Omega receiver

In searching for the superior passive receiver

A little note: I have borrowed many information on this page from other websites, this is other people's work and thank you so much about sharing this information.
I have done a full one week's research on this information to try to categorize the different topologies and file them, this is my work which I would like to share with the readers. Enjoy..
 

Preface (there were much older devices than crystals)

The idea of the passive receiver is more than 100 years old. The first receiver type that is known to detect radio energy is the coherer type. The coherer invented by Edouard Branly and it is a primitive form of radio signal detector, used in the late 19th century, consisting of a capsule of metal filings in the space, sometimes evacuated, between two electrodes. It was a key enabling technology that led to radio, and was the first device used to detect radio signals in practical spark gap transmitter wireless telegraphy. Below is a typical schematic of a coherer type receiver.

Although the coherer is satisfactory for responding to the "on-off keying" characteristic of an early spark gap transmitter, it cannot follow the complex waveforms of audio broadcasting. This problem was solved by the demodulation capability enabled by the hot wire barretter and the electrolytic detector. 
Although the coherer is satisfactory for responding to the "on-off keying" characteristic of an early spark gap transmitter, it cannot follow the complex waveforms of audio broadcasting. This problem was solved by the demodulation capability enabled by the hot wire barretter and the electrolytic detector. Years later he invented the demodulating detector in 1902 that found limited use in early radio receivers. In effect it was a highly sensitive thermo resistor developed to permit the reception of amplitude modulated signals,  something that the coherer (the standard detector of the time) could not do.
The hot wire barretter was a demodulating detector invented in 1902 by Reginald Fessenden that found limited use in early radio receivers. In effect it was a highly sensitive thermo resistor developed to permit the reception of amplitude modulated signals, something that the coherer (the standard detector of the time) could not do.

The first device used to demodulate audio signals, it was later superseded by the electrolytic detector, also generally attributed to Fessenden. The electrolytic detector, or the bare-point electrolytic detector as it was also called, was a type of wet demodulator used in early radio receivers. This form of detector was in extensive use, and was very sensitive and reliable.

The last and most known part of passive ratio came with the crystal detector. Cat’s whisker crystal detector refers to a thin wire that lightly touches a semi conducting crystal to make an imperfect contact-junction detector in a crystal radio. While originally a figurative description of a mechanical part, the term grew to encompass the entire detector assembly and also in some English speaking communities to describe the receiver itself.

Since then, the technology has been advanced and the materials that composed the crystal detector were packaged into small parts called diodes. One of the most common materials used for the diodes is germanium and so these diodes are called germanium diodes or germanium detectors.

Introduction

Passive receivers have been always fascinated people. The absence of electrical power and the simplicity of the circuits were two of the most important reasons. Other reasons may include the nostalgia for the old, the beauty of the sets and the cabinets etc. In this page I present you the research that I did using internet resources in my try to build a superior crystal receiver. I have been based only on internet resources and it took me one week of continuous search to understand the different circuit topologies and the basics behind the crystal receiver. I made a search package zipped of around 150Mb size. It is available for you upon request. Instead of presenting you the super duper circuit and try to convince you that it is the best (as some home brewers will do), I will try to explain you the reasons why my schematic approach is superior to many others.
I am a person of the new generation but I personally like the way some things were built at the past, but I also consider the disadvantages and the bad points (where they existed) of some of the circuits of this era. I am not the person that will give an ironic look to the many decades home brewers' constructions and I highly respect their opinions but... some things have to be better than the classical way of making them. I always try to support my opinion with arguments and I do not blindly believe a construction to be good if I do not try it myself. That is how I am going to approach the topic in this article.
I am not going to give lot of theory behind this consideration because I want to keep the page simple for the reader. Everything, including the theory, is on the search package for you to read if you want so. Of course nothing is perfect and there is a lot of experimentation that can be done in order to achieve even better results. I will greatly appreciate your comments on any part of the text, at my email address at the beginning of the page.

