I'm trying to simulate a frequency counter pre-amp, taken from "Arduino Projects for Amateur Radio". I don't understand why the conditioned signal is AM modulated. I verified the circuit is the same as the book.

 

Is there resonance happening with L1/C2, L2/C4? Try shorting out the inductors and see what happens.

 

I shorted out both inductors and the output looks good. Why the inductors then?

 

I added this command. .ic V(N002)=5 V(N003)=5
Power up time =0nS. Power must charge up C2,4 and this causes L1,2 to have very high current at startup.
I set the two nodes (N002, N003) to 5V at start up. This keeps the power up power supply surge from "ringing" the LCs.

 

So this is more of a simulation issue that a design issue? I assumed the inductors are there to prevent the input signal from getting on the power rail.

 

Thanks for that help. That was something new to me. It looks good.

 

See

 

I would use a FET on the first stage for higher input impedance.

 

Thanks, @Bordodynov. I'll have to read about the step directive. As well as the ic directive @ronsimpson added. Is the spec'd Wurth inductor necessary?

 

No, it is not necessary, but it is necessary to consider the parasitic parameters of the choke. In particular, the series resistance of the inductance. I took the choke designed for the smallest current in the LTspice program base. In this case this is a plus. The probability of parasitic oscillation is lower.

 

I would use a FET on the first stage for higher input impedance.

Thanks, I'll try it. Doesn't the 1nF input coupling cap limit the loading?

 

Thanks, I'll try it. Doesn't the 1nF input coupling cap limit the loading?

The reactance of 1nF capacitance at 7MHz is 23Ω.
In comparison, the input impedance of a MPF102 preamp is in millions of ohms.

 

Oh, right. And there's no emitter resistor either in my circuit.