amradio
I found a very basic AM transmitter circuit on the youtube video here. The schematic is below:
I'm a bit confused on the op-amp circuit on the right. It seems that the op-amp is really only taking the difference between the message and carrier signals – not multiplying them together like it should be doing.
I'm not at all convinced that this part of the circuit is a frequency mixer. Am I just crazy?
A lot of questions but if you are looking for a WWV receiver design here is one and below is one of the circuits contained on the link: -
YES but it's probably not too critical if you are receiving a decent signal. A quarter wave dipole at 10MHz is optimum but this will be 7.5m long so try a couple of metres.
NO, you need to tailor the inductance capacitance ratio to give a decent Q factor but not be susceptible to parasitic components. Try for a resonant circuit that uses no less than 50pF - 50pF and 5060nH resonate at 10MHz - if you half C, double L for same resonance at an improvement in Q of 2:1. BTW it's "M" not "m" in MHz
Tricky, suck it and see
See design linked to for best guess at what you should be aiming for.
WWV uses up to 100% modulated AM (or normal DSB) transmission. Use a diode detector (as per link) to produce the demodulated waveforms.
The voltage from pin 8 of SCTX2BC changes the DC component of the voltage on the base of Q2 transistor, modifying its quiescent IC current and its gain. So this is how the RF signal amplitude is modulated.
The RF signal from the Q1 based oscillator is connected via the capacitive voltage divider consisiting of C5 and C6.
The bias of Q1 may be found by the following equation:
$$V_{bias} = R7\,\frac{I_E}{\beta+1}+0.65V+I_E\times R8 = 9V$$
$$V_{CE} = 9V - I_E \times R8$$
$$I_C = \frac{\beta}{\beta + 1}I_E$$
C4 assures the positive feedback.
Best Answer
Further to my comment. You are quite correct about needing some form of multiplier. Perhaps the following addition could be made to improve the circuit. The FET acts as a voltage controlled resistor altering the gain of the op amp creating an AM signal.