First of all a few introductory words about crystal receivers.
Crystal receivers are passive devices. The term passive means that they do not use any other electrical power apart from the one that comes from the antenna, i.e. the radio signal. If a receiver fails to follow this rule it cannot be considered as passive.
Crystal receivers cannot achieve any type of amplification (except from some special cases that will be presented in this page). The energy of the out coming signal will always be lower than the energy of the incoming, mainly because of the losses of the different parts of the receiver. One main goal of the experimenter is to minimize these losses as much as possible in order to achieve more signal at the receiver output.
Do not expect great performance by any crystal receiver. The best crystal receiver you can find will be much more inferior to a simple pocket radio of today's technology. I am not telling this to disappoint you but it is the truth. The superheterodyne topology itself is much more inferior than the envelope detector used in crystal receivers. Unless you really need a set that will operate without power and you are able to have a big antenna and good earth, do not go for a crystal receiver, as far as concern performance issues.
After all this introduction let's come to the point of the page.

Sensitivity versus selectivity

Crystal-set DXers are up against two problems: selectivity versus sensitivity. Sensitivity describes the ability of the receiver to receive weak signals. The more sensitive a receiver is, the weaker signals it can receive. Selectivity is the ability of the receiver to distinguish a wanted signal from unwanted others, so you can listen to only this signal and not the unwanted ones. If you have not any means of filtering in your receiver, maybe you could make it more selective by lowering it's sensitivity. But then you would not be able to hear the weaker stations but only the loud ones. Also if two loud stations are close together in frequency, lowering the sensitivity, would not necessarily improve the selectivity. If you live in a city or near a large city that has several AM signals, your main focus will be on separating signals so that the weaker DX can be heard between the blowtorches. On the other hand, if you live in the country, you don't have to worry so much about separating strong locals; you can run things 'wide-open' enabling you to log some of the weaker DX signals. Living away from the city certainly makes life simpler for crystal radio DXing but don't despair if you are a city-dweller.

Topologies of crystal receivers

There are many topologies that can be used in crystal receivers but all of them consist of some main parts. There is always an antenna and an earth, a means of selectivity, a means of detection and a means of audio conversion. The selectivity part is usually composed of coils and capacitors and its purpose is to select the desired signal among unwanted ones. The detection part is usually composed of a detector (crystal ore or diode) and its purpose is to convert the high radio frequency signal to low audio frequency one. The audio conversion part is usually composed of headphones or other devices and its purpose is to convert the audio waveform into air pulses (i.e. sound) for your ears to listen. Below I will present you some basic topologies of crystal receivers that I have come across.

The diagram shows a simple basic crystal radio. It is probably the simplest you can find.
In the city, with a good antenna, such a set will hear plenty of locals but it is unlikely to hear any DX. Take it out to the campground for the weekend however and even a simple set like this is capable of logging DX. Since most of us live in or near the city, we need to look at ways of transforming the basic set into a DX machine!
In order to separate the strong locals, the tuned circuit (L-C) must have as high a 'Q' as possible. Q, the quality factor of a coil, is the ratio of the inductive reactance (i.e. "AC resistance") of a coil at a particular frequency to the resistive losses of the coil. This resistive component is the sum of the DC resistance and other losses such a dielectric losses in non-conductive material close to the coil and eddy-current losses in nearby conductors or the coil wire itself. High Q not only yields better selectivity, but also raises the impedance of the tuned circuit, which can allow more voltage to be applied to the detector.
Placing the diode and headphone load at the top of this circuit will result in strong signals but poor selectivity. The impedance of the headphones will load down the tuned circuit, lowering its 'Q'. The result is very poor selectivity. Tapping the diode/headphone load at a lower point in the tuned circuit results in less 'Q-lowering' of the circuit. The result is a higher degree of selectivity. You don't, however, get a free ride! The higher selectivity diode tap point results in less overall voltage being fed to the headphones. Signals are weaker but easier to separate! The challenge of building an efficient tuner is to find the right balance between selectivity and sensitivity!





 

There are a few more improvements that will help the 'basic set'. More sensitivity as well as more selectivity can be realized in the antenna circuit. A separate antenna coupling coil that will couple the antenna into the main tuning coil will result in a further overall improvement. If the position of the antenna coil can be adjusted in relation to the main tuning (detector) coil, a degree of coupling can be reached that offers a good balance point between adequate sensitivity and improved selectivity. Often the antenna coil and the detector coils are mounted on a long rod or dowel that allows easy adjustment of the 'loose-coupling' circuits. Also note that improved detector tuning can be achieved by optimizing the diode 'tap' location on the main tuning coil.
 

