Building a high frequency VCO (Voltage Controlled Oscillator) is not a difficult task anymore! Maxim provides fully monolithic voltage – controlled oscillators. Here, we use a MAX2623 Monolithic Voltage Controlled oscillator and a MAX2402 IC as a power amplifier, to build a small ISM band (902MHz to 928MHz) transmitter with more than 100mW of output power.
Looking at the electronic schematic, you will notice the absence of any modulation input. However, you can easily modulate this small transmitter. Just apply a weak modulating signal in the #2 pin of the MAX2623, together with the DC bias, and you will have frequency modulation (FM). If you want AM-DSB, PSK, ASK or any other modulation scheme, you can use the internal double balanced mixer of the MAX2402. This mixer is linear over a 2V range and it is optimized for high carrier rejection. The baseband modulation input to the mixer is the #2 pin of the MAX2402 IC. Looking at the schematic, you will notice that this pin is grounded since we do not use modulation in the prototype.
Output power adjustment in the prototype is achieved by a small trimmer potentiometer which is used to bias the Variable Gain Control Input (VGC pin) off the MAX2402 The same result could be achieved by connecting the Bias Adjustment Control pin of the MAX2402 (BADJ pin) to a single pullup or pull-down resistor to VCC or GND, respectively. For more details, refer to MAX2402 datasheet.
If you decide to build this transmitter, please notice that you must use only surface mount components.
Carrier Frequency: 885 - 950 MHz / adjustable from P1
Output Power: 0 - 20dbm / adjustable from P2
Power supply: 3.5 - 5V DC
Maximum Supply Current: 230mA
Maximum Power Dissipation: 1.15W