Thursday, March 13, 2014

Minima - BFO Module

I have finished constructing the Farhan Minima Transceiver BFO Oscillator Module for my Transceiver Experiments.

I used the same Toner Transfer Method that I have used for all of my other modules. The PCB is double sided, with oscillator power connection and ground plane on the backside. The size of this module is 1.0x1.5 inches.

Mimima - 20MHz BFO
This module supplies 20MHz to the Transceiver's BFO Mixer (via SMA connector on right) with frequency off sets for Upper Sideband (USB) and Lower Sideband (LSB). The Relay selects one of the two Sidebands via one of the two adjustment Trimmer Caps used with the oscillator.

On the initial power-up, the measured output was about 800mV PP (will this be enough?).

My circuit is slightly modified version of Farhan's published circuit, I wanted to use the colpitts oscillator and adjustment strategy that I have used in many of my other projects. I have kept the circuit's interface the same to maintain compatibility with Farhan's Minima Transceiver circuit.

The upper left pin header is for power (12v). The lower left header selects the Sideband, the extra (upper) pin in this header provides a quick way to select the Sideband with just a simple jumper, which will be useful while testing this module as a stand alone circuit. The white block in the center left is the Relay with its control circuit far center left. The BFO colpitts oscillator and output transistor are center right. The crystal (of course) is lower right. A red power-applied LED is upper left.

Note: I use EB2-3NU 3Volt Relays in most of my projects, and therefore when they are used in 12Volt circuits a limiting resistor is needed.

So now, after doing a little head scratching, my questions are:

  • What should the actual BFO Frequencies be for generating USB and LSB with a 20MHz IF?
  • Or at least, What is the expected difference between the two Frequencies?
  • Does this all depend on the width of the Crystal Filter?
  • In normal Transceiver operations, does the VFO Frequency also have to change (a little) when selecting the opposite Sideband?

More research about Superheterodyne Receivers on my part is necessary (and I thought I knew this stuff :-).

Any feed back via comments would be appreciated.

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1 comment:

Charlie Vest said...

Hi Eldon, Did you use the same relay for all of the relays ?

Charlie, W5COV