Regenative Receiver Question [27 MHz]

[EE_Chris]

Hi Guys,

I've been staring at this schematic for a few days trying to get a better feeling for how this receiver is working, specifically the part in the red box as the rest is just amplification and H-bridge motor control. The transmitter pair for this receiver has a carrier frequency at 27.145 MHz.

My question: The tuning circuit seen in the schematic has an 18pF and 7T 0.3mm x 5mm(core) inductor, which seems to be somewhere in the 100-300 nH range. Wouldn't that be tuning for a much higher frequency than 27 MHz? Something in the range of 70 - 120 MHz?

It seems like the proper tuning circuit would need an 18pF cap and 1.9uH inductor for proper tuning? Any comments or help on understanding this receiver circuit better would be appreciated. Thanks.

Schematic: **broken link removed**

IC Datasheet: **broken link removed**

 

[mister_rf]

I think it’s not a simple air core coil, they use a ferrite core and the presence of the core can increase the magnetic field of a coil by a factor of several tens/ hundreds over what it would be without the core…

 

[EE_Chris]

Thanks for the reply.

It's a 5mm ferrite core x 0.3mm wire and has 7 turns. This could potentially be in the uH range?

When building these tuning circuits, do you have to use a wire coil? or can you simpley use one of those ceramic inductors that looks like a resistor?

 

[mister_rf]

In this case you need fine tuning of the circuit so use an adjustable coil

**broken link removed**
plus a trimmer capacitor
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[tony_lth]

Coilcraft has many very good tuning inductor with shield.

 

[jiripolivka]

The LC circuit alone does not determine the resonance frequency! There is also transistor c-b capacitance, and stray capacitance of the components.

 

[mister_rf]

Yes, in you're right, but we must remember this discussion has as the starting point some questions about number of coil number of turns, if using 7 turns it’s a correct choice, here given the small number of turns and low frequency signal used. In my opinion for this application, without using a ferrite core (and with or without the additional capacity of bipolar transistor circuit junctions), frequency is still very high, so it should be noted that this application need some adjustable ferrite core coil to obtain the required 27Mhz.