SA602, SA612 Mixer This side will present some of my ongoing 612 project.
The SA602 is a low-power VHFmonolithic double balanced mixer with input amplifier, on-board oscillator, and voltage regulator.
It is intended for high performance, low powercommunication systems.
Step one is to connect a crystall to the oscillator. Figure below shows the schematic for a 44MHz crystall.
R1 has been added to increase the current into the tank.
The schematic inside the dashed lines are just a probe (high impedance amplifier),
if you want to measure the oscillation frequency, wich can be nessesary if you want to conect the osc to a frequency counter or to control the
oscillator with a PLL frequency synthesizer. If you are not going to do this you can just exclude this part. The Output (Fout)will have about 100mVpp.
Instead of the L2 coil, you can use a resistor of 1.5k ohm. The output-amplitud will then drop a bit.
I have choosen a JFET BF245 for no reason at all, I just had some of them. The type of FET is not critical. You can use BF981 dualgate also. Just remeber to connect gate 2
to a suitable voltage and decoupple it with a capacitor.
To fine-calibrate the frequence you can add a coil in serie with the crystall. The inductance of the coil should be from 500nH-1500nH depending on how much you need to
adjust the frequency. The best is to use a slug-coil where you can adjust the inductance by turning the slug.
Experience: My 45.18125MHz crystall oscillated 6600Hz to high, so I
had to add 700nH coil to make it oscillate at the right frequency.