The Project: 

After successfully completing my QRP transmitter project it was time to consider building a matching receiver.  I wanted to build one that could be housed in the same enclosure and that would work with the transmitter in a transceiver-like mode of operation but still have the ability to stand on its own as a separate receiver. 

Like all my projects, the design phase starts with a search of my local library and the internet for as much material as I can find on the subject matter at hand, in this case receivers.  Early in my search I was faced with making a decision of the type of receiver that I wanted to build, then decided on building a superhet which narrowed the search considerably.  I do not have much experience building receivers and realized that it would be necessary to learn about some of the design criteria used in various schematics that I reviewed. 

Design Process: 

The design process was begun by choosing a receiver circuit that looked easy to build and would provide good performance as a companion to my transmitter.  The circuit that I used was in the article ‘Revisiting the 40-40’ by Mitchell Lee and Dennis Monticelli, in the book “QRP Power” published by ARRL and I believe the original receiver circuit was designed by K1SWL, Dave Benson of Small Wonder Labs.  After contacting Dave with some questions about the receiver, he was extremely helpful with information and some suggestions. 

In studying the circuit I began to see some areas where  some changes needed to be made.  To begin with, I decided to use a double tuned band pass filter on the input.  Next I thought a little sharper filter would be nice, so I settled on a 4 pole crystal filter.  The product detector and limiter amp looked pretty standard so these circuits were used without any modification.  Since the audio limiting amp uses ½ of a NE5532, I decided to use the second half of the 5532 as a pre-amp to the final audio power amp stage.  Since I had so much success with the VFO circuit in my QRP transmitter, the same circuit was incorporated in the receiver because it’s so stable and drift free.  Using this combination of circuits would create a complete stand-alone receiver that can be used in conjunction with my QRP transmitter in a transceiver-like mode of operation.   

The paring is made possible by the muting or more accurately stated, attenuating the audio path prior to the pre-amp by inserting a 4.7 Meg resistor in series with the audio, utilizing a FET switch activated at the keying rate by a key line from the transmitter.  Each time the transmitter is keyed, a DC ground is applied to the FET switch in the receiver which attenuates the audio.  The transmitter is designed with this feature and provides the necessary ground-line through a RCA connector on the rear of the transmitter. Without the attenuation the audio level through headphones would cause your eyes to roll around and smoke come from your ears. This feature also allows you to monitor the actual transmitted signal instead of an audio oscillator used as a side tone monitor.  

I feel compelled to mention that I was at a complete loss when it came to designing the crystal filter.  One of my many reference books is “Experimental Methods in RF Design” by, Wes Hayward, W7ZOI, Rick Campbell, KK7B and Bob Larkin, W7PUA.  This book comes with a CD packed full of some great software.  However if you don’t understand what to do with the software then it’s not much help.   

After contacting Wes, W7ZOI, with many questions about filter design and the software,makeing a long story short, this man demonstrated patience beyond anything I expected.  There were some things I just didn’t understand and Wes did not give up and stayed with me until I finally saw the light and was able to actually understand how to design a crystal filter that really works.  Simply saying thank you does not seem adequate. 

Readers should know that not only does this final design reflect many hours of my time, but countless hours of others who have traveled these roads before me and made my trip possible.  I have discovered that the guys who are giants in our ham community are always willing to take their time to give us a lending hand when needed, and that’s what makes this hobby so great, the great people in the hobby. 

Once settled in with the final design I would have to gather up my notes and pieces of tablet paper with scribbled drawings of various circuits and draw a real schematic that could be used as a basis of design for the printed circuit board layout.  Here is the schematic drawing:

   Having completed the schematic I could now begin the pc board layout, using ExpressPCB software for all the pc board layouts. Their software is easy to learn, easy to use and best of all it’s FREE.  You can download a copy at http://www.expresspxb.com  

 After finishing the board layout you simply upload the file to ExpressPCB and in a few days you will receive your professionally manufactured circuit boards with plated through holes, tinned backplane, ready to build.  I do not use the manual assist routing built into ExpressPCB software.  I prefer doing the pc board layout and wiring myself. Better control over interaction between circuits caused layout problems is achieved by doing the layout manually.  Of course this train of thought may be due to my inexperience with more sophisticated software employing auto routers. Just to set the record straight, my only affiliation with ExpressPCB is that of a satisfied customer. 

