Monday, January 05, 2015

Another VFO (7.000 - 7.100Mhz)

Was kind of bored last Sunday so decided to build something just for fun and no particular use.

 After soldering it worked first time:, it's not the frequency counter, is the circuit behind it!

General view of the assembly:
Excluding the oscilloscope, the counter on the image is now my bench frequency counter!

Focus on the oscillating circuit part:

Basically it's a Bitx V3 VFO  with an J310 instead of the 2N3819 and with a double varicap in anti-fase (I could had done another arrangement with just one). The serial cap for the varicap part is 100pF and not 120pF as on the original, instead of 4-40 for the trimmer I used 10-60, the coil is a red core with around 25 turns (give or take). I used a 10K multiturn pot (5K on the original) and placed in series one 1N4001 diode on the ground leg to create a small minimum voltage for not getting too close to the varicap minimal bias.


It has around 100Khz of span, removing the series diode, using other type of variacp's and also a higher value in the series capacitor will extend the range.

I might add a mixer for a direct conversion receiver if I get bored again.

original schematic:

Have a nice week!

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