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40m DC-receiver – VFO

So the receiver is alive and kicking and I have received amateurs from The Netherlands, UK and Italy on a simple 10m wire antenna. However, the VFO is comprised of a Siglent SDG1032X function generator from work. Hardly a self-contained solution!
After an article by Bill Merea in the GQRP Sprat magazine, I’ve decided to go for a digital VFO. The clock generator is based on a Si5351 module. The I2C interface is controlled via an Arduino Nano board together with a Groove I2C LCD.

This all seems pretty straight forward. See the circuit below:

With the code Bill Merea provided the stuff was up and running in an evening. A couple of things I learned:

  • USB – Arduino Nano does NOT work on USB 2.0 ports. it is just not recognized by my, up-to-date Windows 10 host.
  • I2C LCD – there are different type of I2C LCDs with different drivers….. Only after successful capturing data from the build-in UART I realized the code was actually running.
  • Si5351 3V3 – This module has a build-in 3V3 Low Drop Out regulator and I2C level shifter.

The last one, was pretty nasty. This means that the Si5351 chip output a signal with varies between 0V and 3V. The whole mixer around the TSA5A63157 is build around the assumption of 5V signal levels. Thus the input logic low should be less than V+ x 0.3= 1.5V. The input logic high signal should be V+ x 0.7 = 3.5V minimum. So this requirement is not being met in this set-up. Now, these are the minimum guaranteed specifications. So I could take a gamble and hope that it will work. However, if it works at all, there is no guarantee it will keep on working over time and temperature. Therefor a level-shifter or amplifier is needed.

Various options were evaluated. In the end the choice was made to use another TS5A63157 analogue SPDT switch. The power supply is set right between 5V for the mixer and the 3V output from the Si5351 clock generator. In this way, it can be switched by the Si5351 clock IC and the output can be used to switch the RF mixer. The output showed some ringing which is not unexpected because of the unmatched impedance’s and the sharp edges. Via 10cm RG-174 cable the VFO board was connected to the receiver. The antenna was hooked up and reception was loud and clear!

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