4MHz Oscillator

4MHz Oscillator

The crystal oscillator uses a crystal that is 455kHz from the crystal filter value. The easiest to find are the 3.547MHz and the 4.000MHz crystals. The best CW performance comes from the 3.457MHz crystals in the filter and a 4MHz crystal in the oscillator. However, when using the 3.457MHz crystal in the filter, it's difficult to calculate the frequency you're at when using a frequency counter to read your received frequency. Check out the kit from Almost All Digital Electronics for a frequency counter that makes reading IF offsets a snap. Recommended if you use the 3.457MHz crystal in the filter.

When using the 4MHz crystal in the crystal filter, reading the received frequency is easy, and your bandwidth really doesn't suffer that much. A 4MHz crystal filter should be used if you want to receive SSB signals as your main interest.

A single RF MOSFET amplifier is used to amplify/buffer the oscillator. The buffer also brings up the signal level to 5-6 volts necessary for the single balanced MOSFET mixer. A one crystal filter is used at the output of the buffer to eliminate harmonics and reduce spurious responses. Adjust the trimmer that connects to the crystal for maximum output through the one-crystal filter on the output of the MOSFET amplifier.

One problem with this oscillator is feedthrough to the first mixer. If this oscillator feeds through the 12 Volt line or ground loops to the first mixer, you will have a signal in the output that cannot be tuned by the VFO. You can verify that it is this oscillator by tuning another receiver to around 4MHz and listening to see if it is the same station.

A way to help cure this problem is to shield the first Single Balanced Mixer. The shield needs to go above the transformers and the LEDs. When I built a broadband version of the IF strip, shielding of the Single Balance Mixer was absolutely essential. Be sure the shield goes above the transformers and the LEDs. A 1" high shield works fine. Use copper squares with "mouse-holes" or 1/8" drilled holes on the input and output ends to connect to the other circuits.

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Last Update: 01/22/2000
Web Author: David White, WN5Y