rf transformer question

[obrien135]

Hi,

I built a diode ring modulator and made the mistake of using a toroid transformer with an AL factor that was too low, so that it loaded down the audio signal from the microphone too much. The transformer that inputs the audio signal to the diode ring is OK (it's an audio transformer) but the one that outputs the modulated signal to the IF section has too low of an inductance. Based on what I have seen on the internet as far as toroid cores are concerned, I would need a core about 2.4 inched O.D. and about 50 turns on the primary and secondary to achieve an impedance of about 50 ohms. I haven't seen radios with transformers that big very often. Is there some other way to solve the problem? The IF frequency is 3MHz.

George
KB1PDK

 

[FvM]

2.4" toroid core would be rather used for 100W or kW transformers, I think. Actually, neither impedance matching nor an intended inductance requires a particular diameter. It's a matter of AL value and number of turns. I use 0.5" or smaller for signal transformers. Generally, your question would be more clear, if you show the complete circuit.

 

[obrien135]

Here it is.

 

[obrien135]

Isn't the AL factor somewhat dependant on the size?

 

[FvM]

It depends on the ferrite's µr and and the Σl/A geometry factor. So scaling only the inner and outer diameter keeps AL, increasing
the height increases A and AL respectively. Using a higher µr ferrite may be an alternative.

On the other hand, a large transformer has typically a higher windings capacitance, resulting in a lower transformer bandwidth. Thus
high bandwidth RF transformers (and coils as well) are preferably small, unless a high power handling is required.

 

[vfone]

Use the attached modulator topology and you don't have to care about audio transformers.

 

[obrien135]

Thanks, I was able to get some sound out of it. Sometimes it does and other times it doesn't. My L.O. is unstable. There is a lot of noise on the power supply. I have been trying to get rid of it, but I think I might have to use some other type of supply. Maybe if it was more stable it would transmit better. I'm not sure if there is always a signal getting through the IF section. The only way I have to check it is with a small receiver (no O'scope) and since it is close to the transmitter, perhaps it is picking up one or more of the oscillators (hence the hiss) but not the modulated signal. I'm not sure what else could be the problem.

 

[obrien135]

I wonder if I used a battery, if that would be quieter. Maybe when I have the money I will buy one.

 

[obrien135]

Do you think that could be the problem?