost 10 times.

What about the RC filter that I mentioned previously? After the big filter capacitor, RC filter is added (just like low pass filter) to reduce the ripple.

 

[RadioRon]

No wonder last time before I start doing the transmitter, the manufactured inductor is not encouraged.
You said that not more than 10% different, but from calculation, the inductance of the coil is 760 nH... almost 10 times.

What about the RC filter that I mentioned previously? After the big filter capacitor, RC filter is added (just like low pass filter) to reduce the ripple.

I think that you misunderstand my point. If the manufactured inductor has a rating of 100nH then it will be 100nH +/- 10% in your application. If you connect this inductor into your oscillator, it will probably work.

If your calculations indicate that 760 nH is needed then I think that your calculation is in error. When I do a simple calculation for a tank circuit for 100MHz, I calculate that 100nH, not 760nH is needed. Please check your calculation again.

The RC filter will indeed reduce the output current available from a DC power supply. There will be a voltage drop across the series R depending on how much current passes through it. It is common to use simple RC filters where the amount of current going through the resistor is small, resulting in an acceptably modest voltage drop. The R value is usually quite low.

 

[bananasiong]

If your calculations indicate that 760 nH is needed then I think that your calculation is in error. When I do a simple calculation for a tank circuit for 100MHz, I calculate that 100nH, not 760nH is needed. Please check your calculation again.

Maybe you have misunderstood mine? I didn't mean that from the tank, 760 nH is needed but a 76 nH. I meant the inductance of the air core coil I made. From calculation, i is 760 nH. 10 turns, 4.5 mm radius and 10 mm of length. So it is around 800 nH from calculation. Perhaps the trimmer cap is not the same value as it rated?

I've read a thread that say about the value of the filter capacitor of the regulated power supply. The value of the filter capacitor shouldn't be too big because of the rectifier diode? Then what is the limit of it?

Thanks

 

[RadioRon]

You're right, I did misunderstand you! Now, my quick check also shows that a coil of that size should be around 570nH, which is also quite high, so I'm wondering what the answer to the problem is.

One of the problems that occurs when you use a coil that is much bigger than it should be is that the parasitic capacitance in the coil can cause it to not behave as expected. I'm wondering if this is a case where the parasitic capacitance in this coil is so large that much of the inductance is cancelled out and the real inductance at 100 MHz is actually much lower. This is a common problem and it is necessary to choose an appropriate range of inductance. 800nH is a very high value for use at 100 Mhz because it's impedance is about 500 ohms which is high for anything other than a choke in most RF circuits.


Do I understand correctly to say that when you put this large coil in the circuit it works correctly? How do you determine that it is working correctly?

It came to me later that perhaps your capacitor is broken or not properly connected, and in fact you are resonating your circuit only with that big coil. The capacitive reactance in the coil may be resonating with the inductance of the coil, at 100Mhz.

 

[bananasiong]

I can listen my voice from the FM radio, so the range must be in FM band. The capacitor is not broken nor not properly connected. I can tune it to change the transmitting frequency, maybe its value is not as rated?

 

[RadioRon]

Perhaps the capacitance is actually 3.3pF? Can you provide a close-up photo of the capacitor? Is it a junkbox part that you found or did you buy the capacitor?

One thing that I would do is to replace the variable capacitor with a fixed value type (ceramic) of about 22 pF or 27 pF (within the range of 33pF), then see if your signal can be heard anywhere on the FM radio band. If not, then I would try making a much smaller coil, something that theoretically should come closer to 100 nH, and solder that in place of your big coil. You can tune the frequency around by pulling the coil more open (making it longer) or pushing it tighter together, to help you find your signal on the FM receiver. This idea would help confirm if your variable capacitor is right or not.

Another thing that would be very useful for us to help you is to show us the appearance of your construction of the transmitter. Can you make some photos of the transmitter showing how you connected each component?

OH, and also important, can you please provide a schematic diagram of your transmitter?

 

[bananasiong]

I bought the trimmer cap from an electronics shop. The Fm transmitter is mod4 by audioguru and the photo of it.

 

[RadioRon]

Thanks for posting the picture and the other info. I think the trimmer is probably OK.

It is still difficult to see if there are hookup problems so a picture of the bottom side would be useful too. However, one thing I am wondering about first is whether the turns on your inductor might be touching each other. It is critical that each turn of the coil not touch either of the turns before or after it. This can happen if you use bare copper wire. Notice that Audioguru's example in his photo uses coated wire (that's why the wire is red) which insulates one turn from another. Take something thin and push it between each turn. This would put a small air gap between each turn. See if that is the problem.

I notice that your coils are indeed larger than Audioguru's example. His appear to be 0.4inches long, 9 turns and 0.2 inches outside diameter. His layout is also much easier to follow against the schematic. Why did you not follow his component placement and wiring example? At these high frequencies, component placement and wiring is as important to the performance as are the component values.

 

[bananasiong]

Yes, I'm using enamel coated wire, pretty sure. Are you sure the diameter of his coil is only 0.2 inches? Then mine is almost twice of 0.2 inches, the rest of the measurements are almost the same.
For his layout design, if I've not mistaken, one or two components are placed at the bottom of the veroboard. My placing of my layout is almost the same as the schematic.

 

[RadioRon]

It is easy to see in his photo what the sizes are because the holes in the pcb are spaced by 0.1 inches

**broken link removed**

I am puzzled by those parts near the collector of Q3 on your board. Looks like two resistors and a small capacitor. What are those parts?

I don't know what else to suggest at this point. One other minor issue is that the components close to the coil can alter the inductance but usually the alteration is small. You have components right up against the coil, but I don't think this would vary the inductance enough to explain the problem. Can you take a picture of the other side of your board?

If I had this board in front of me, I would first inspect it very carefully especially around the tank circuit of Q2, which determines the frequency. I would make sure that the connections are exactly as shown in the schematic, and that the capacitance and inductance connected to Q2's collector is as they should be. I think I would also disconnect C12 to simplify the circuit temporarily and see if this changes anything. I might also temporarily disconnect R2 as well to shut off Q1 to help simplify and isolate the oscillator, Q2.

 

[bananasiong]

Oh ya, looked thinner.

I am puzzled by those parts near the collector of Q3 on your board. Looks like two resistors and a small capacitor. What are those parts?

Ha.. The capacitor and the resistor in horizontal are not shown in mod4, I added them at the emitter of Q3 to reduce the power dissipation of the transistor as I felt it is getting a little warm when it is turned on. While the 45 degree 33 khm: resistor is the R3 for Q1. I used 180 khm: and 33 khm: which are available for me to form the voltage devider.

 

[audioguru]

Banana,
Your coils are much bigger than mine.
You have the RF amplifier spaced very far away from the RF oscillator so the stray capacitance and stray inductance is increased a lot.

 

[bananasiong]

Now only I realize that my coils are bigger. But in logically thinking, my coil has higher inductance than it is supposed to be, plus the stray capacitance and inductance, the frequency should be much lower than expected right?