I simulated the single balanced mixer circuit is LTSPICE. I wanted to see if the LO amplitude had a large effect on the mixer gain.
- I was able to calculate models for the L1 to L5 input circuit toroids. I lumped the tuning network into the gate drive transformer and found the tuning capacitance Ctune by trial and error.
- According to the 2N2222 datasheet the small signal input impedance should be between 2Kohm a nd 8Kohm. I used 2 Kohm, shown as RQ3 in the circuit.
- The audio transformer T1 is a Calectro D1-711. I searched the Internet for a couple of hours trying to find the specs for it, but eventually I gave up and pulled T1 off the board and measured its parameters with my LCR meter. I was surprised that something the size of a small acorn could be 2.5 Henry, but that's what it was. See the circuit for the parameters. I measured the parameters at 1 kHz. The transformer had significant series resistances, which are included in the model.
- There is a 1:2 step up from the antenna across the gate driver transistor. I modeled a 1 uV rms input signal and scaled it accordingly.
- The LO is simulated at 14.000 Mhz, and the signal simulated at 14.001 MHz. The baseband frequency is 1 kHz.
- The conversion gain from 1 uV RF in to the baseband 1 kHz detected waveform across RQ3 is 12 dB. Doug DeMaw, is his "QRP Notebook" states that this kind of single balanced mixer usually gets about 10 dB of gain. So 12 dB feels reasonable
- I simulated with LO amplitude of 2V rms, given in the magazine article, and 0.87 V rms as actually measured on the board. There was only a very slight, barely noticeable drop in conversion gain.
Base band output waveform is given below.
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