Further improvements in both sensitivity and selectivity can be achieved by actually tuning the antenna system to resonance. Your type of antenna system as well as the part of the AM band that you are tuning will determine which system works best. The series-tuned antenna coupler is best suited for tuning large antennas, especially those with large horizontal runs ("top hats"). The parallel-tuned antenna coupler is better suited for tuning shorter antennas, such as vertical wires, masts or whips - those without top hats. A series of test clips or a simple switching system can be used experimentally to determine what works best for you. Making this effort is important as it goes a long way towards avoiding initial disappointment with the crystal set's performance. Such a system as this is called a 'double-tuned' set. With care and attention to construction, such systems are capable of hearing DX signals.
 

Adding a tuning capacitor in series with the antenna permits a long antenna to be tuned very effectively by a parallel-tuned antenna coupler. In fact, this is the method-of-choice for many crystal set listeners. To avoid the complication of an extra tuning knob, a single-knob, two-gang variable capacitor can be used as shown here. The added variable capacitor can be placed instead in the ground leg. This 'enhanced parallel-tuned' coupler can be very effective in peaking up desired signals while knocking down nearby interference. The use of a tapped coil in this circuit can be very beneficial in achieving optimal Q for the portion of band being tuned. One might even decide on using separate 'high-band' and 'low-band' coils for this circuit.

Although you may now be able to hear some DX, chances are you are still bothered by at least one (if not several) local 'pest' signals. Further gains in selectivity can be had by the addition of another tuned circuit, called a 'wavetrap'. By coupling this circuit to the detector coil, this circuit can be tuned to the same frequency of the pest signal, absorbing much of its signal-killing strength at the detector, while not adversely affecting the strength of wanted signals. More than one wavetrap may be employed to tackle more than one pest signal. With a high Q wavetrap, very loose coupling is usually best.
 

Another method that some find successful is the use of an 'in-line' wavetrap in the antenna circuit. The smaller secondary winding can be wound either on top of the primary or on one end. One or two in-line wavetraps can often be used to effectively knockdown the strong pest signals.
 

Another important improvement can be made in the headphone circuit. If you are using the old standard style of headphones (Brush, Baldwin etc) they must be high impedance phones. The modern low-impedance 'walkman' style phones will load-down the detector circuit too much, seriously degrading selectivity and signal strength. Even high impedance phones will cause unwanted loading of the detector circuit. Using a low-loss audio transformer with a very high impedance primary between the phones and the detector will reduce the problem and often great gains in selectivity can be achieved.


 
 

This is the schematic diagram of a good performance receiver based on the above considerations.

This is a 'triple-tuned' arrangement. There are vernier reduction drives on all tuning capacitors as tuning is very sharp on all circuits. Because of the great abundance of 'blowtorch' signals within 15 miles of my location (seven 50 KW locals) heavy trapping of pests is required. Both sensitivity and selectivity are quite good.
Below many other additional approaches are presented. There is no better description than the original articles so please click on each picture to view or download the relevant article.
The second picture below uses a band pass filter to select the desired station. The fourth picture uses a wave trap and offers both broad and selective tuning to be made.

More classic schematics.

The schematic below uses double tuned circuits linked together by a link line, so it could be build in two boxes maby.

The three schematics below incorporate the use of a variometer (variable coil inductively varied, not selective).

Many other circuits in the boy's book of crystal sets.

The next seven circuits use band pass filters in order to improve selectivity. A band-pass filter is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. These filters can be created by combining a low-pass filter with a high-pass filter and that is the case here. Band pass filters are quite sharp and they are used extensively not only in crystal radio but in all forms of receivers.

The next three circuits show you how you can construct your crystal receiver in order to have memories of pre-selected stations. A crystal receiver can have memories? Of course it can. Not electronic memories but rather a set of multiple tuned circuits, each one tuned to a predetermined station in the band. Using a multi-switch you can select the desired stored station by selecting the relevant tuned circuit. That is a clever technique but it requires you to multiply the tuned circuits of the receiver by the number of the memories you want to have.

