Communications Receivers.

Construction Tips.

If you don't intend to build a receiver you might as well skip this page. Other pages in this article contain general information about receivers and later pages give specific diagrams of various sub-systems of a receiver. The following is a discussion about how to go about building a receiver from scratch.

This is not a step by step construction article in the manner of the IM Distortion analyzer and some of the amplifier projects. I have not built a complete receiver myself so I can't give you any useful guidance. Maybe someday that will change but at this time I don't have the time with all the other projects on my to do list.

Forward or Backward.

A communications receiver is such a complex project that most builders don't build the whole thing then turn it on to see if it works. The best way to proceed is to divide the project into bite sized chunks, build and troubleshoot each one before going on to the next.

I like to build the hardest part of a project first so It will be out of the way. The hardest part of a receiver to build is the front end which means building the receiver from antenna to speaker, forward. That is from RF amplifier to audio circuit. OK, you have finished the RF amplifier, First converter, and first oscillator with all of the necessary band switching. Now, how are you going to test it. The answer is you can't until you have finished the IF amplifiers, detectors, audio amplifier, and of course the power supply.

For this reason I recommend building a receiver backward. Build the power supply first and test it. Then the audio section. Then the detector or detectors, followed by the last IF amplifier and filter etc. You will need a signal generator to test the stand alone IF and audio system but I can't imagine anyone tackling a receiver project without one. Nice suggestion coming from someone who has never built anything more complex than an AM broadcast radio. Although I am not building a receiver as I go just writing these words has made me aware that I have painted myself into a corner with my advice to work backwards. I wrote the article on the power supply, the audio amplifier, and the detectors. While writing a section on amplified delayed AGC it hit me and I realized that someone thinking seriously about building a receiver would be shouting to his computer screen "How the hell does he expect me to test an AGC system when I don't have any IF or RF amplifiers to control the gain of". Even the product detector cannot be fully tested working backwards. Some product detectors sound better than others and the only way to tell the difference is to listen to a real off the air SSB signal with a real person behind the microphone. But the AGC can have an overwhelming influence on the quality of sound. The NC300, which I owned at one time, had a very good product detector but a lousy AGC for SSB. It was so bad I had to operate it like a pre SSB receiver with RF gain cut back and AF gain cranked up.

If you are building a receiver chances are you already have a factory built one. But unless the one you are building and the one you have employ the same intermediate frequencies it is still quite useless.

Maybe you have a silicon based receiver that you want to duplicate in tubes. That would be an understandable goal but I don't recommend probing around in your expensive receiver to find a signal you can pull out for testing a newly built product detector. And there is still the problem of testing the AGC. Tube and transistor voltages are highly incompatible and it only takes a small slip to blow out half the transistors in your transistorized receiver.

If you own one of the classic ham band receivers you might be able to make use of it but only if you are using the same intermediate frequencies. I already have a Drake 2B so I wouldn't be very interested in building a duplicate of it. There seems to be little fun in exactly duplicating a commercially made receiver.

On the other hand if you are a neophyte who wants to build a receiver just for the experience and would like to start with something easy, duplicating an existing receiver is a very good way to start.

If you have an old NC300, SX101, HQ170, or 2B, go ahead and duplicate it. I recommend building from antenna to speaker but build the power supply first of all. Build the RF amplifier first and when it is finished test it by unplugging the RF tube and feed the output from your home built version in at the corresponding point. It may be necessary to unsolder a wire in the factory version but remember where it came from. Better yet take pictures. You can proceed stage by stage in this way until you have duplicated the entire receiver.

If you are working on a unique receiver and have a general coverage one you can use it to develop the front end. By tuning it to the IF. Example if you are going to build a tunable IF receiver you could build the crystal controlled or synthesized converter and use your general coverage receiver as the tunable IF for testing. If the first fixed IF is above 540 you can still use your general coverage receiver to test the second converter. But if you are going to use Ifs that are below the AM broadcast band you may be out of luck. You could build an up converter for your receiver. This could also render a ham band only receiver useable for this purpose. Of course the alternative is to forge ahead and build the entire receiver knowing that you have a working power supply and audio amplifier and figure on some troubleshooting of the final product. This could be difficult if you are not experienced in troubleshooting.

Modular construction.

I have used modular construction with other projects and I know how easy it is to correct design mistakes. If one of the subsystems doesn't work properly in concert with all the others it is easy enough to build another one and drop it in place. It does require careful planning along with some knowledge about how big a given module needs to be. You shouldn't begin planning a modular receiver at this point. For example if all you want is a diode detector for AM and a simple one tube product detector for SSB the detector module may be quite small. An S meter will add one more tube. Amplified AGC will require another tube. And if you are interested in the universal detector witch with minor switching will detect SSB, NB FM, and synchronously detect AM, that adds three or four more tubes. (Note; I have no tested circuits for such a detector but it seems possible that the integrated circuit that does all of those things could be translated to vacuum tubes without needing the entire output of Boulder dam to power it).

