SRF of inductor

[Powzoom]

I'm designing an AM radio. For the local oscillator, I'm using a clapp design with a BJT. I've calculated that I need a 1mH inductor. The frequency range of the radio is standard comercial AM, so .5-1.6Mhz. I can only find inductors with a SRF of 3.8 or 4.2Mhz. 10X is the recommended SRF value which is 16Mhz, but I can't find that. I don't have the equipment to work with SMD parts so it needs to be through hole.

Will the lower SRF have a great effect on my circuit? Also, other than a lower Q value, is there any difference between an inductor and a choke?

 

[JimB]

I'm designing an AM radio. For the local oscillator, I'm using a clapp design with a BJT. I've calculated that I need a 1mH inductor.

1mH seems rather high, how did you arrive at this value?

The frequency range of the radio is standard comercial AM, so .5-1.6Mhz. I can only find inductors with a SRF of 3.8 or 4.2Mhz. 10X is the recommended SRF value which is 16Mhz, but I can't find that.

Where did you get this factor of 10 from, this sounds unrealistic.
A self resonant frequency of 4Mhz sounds more practical.

Will the lower SRF have a great effect on my circuit?

Most unlikely.

Also, other than a lower Q value, is there any difference between an inductor and a choke?

Not really.

JimB

 

[dknguyen]

Except for common mode chokes. They can't be used as inductors of the same current capacity for non-common mode signals...the reason they have such high filtering and current capacities relative to regular inductors is they depend on a common signal through both ends cancelling each other out. Without it...they're nothing.

 

[Roff]

220uH-400uH is more common for AM radios.

 

[MrAl]

Hi,


I think a lower value for L would be better too due to the bandwidth of an
AM station.

The value of L not only affects the center frequency but also the bandwidth,
and this can vary considerably even though tuned to the same frequency.

Here is a list of some values and bandwidths for a radio tuned to a station at
1MHz:

200uH, 40kHz
400uH, 20kHz
800uH, 10kHz
1mH, 8kHz

From this we can see that 1mH will probably be too big and so might damp out
the higher frequencies a little too much. It might also be harder to tune
the station in with a narrower bandwidth.

For modern transmitters in the USA, you would probably want 400uH max,
and that may even be a little too high for the upper frequency stations.
300uH better.

 

[Roff]

MrAl, how did you calculate bandwidth? Did you make some assumptions about the Q of the resonant circuit?

 

[MrAl]

Hi Roff,


Yes, i was using theoretically perfect circuit elements just to give some
idea how the bandwidth changed with the inductance value. My
original intent was just to show how the bandwidth varies with
choice of inductor value, not to be as specific as i ended up being.

In any case, if you have an alternate method that would yield some
real world results just let me know and i'll use that method and post
some results.

Thanks, and hope your New Year is better than last.

 

[Roff]

Hi Roff,


Yes, i was using theoretically perfect circuit elements just to give some
idea how the bandwidth changed with the inductance value. My
original intent was just to show how the bandwidth varies with
choice of inductor value, not to be as specific as i ended up being.

In any case, if you have an alternate method that would yield some
real world results just let me know and i'll use that method and post
some results.

Thanks, and hope your New Year is better than last.

You must have had some resistance (series or parallel) in the tank circuit. Your Q values, assuming those are 3dB bandwidths, are very low for ideal components. I get Qs of over a million when I sim with ideal components.

 

[MrAl]

Hi Ron,


OH yes, perfect components have no loses. I used a termination of 50 ohms.
The inductor and cap are in series with this resistance too.

 

[Roff]

Hi Ron,


OH yes, perfect components have no loses. I used a termination of 50 ohms.
The inductor and cap are in series with this resistance too.

OK, now I understand.