Up to this point I have shown you most of the more conventional crystal receiver topologies. Now I am going to show you a different approach to the topic. The next circuits could be described as more modern circuits.
In a conventional crystal receiver the diode delivers power to the headphone during the positive part of the RF cycle. The negative part of the RF cycle goes nowhere. We went to all the trouble to put up a 100 foot antenna and then waste half the energy we manage to capture. In the first picture below the top diode delivers power to the headphone during the positive part of the RF cycle and the bottom diode delivers power during the negative part of the RF cycle. In your typical crystal radio the voltage across the headphone comes from Det1 and ground (0 volts), so it has a magnitude equal to that of Det1. By putting the voltage from Det2 at the other side of the headphone, we get a difference of Det1-Det2 across the headphone (i.e. a greater voltage). Of course, this isn't exactly how it works. Really, the forward voltage drop of the diodes has a big effect on the output voltage, and all kinds of impedance interactions probably show up when you hook up another detector. So I do not know how efficient this circuit can be. But, this should get the idea across. A person could go through a lot of math to figure out exactly what happens. I think it's easier and more satisfying to just build the thing and see how good it works.

The next two circuits use another technique called voltage doubling to double the voltage that comes into the headphones. These are generally more effective detectors.

In order to understand better the how voltage doubling works consider the following. There are two types of voltage doublers. The Greinacher and the Villard.

Greinacher voltage doubler

The Greinacher doubler circuit (a) transforms a grounded AC voltage (peak voltage Up) into symmetrical DC voltages of 1x Up each, thus producing 2x Up between outputs. If the input voltage is already symmetrical (e.g. as in an Obit), the Greinacher circuit may be doubled according to (b). This does not change the output voltages but increases the possible output current by a factor of two.

Villard voltage doubler

In contrast to the Greinacher circuit, the Villard doubler circuit produces 2x Up towards ground at a single output. Besides this, the advantage of the Villard circuit is that it may be cascaded to form a voltage multiplier with (in principle) arbitrary output voltage. The way this works is that after C1 and C2 are fully charged to 1x and 2x Up respectively, there is a pulsating DC voltage oscillating between 0 and 2x Up across D2, which is converted to AC by the subsequent cap and thus acts as input AC of the next stage, and so forth.

Villard cascade

An n-stage cascade produces 2n x Up output voltage. By choosing an appropriate number of stages, any voltage can be reached. And that can give you a hint in your future experiments. However, this is only valid for negligible current draw. As soon as there is output current, there is also an AC current through the caps, resulting in a voltage drop and a lower input voltage for subsequent stages. In fact, numbers much higher than, say, 10 or 20, are not sensible in practice. More specifically, the formula for the voltage drop is

This means, the voltage drop is the higher, the higher the output current I, the lower the frequency f and the lower the capacity C. The voltage drop also increases with the number of stages cubed, which means for 10 stages it is already 1000x as large as for a single stage.

The next three circuits are examples of another type of detection, the full wave detection. These are not similar to the double diode circuits described above and they do not use the previously mentioned voltage doublers, yet they are very efficient and they provide some advantages over the voltage doubler circuits that will be discussed later on.

The best way to understand how this type of detector works is to compare it with the full wave rectification of the power supply units.

In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a one phase supply, or with three diodes in a three-phase supply.

A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient. However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. This has some disadvantages that we will see later on. Four rectifiers arranged this way are called a diode bridge or bridge rectifier:

For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (i.e. anodes-to-anode or cathode-to-cathode) form a full-wave rectifier (in this case, the voltage is half of that for the non-tapped bridge circuit above, and the diagram voltages are not to scale). However this is a very good case for the crystal receiver for reasons that will be discussed later on.

I said earlier that crystal receivers cannot achieve any type of amplification, except from some special cases. The next notes and schematics have to be considered with great interest. They achieve amplification without using external power other than the one comes from the antenna. They are still crystal receivers and they still use detectors. The diodes here, are not used for signal detection but they deliver power to the transistor amplifier/detector. One could argument that this may degrade the wanted signal. But what is actually happening is that the power needed for the transistor is not taken from the wanted signal itself but from the unwanted ones that the antenna receives. In this case, the greater the RF interference in an area, the better.

Two other examples of such receivers are shown below. They use FETs as RF amplifiers and detectors. I do not know if they use unwanted or wanted signals for the power required for the FETs to work and I am not sure if they can be considered as crystal receivers as they do not use any diodes or germanium transistors. But nevertheless, the topic of the project is not only about crystal receivers but generally about passive receivers. The rectification is done by the FET itself. Well no one says that a crystal receiver must use only diodes or germanium transistors for detection, although the word "crystal" denotes this. In the context that they do not use external power they can be considered as crystal receivers but in the context that they do not use diodes/germanium transistors for rectification they cannot. The certain thing is that they do not use external power and this makes them passive. They also offer some advantages over diode crystal sets.

