Amplitude Modulator

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The following circuit is an example of a low level amplitude modulator. It may be used in a multimode transmitter to avoid the need for unbalancing the balanced modulator and is not critical in its adjustment or operation.

Notes:

·	TR1 and TR2 should be RF devices suitable for the required frequency e.g. BF199. TR3 is an audio device like the BC108.

·	Decoupling values shown are suitable for HF operation from 500Khz to 30Mhz.

·	The original circuit used CA3028 or CA3053 ICs which are now difficult to obtain. However, a transistor array IC like the CA3054, which contains six transistors configured as two long tailed pairs, would be fine in principle and is still available.

The upper two devices act as a phase splitter with single ended input, push-pull output and modulation achieved by varying the contant current source. No balancing or matching was attempted or found to be necessary in the discrete version although the transistors in the original ICs would have been fairly closely matched as a result of the manufacturing process.

The primary of T1 should resonate with C1 at the required frequency of operation. The turns ratio of T1 will depend on the load impedance. The original prototype at 9Mhz used 9+9t bifilar on the primary and 3t on the secondary. A screened tuned circuit or a toroid are both suitable forms of construction.

·	The emitter of TR3 is left undecoupled to provide negative feedback at audio frequencies.

·	The circuit was tested with 25mV pk-pk RF input and 1.1v pk-pk audio input which resulted in 100% modulation and 800mV pk-pk modulated output.

·	The circuit should not be over-driven at RF.

·	Some form of RF decoupling may be required at the base of TR3 depending on the application.

·	The 9v supply should be reasonably stable and hum free.

This circuit could be used as a single balanced modulator by swapping the RF and AF inputs but a manual balance control would be required in the emitters of TR1/TR2 and TR3 would need to be an RF device. The CA3054 could be used to provide both an AM modulator and a balanced modulator. The author has not tested this idea yet.

·	In principle, the concept should also work using valves instead of transistors TR1 and TR2 although a negative rail may be required to get sufficient voltage across TR3 to cater for the valve bias voltage.