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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Minima - BFO Mixer

I have started working on the next Farhan Minima Module for my experimental receiver (see previous post). This module will be the BFO Mixer which is used for SSB Modulation and Detection.

The circuit and layout is simple and will use the same SMA connectors as my previous constructed modules.

Proposed BFO Mixer

My proposed BFO Mixer board will 1x1.2 inches, which is laid out with DipTrace.

Farhan's Minima Transceiver requires for two 1N4148 Diodes for the mixer. He suggests that the diodes should be matched, and provided a test fixture and procedure to help find a matching pair.

I have several old 1N4148 Diodes from which to pick from, but I decided to order a fresh batch which hopefully will be manufactured from the same processing batch.

SOT-23

While looking for Diodes to order, I discovered the 1N4148 is manufacture as a SOT-23 MMBD4148 (single diode), and a dual diode as MMBD4814SE. Of course, for my projects I am always looking for the smallest package(s) available, and with two diodes in one package this part is a big win. This diode is available in four configurations; single diode, dual common cathode (CC), dual common anode (CA), and dual one of each (SE). To meet Farhan's circuit connection requirements, my BFO Mixer will use the SE configuration.

I tried to find information about the manufacture of this dual diode, that is; I wanted to know if the two diodes are from the same die, or if they are just two chips put into a common SOT-23 package. The information was not found. I am hoping the two diodes will be a matched pair.

I think the dual diode part will work fine for my project. More details to follow.


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