Simplest FM Receiver Circuit in LTspice

[Electronaut]

Hello,

The attached LTspice schematic models a simple FM receiver circuit that is commonly used for online examples. For example, it is used here: https://www.electronicshub.org/fm-radio-circuit/. The physical circuit works (not very reliably), but the LTspice simulation does not work as expected. In the LTspice circuit a modulator is used to simulate an FM antenna signal with 1 MHz carrier, and 10 kHz message. The 10 kHz message should appear at V(Out), but it does not. I tried 2n3904 and bf494 transistors.

Any ideas how the modify the LTspice circuit so it operates correctly?

Thanks,
Electronaut

 

[alec_t]

The transmitter part is ok, but the receiver part is self-oscillating at >200kHz even when the transmitter is disabled.

 

[Electronaut]

Yes, it is an oscillator, and it should be modulated by the inductively coupled signal from the transmitter. The frequency modulation should interact with the internal capacitance of the transistors to vary the impedance they introduce, resulting in amplitude modulation at V(Out). If the output of the modulator is directly connected to the top of the receiver tank circuit (with C3 and L2 removed), and if a 1nF cap is placed across R4, we get the amplitude modulation, but this is cheating (we are simply using the capacitor to detect the signal by means of its own varying impedance).

 

[Electronaut]

BTW, playing with the physical circuit seems to reveal that the LM386 that appears in the original schematic is more than simply a signal amplifier. It is part of a feedback loop. This suggests that a full analysis of operating principles is not trivial.

 

[unclejed613]

http://www.vk2zay.net/article/195 i think you will find that one works better. the page also gives details on things to try if it doesn't seem to work right. one detail left out is the antenna connects to the emitter of the BF199

in the circuit linked by the OP, the inductance, dimensions, number of turns, or other important details are not given. but it's easy to figure out.

given the 22pf tuning cap and 100Mhz as the center of the band, ...

1) the cap will be about 12-15pf at center tuned position
2) center of the band is 100Mhz
3) find the inductance that resonates with 15pf at 100Mhz


4) result is 150nH .... went to an inductance calculator page and entered the following, coil diameter =8mm (about correct if you wind the coil on a #2 pencil ), number of turns=5, length of coil = 12mm, wire diameter 0.5mm (page script changed it to 0.55), and design frequency 100Mhz. these figures got the coil almost correct for inductance at 125nH. if you squeeze the windings a little closer together you will get 150nH or add one more turn and stretch them out a little...

both circuits are regenerative or superregenerative receivers.
both of these receivers normally work as AM receivers, but for FM the radio is slightly off-center tuned, and the FM signal is demodulated by "slope detection".

NOTE: the antenna is connected directly to an oscillator operating at the receive frequency. this type of receiver can generate interference that radiates from the antenna.

 

[Electronaut]

Thanks for the feedback. My goal was to simulate the simplest FM receiver circuit that I have been able to find using LTspice, or to determine if perhaps this is not possible due to limitations of LTspice models. I have seen LTspice examples of slightly more complicated circuits (Foster-Seeley or Ratio Detector) that work as expected, but have not been able to get this (simplest?) regenerative receiver working. I suspect that the Miller effect is important here. I'm not sure what you mean by normal operation as AM receiver, as there is no envelope detection.

 

[audioguru]

Yes, the extremely simple regenerative receiver detects AM and lots of AM pops and clicks as interference since it detects the amplitude fluctuations, not frequency fluctuations. It "slope detects" FM since as the transmitted frequency moves away from the LC frequency the level drops and is AM detected as dropping then the FM transmitter frequency moves towards the LC frequency the level increases and is AM detected as increasing.

I think the transistors in the circuit from India are not biased correctly.