EME205 9MHz BFO Demodulator Kit

EME205 Module Description:

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.

Some of these performance figures may change with future tests.

Kit Constructors Alert:

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.

EME205 Construction Notes:

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..

  • EME205 Ver1 BFO Demodulator Top View
    Top view of the EME205 BFO Demodulator Module
  • EME205 Fitting TF1
    Fitting of the TF1 transformer
  • EME205 Fitting TF1
    Fitting of the TF1 transformer
  • EME205 TF1 Cable Tie
    Fitting of the TO220 transistor insulator for L1
  • EME205 TO220 Insulator for L1
    EME204 AGC Gain curve comparing the RF ouput level to input level
EME205 Performance Tests:

  • EME205 BFO Frequencies
    BFO frequencies measured on the prototype module when tuned to suit the 9MHz 2.5kHz 8pole SSB filter. LSB, Red, CW Green, USB Blue
  • EME205 BFO Frequency Response
    Shows the 9MHz 2.5kHz 8pole SSB filter and the LSB, Red and USB, Blue carrier frequencies on the prototype modules
Changes and Updates:

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.