TRF receiver....

[elec123]

Hi,

Please see the file attached.

In this circuit the tuning capacitor varies from 4 to 176pF.

I need to calculate the inductance (L) required to tune to a minimum frequency of 600kHz.

Using w=1/root L * C w=2*pi*f

Do i take C=172pF? f= 600kHz?

putting the values in i get L=409microH.

Don't really understand what difference tuning to a min or max makes to the calculation...

Would appreciate if someone cld go over this...

Thanks in advance

 

[Nigel Goodwin]

For a start, don't post diagrams as JPG, use GIF or PNG instead, these are much smaller and of better quality - also don't post a small diagram and a huge white area - trim the diagram sensibly!.

As for your question, you calculate the inductance based on the lowest frequency and the highest value of the capacitor (in this case 600KHz and 176pF).

Once you have the inductance value, you use that, along with the minimum capacitance (4pF), and calculate the highest frequency it will then tune to. Bear in mind that stray capacitance will actually increase the 4pF figure, so it won't tune as high as theory indicates.

 

[elec123]

Thanks for your reply, and sorry for being dumb with picture!!!

 

[k7elp60]

Since you know the minimum and the maximum values of the variable capacitor there is a easier way to calculate the maximum frequency with the values shown. The capacitance ratio is maximum capacitance/minimum capacitance. In this case 176/4 or 44. The tuning ratio of the capacitor is the sqt(square root of the capacitance ratio. In this case the sqt of 44 is 6.63. Since the lowest frequency is 600Khz. Multiplying 600Khz by 6.63 one gets 3.98Mhz. If you do the math the other way using the 400 uh inductor it comes out the same.
What I am saying in simple terms is the square root of the capacitance ratio times the lowest frequency is the maximum frequency.
Looking at it another way if you know the upper and lower frequencies to be tuned you can determine the capacitance ratio needed to tune them.
Upper frequency/Lower frequency = Frequency ratio. Tuning ratio=Frequency ratio squared.