EME205 9MHz BFO Demodulator Kit
The Mini-Kits EME205 9MHz BFO Demodulator Module is part of the M1 series building blocks to construct a high performance M1 HF Transceiver. The M1 HF Series Kits have been designed to allow easy construction due to the use of mainly leaded components for those that don't like surface mount devices. Modern SMD components are only used when required due to availability of components, or for better performance. The RF modules are all designed with a 50ohm impedance making it easy to interconnect with the other M1 series modules, and your own homebrew circuit designs.
The module board is designed as a 9MHz I/F SSB and CW demodulator, and uses a SA612 Gilbert Cell Mixer to demodulate the 9MHz IF down to a filtered audio output. The module includes an on board Beat Frequency Oscillator BFO, that uses switched multiturn trimpots to set the carrier frequencies for LSB, CW and USB. The BFO is also available as an output on the board for connetion to to the M1 Series SSB modulator which uses the MC1496.
The module is controlled through a single 16 Pin IDC header connector, making it easier to interface to logic circuitry. See the Block diagram for the connections to this module.
1/ Transformer T1 should be 9.5 Trifilar turns on the BN43-1502 core for around 130uH not 17 turns as shown on early circuits.
The images below show both sides of the board. The BFO Module is very easy to construct as it mainly uses conventional leaded components. There are a few SMD components on the bottom side of the board which are quite easily fitted. The Transformer TF1 is easily would using 10 turns of 0.3mm bi-filar wire on a T37-43 Toroid. The Toroid can be secured to the board with hot melt glue, or non acidic silicone if required..
1/ If you are using this Kit for your own project and are not using the M1 logic controller to switch the BFO through the E4 connector, then there could be erratic operation of the BFO due to there being no pull down resistors on the gate connections of the T1 to T3 FETs. If the gates are left floating above ground, then this could turn the FETs on and change the BFO frequency. 100k 1/4 watt resistors can easily be connected on the gates of each of the T1 to T3 FETs to ground.