I couldn't resist testing a 4 pole circuit. The 3 pole filter worked very well, but some of the opposite side of the signal could be heard with very strong signals. The first crystal filter, referenced above, was a 3 pole filter. This probably makes the filter a bit more narrow than 500 Hz.ģ-Pole 500 Hz Cohn Filter Performance Graph The computer generated filter specified 213pF capacitors, but I chose to use the standard value 220pF caps. With W7ZOI's help and with his filter design programs, I built this circuit. One of my goals I wanted to achieve was a practical understanding of crystal filter design. I designed my own crystal filter for this rig. IK0VSV's design works very well: Nominal 3dB bandwidth = 10% of center frequency, Nominal mid-band loss = 1dB, and Attenuation is greater than 60dB out of band. I incorporated a receive input filter designed by Marco Eleuteri, IK0VSV, published in SPRAT 105. VFO Schematic The 40673 dual gate MOSFET symbol is a bit artificial since my schematic drawing program didn't have a symbol for this "obsolete" device. Maximum output is about 1.5 Watts, but I set the gain for a little over 1 Watt. I also replaced the final transistor in the original Cub schematic (2n5109) with a 2n3553. I also included a simple 741 Op Amp sidetone oscillator. The schematic for the bandpass filter can be viewed with the link below. I changed the bandpass filter between the mixer and the PA because the Cub utilizes tuned slugs which I didn't have in my junkbox. The Transmitter is a modified MFJ Cub circuit. The small board left of the transmitter, toward the front, is a sidetone oscillator. Photo of the completed transceiver (left) and the transmitter (right). in the receive line which serves as an RF attenuator to help control very loud signals.Ībout the only thing absolutely original to me, apart from the unique combination of circuits, is the 3 pole 500Hz crystal filter that I designed using Hayward's G87 and X filter computer design programs.Īt this point the receiver is working very well and the homebrew 500Hz filter is performing as expected with reasonable rejection of the opposite side of the signals. Receive input filtering is also different in my rig.
(I choose this frequency because I had a collection of matched crystals in my junk box.) The VFO is scaled downward to function at about 1.820-1.895MHz. The Receiver is essentially a Progressive Receiver (Wes Hayward's "High Performance Receiver") with a few modifications. Special thanks to Mike, W3TS, and to Wes, W7ZOI, for helpful and much appreciated elmering. This was my first attempt at putting together a few "pet" circuits to make a transceiver.
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