Modulator and Demodulator are RF circuit components which mixes the carrier signal and the modulation signal to produce output which has either higher frequency signal than that of the input carrier and/or modulating signal or produce output signal which has lower frequency than either or both input and modulating signal. One such modulator/demodulator IC(Integrated Circuit) is MC1496(or LM1496). It is old but popular modulator/demodulator IC used in professional communication equipment and by radio HAM hobbyist. It can still be found in the electronics stores and it is useful for learning the basic of RF circuits and for radio electronics projects.
The MC1496 modulator/demodulator can be used for AM signal production and for detection and reception circuits. It can used for both standard AM, double and single suppressed carrier AM signal generation and detection. Beside this it can also be used FM detection, Phase detection and chopper application.
It has 14 pins out of which 4 are not used(NC pins). The other are for carrier and modulating signal input, for output, for gain adjust, for adjusting bias and power supply.
The circuit consist basically of a quad differential amplifier which is driven by a standard differential amplifier with dual current source. The output is taken from the collectors of the quad differential amplifier. The collectors are cross coupled due to which balanced multiplication of the carrier signal and modulating signal occurs. At low level signal frequencies the output is normally signal with sum and difference of the two input signal. At higher frequencies harmonic products are produced. The lower differential amplifier has emitter connection for gain adjust(pin 2 and 3). Here we place an external emitter resistance Re to adjust the gain. With this external emitter resistance the maximum input voltage for linear operation is given by the following equation.
\[V_{max} = I_5 \times R_e\]
where Re is the bias current at pin 5.
Balanced Modulator
The following shows typical MC1496 modulator circuit diagram that produces Double Sideband Suppressed Carrier(DSB-SC) AM signal. More on this RF circuit application can be found in the tutorial How MC1496 Balanced Modulator Works?.
For optimum carrier suppression of with minimum spurious sideband, it is recommended that the the amplitude of the carrier signal amplitude be 60mVrms at carrier frequency of 500KHz(AM band) while using power supply of 12V and -8V. But the general rule applies, that the higher the signal level the higher will be the suppression. For higher frequencies, the PCB layout becomes important because of the induced capacitive couplings introduced between input and output leads when operating at higher frequencies. Here shielding should be applied.
Single supply balanced modulator
The MC1496 can be used with single supply. The following shows balanced modulator circuit diagram to produce double sideband AM signal (DSB-AM) using single +12V power supply.
AM modulator
The MC1496 IC can be used AM modulator. The following shows the circuit implementation.
In the above circuit the 50K potentiometer is used to adjust amount of carrier signal insertion in the output AM signal. The 50K carrier null adjustment potentiometer might not provide sufficient range and for this we can use 750Ohm resistors in the carrier null circuit as shown in the following modified AM modulator circuit.
Product Detector
Another application of MC1496 is product detector for SSB signal reception. The following shows the circuit diagram.
