“5 By 7” CW Transceiver: The Receiver

“5 By 7” CW transceiver you might ask. Well i have been kicking about a name for this project for some time. For a long time it has been called Robs Improved Pixie as it pretty much started with a collpits oscillator and had grown from there. A lot of what is going into this rig has been built up Manhattan style on scraps of single sided FR4 and found to have done the job nicely.

So what started out a couple of weeks ago as learn some CAD software to design the CW Audio Filter, has grown into a full blown all out assault on laying out circuits using Sprint Layout, and etching the boards and learning from all the little mistakes you make along the way. All going well I will etch these boards tomorrow and use up the last of the toner transfer paper I have, a sum total of 2 sheets. I have more on order from China but they will not arrive till mid January, as there is no way Im paying $25 for 10 sheets from Jaycar, when i can get 50 sheets for $10 off Ebay.

Oh and the name, “5 by 7” while it sounds like a signal report, its really about the size of each board. Part of the design of this has been that each stage needs to fit on a 5cm x 7cm piece of single sided FR4 PCB. I just want to also mention that none of this is really revolutionary, it has all been taken from various sources on the internet and Experimental Methods in RF Design and other books. All i have done is pick and choose bits that are easy to build and work well enough and are easy enough to build as a stand alone module.


First stage is the Bandpass filter and RF preamp, there is a switch missing off the 180 ohm resistor which allows for muting the preamp for large signals. This circuit was taken out of the DC Receiver section of EMRFD.


Now i know this is 2 5 by 7 boards in one, it was 2, but it was jumper madness and I might yet make this a double sided board and send off to have 10 of these professionally done and sell off the extras. I should also mention that board edge SMA connectors and coax are used throughout to join most of the stages together, in other places 2 pin jumpers are used.

The Arduino Nano is configured as a DDS VFO, with LCD screen and controls the AD9850, the sine wave is boosted by a buffer amp to an amplitude suitable for the ADE-1 Mini Circuits mixer, mixed with the RF from the Bandpass RF Amp, giving audio out. This circuit is a common one found on the internet. The code came from AD7C originally, but i have added a number of features to it to suit my own needs.


Audio from the mixer is run through an audio diplexer to narrow up the bandpass and improve selectivity. It is also amplified somewhat before being sent to the filters. From here things get uncertain, as im not sure how much gain the audio filter will take without causing distortion. This circuit was taken from EMRFD.


Audio from the diplexer then comes to the OP Amp Audio filter. A quad OP amp is used to cascade 4, 2 pole filters to make an 8th order Butterworth CW filter 200hz wide. Centre frequency is 600hz, where i like to listen to code and the bandstop is -35db down at 600hz.


Finally, narrow filtered audio is pre-amped and finally amplified in the last stage and should drive a speaker with very loud audio. Both circuits here are very common ones that can be found all over the internet.

There are a couple of things missing, virtual ground to power the negative rails of the OP Amps, TX RX switching and LO out to the TX. A couple of small things i will need to look into and solve. Maybe by the end of January next year i will have this all built as well as the transmitter, to go with the home brew antenna tuner i have built already. Home brewing, is such fun.


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