SSB Modulator

Posted on Feb 6, 2014

Circuit detail is shown in figure 3. The main elements of the system are a speech amplifier, the audio phase shift network with balanced driver circuit (N1A, N2A), two balanced modulator stages (N1B & N2B), an RF carrier phase shift circuit (N3) and an output RF driver stage incorporating a lowpass filter (N4). A number of different well known inte

SSB Modulator
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grated circuit packages such as the MC1496, NE602 and the SL1640 could have been used as balanced modulators. However I happened to have a number of XR2228 Monolithic Multiplier packages and two of these (N1 & N2) are used for these functions. These devices have balanced X and Y input circuits, a balanced output and a circuit arrangement which looked ideal for a balanced modulator. They can also handle frequencies up to around 3 MHz, well above our LF range. The XR2228 packages also include an operational amplifier each (N1A & N1B) and the pair of these are used to form a balanced low impedance driver for the audio phase network. The amplifier outputs are directly coupled through the network to the X inputs of the balanced modulator sections of the packages (N1B & N2B). Precise DC balance of the complete circuit (necessary to set to for best carrier rejection) is set by trimpot RV2. In retrospect, my thoughts are that it might have been better to use a separate op amp package for the driver as the arrangement resulted in the concentration of a lot of components around the two XR2228 units. Concerning residual carrier balance, a later addition was the inclusion of trimpots RV3 and RV4 which allow individual adjustment balance of the two balanced modulators and further improvement in the rejection of residual carrier. A 90 degree phase shift for one of the RF carriers can easily be achieved with simple reactive circuits but...

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