I have found through experience that it is much easier to design the circuit board to a specific size that will fit a readymade enclosure rather than to try to fabricate an enclosure for the completed board.  With this in mind I used the same board size as my QRP transmitter so the same size enclosure can be utilized.  I also like to design the pc boards so that with all the parts are mounted, there is no external wiring necessary.  This makes it much easier to work on since there are no wires running all over the work bench to switches, pots, connectors etc. 

In order to keep the cost of manufacturing the pc boards as low as possible I never get silk-screened parts legends stenciled to the top side of the boards.  Instead,  use the hole patterns to determine parts locations.  I print a full-page copy of the pc board layout with parts and pads on the drawing.  It is very simple to establish the proper mounting holes to use for various parts. 

I always begin by mounting all the resistors on the board first, then mount all the disc ceramic caps.  After the caps are mounted, the remaining parts are installed.  Here is a picture of the completed board.

Here is a compiled a parts list with Mouser Electronic part numbers to aid in gathering parts for this project.  Of course you can always use what you have available in your junk box. The home brewers among us always have quite an inventory of parts and can’t wait to find a project to use them.  Here is the parts list: 

 I usually build transmitters one stage at a time then get that stage working before moving onto the next. The reason for doing it this way eliminates a lot of trouble shooting when you power up the finished transmitter and nothing works.  This receiver can be built with minimal test equipment.  The only test equipment I own is a scope and frequency counter.  Not having a signal generator makes it difficult to test each stage of a receiver while it’s being built, but the simplistic design of my receiver makes it pretty simple to trouble shoot, either you hear signals or you don’t! 

Testing and Alignment: 

            Upon completion apply power and connect a 40 meter antenna.  Tune C15 and C17 for a peak in background noise.  If you hear background noise chances are that you will be able to tune across the band and hear signals.  Once you hear a signal tune C34 for the tone frequency that you find most pleasing and easy to copy.  That’s it; you’re ready to begin using your new receiver.   

            Now, if you build the accompanying transmitter, you must connect the two units together with a cable having RCA connectors on each end.  This is the mute line for the receiver audio during transmit.  If you’re going to use this receiver with a transmitter which does not provide a ground during the transmit mode, then please manually turn down the audio prior to transmitting.  Failure to do so will present a bone chilling blast through your headset that you won’t soon forget. 

Enclosure: 

            The enclosure chosen at the very beginning of this project provides a nice looking, low profile piece of equipment.   To finalize the receiver it’s a simple matter of drilling holes in the front and rear panels at the proper locations to fit the connectors and controls.  Here is a picture of the front and rear of my receiver.   

Front View	Rear View

            Controls on the front panel are left to right, headphone jack, volume control and tuning.  On the rear panel left to right are antenna connector, audio mute from transmitter, speaker switch and DC power.  I really enjoy using my QRP twins and hope you enjoy the project as much as I have.  There’s nothing like the feeling you get from making a contact with a rig that you built yourself.  Best 73 and I’ll be looking for you on 40.

            This website version of the 40 meter receiver is a little different than the version published in QST in as much as it includes all the parts list and PC board layout and photo's.  You can download a file that contains various helpful files such as the schematic, parts list, PC Board parts legend, etc..  I have also included a Bill of Materials file prepared in Excel.  This will keep you from manually typing the parts list into the Mouser site to order parts.  Simply load the file into Excel, run the file and copy and paste to the Mouser site.  The download file contains the following:

Receiver Schematic - PDF format

Receiver file - ExpressPCB format

Bill of Materials - Microswoft XLS format

Parts List - PDF format

PC Board parts layout with legend - PDF format