You probably don't understand a lot of the above paragraph but as you work your way through this article page by page the light will slowly dawn.

Have you ever heard someone say "the light goes off in my head"? That's why they don't really understand what is being explained to them. The light turns off leaving them in total darkness. If they understood that the light has to go on to illuminate the subject they might be able to comprehend the world. As it is these people don't have a clue.

Band Switching Troubles.

This section appeared on the block diagrams page and is repeated here because it seems to fit.

There is one feature that separates all of the classical receivers such as the National NC300 from tunable IF receivers such as the Drake 2B. That is band switching of the oscillator (VFO) coil. When the first local oscillator has to operate at a fixed frequency above the incoming frequency the oscillator inductance has to be switched along with the antenna and RF amplifier coils. In a tunable IF receiver the VFO always tunes the same range regardless of the band being tuned. Switching crystals isn't the same as switching coils and capacitors. It takes a really bad switch to effect the frequency of a crystal oscillator. In fact it is the case that the oscillator either runs or it doesn't. If it is running it is on frequency.

My experience with receivers old and new made me painfully aware of this problem. After I got my novice ticket my first two receivers were prewar, WW II, a National NC45 and an NC100. Both were 20 years old if they were a day and both had a tendency to make random jumps in frequency. Cleaning the band switch helped a little for a little while but copying CW on a receiver that was prone to jump around in frequency was not a happy situation. My purchase of a new NC300 solved this problem. My two old receivers had no doubt passed through many hands and had been heavily used. No amount of cleaning can restore a worn out band switch. Allowing the VFO to operate in a fixed frequency range eliminates a major source of instability in the oscillator. Also it doesn't hurt that it can be made to operate in a part of the HF band where it is easy to make stable and high Q inductors. However if you want to build a classical general coverage receiver you have no choice but to switch oscillator coils.

Equipment required.

1. Signal generator.
2. DC Voltmeter or DMM.
3. RF Probe.
4. Digital RLC Meter.
5. Soldering Iron.
6. Hand Drill.

1. Whether you are a novice or a gray beard you need one piece of test equipment. That is a signal generator. It is sheer folly to begin a receiver project without one. It doesn't have to be an HP 606 although that is a very nice generator. An old Heath or Eico bought from eBay will do. One with output calibrated in terms of microvolts and millivolts is helpful but isn't an absolute necessity.

2. Some sort of voltmeter or multi tester is also required. DMMs are so inexpensive these days there is no excuse for not having one. Even the cheapest DMM is as accurate as a good analog meter.

3. RF probe. They are easy to build and a diagram will be given when we start building the receiver's front end. An RF probe may be used in conjunction with an oscilloscope or a DC voltmeter. To detect the presents or absents of RF and can even be used for rough measurements.

4. Digital RLC meter. Although this is not a common instrument in most ham shacks you really can't wind your own coils without one. If you shop carefully they aren't all that expensive and although you might be able to get along without one building a receiver is frustrating enough without the uncertainty of "what actually is the inductance of this coil I have just wound". There are alternatives you might already own. A grid dip meter can be used but many have accuracy problems particularly if they are elderly. A Boonton or Heathkit Q meter can be useful if it has been recently repaired and calibrated

5 and 6. The only other vital pieces of equipment are a soldering iron and a hand drill. But don't solder your fingers and don't drill your hand.

Conclusion.

It would seem that I am proposing an experimental platform instead of a receiver that is meant to be used for rag chewing, passing traffic, or chasing DX. The decision about what and how to build is yours. Standard construction can be done by starting with a new large chassis and drilling as you go. If you have a pretty good idea of what you want you could probably punch all the tube socket holes at the start leaving only small holes to be drilled for terminal strips, coil forms, and shielding. You might have to do a little experimenting to find optimum resistor and capacitor values but when you finish you will have a receiver you can be proud of. You can tell the fellow or gal on the other end of the contact " I built it from scratch myself". Good luck and may your oscillators oscillate and your amplifiers not.

I hope this series of pages will help those of you who have always wanted to build a receiver but never quite got up the courage to do so to decide to just do it and let the sparks fall where they may. And always remember to have fun.

Introduction
Image Rejection, IF Bandwidth, and Number of Conversions
Block Diagram
Construction Tips. You are here.
Power Supply
Audio Amplifier
RF Amplifier and Preselector
Converters, Mixers, and Local Oscillators
IF Amplifiers and Band-pass Filters
Detectors
AGC (Automatic Gain Control), and S Meter (Signal Meter)