Here you can find more details of how to build such circuits

Below there are some more schematics and descriptions (above them) about such circuits.

Simple Crystal set for local transmitter

You can use also a crystal phone (x = 3 K OHM). For weak signals you can shorten the amplifier switch. Transistors: AC151, AC104, OC76, SB187. Rectifiers: AA112, AA114, AA132, OA85. L= 100 or 120 wdg with diameter 2 cm, C= 500pf for MW.

Crystal receiver with AF-amplifier for your local broadcast transmitter. Model 1

The rectified RF is smoothed with R1 and C3 and is the "source" for the voltage energy of the AF - transistor. Disadvantage of the receiver: The voltage stands only at strong transmitter at the disposal. On the other hand is loudspeaker - reception realizable. L= 100 or 120 wdg with diameter 2 cm, C1= 500pf, C2= 1nf, C3= 3uf, C4= 0.1 uf, C5= 500 500pf, R1= 1M, R2= 1M, transformer 6/1 or 4/1, transistor: AC151, AC104, OC76, SB187 rectifier: AA112, AA114, AA132, OA85.

Crystal receiver with AF-amplifier for your local broadcast transmitter. Model 2

The second concept is more profitable. The transistor - energy is produced from a separate resonance-circuit. This circuit (L1/C3) is tuned firmly on the local transmitter, Circuit C4/L2 is tuned for radio reception. Good loudspeaker - reception, also at weaker transmitters, is the result. As transistors are indeed germanium - types necessary. You need a operation voltage of about 800 mV at C8 for well working.

Transistors: AF preamplifier T1 = OC71 ( NKT214, 2N280, 2SB244) , AF final amplifier T2 = OC 72 (2SB56, NKT212, 2N281). R1 = 75 KOhm, R2 = 100 KOhms, C8 = 100 uF, C5 = 500 pF, C6 und C7 = 10 uF, transformer Tr1 = 1/1 ratio Z approx. 4000/4000 Ohms, transformer Tr2 = 2/1 ratio Z =2000/1000 Ohms, transformer Tr3 = Z 200/4-5 Ohms. L1 and L 2= 100 wdg with diameter 2 cm (tap for D1 80 wdg from ground) , C3/C4= 500pf, rectifiers: AA112, AA114, AA132, OA85. I think, you have problems to buy the transformers. Perhaps you find an old low price transistor portable with some transformers.

Adjust the trimmers R1 and R2 for best audio quality (bias voltage between basis and emitter of approx. 350-450 mV).

Version 2: For best performance use separate antennas for power supply detector and receiver detector. L1 except tap. A1 for power supply detector. C9 = ceramic 1nF. With the (high ohm) voltage meter you can adjust to highest voltage. Use a instrument with a full range of 3 to 10 V in relation to the max. voltage on your location..

Crystal receiver with AF-amplifier also for long-distance broadcast reception.

This is a high performance project because the supply voltage needed for the transistor amplifier is generated by the local broadcast station to which crystal receiver number 1 (left side of the schematic) is fix-tuned. Tune with the aid of a meter and adjust for maximum detection. Good AF amplification also of low-level stations with the crystal receiver number two (right side of schematic). Use a Germanium low signal AF type transistor (AC151 or 2SB187, etc.). A supply voltage as low as 450 mV produces audible signal amplification. Of course you have to use TWO antennas.

I use for the crystal receiver number 2 (right) a loop (frame) antenna, for crystal receiver number 1 (left) I use a 20 meters (66 feet) wire outside antenna. This generates a supply voltage of approx. 900 mV under load of receiver number 2.

Adjust the variable 250 k ohms resistor to approx 50 % of the voltage generated by receiver number 1 voltage (measured at + pole). Best value for 10 k ohms trim pot is 3 to 6 k ohms.

Do not use electrolytic for the 0.1 uF capacitors. Do not forget to install the 1 nF capacitor. The 150 k ohms resistor of receiver number 1 is not really necessary. Attention for the polarity of the diode in receiver number 1.

Crystal receiver with RF-amplifier for long-distance broadcast reception.

This concept is effective since the supply voltage for the transistor amplifier is provided by a separate tuned circuit which is fix-tuned to the local radio station and thus is constantly available (tune to maximum output with a suitable meter). Unlike the previous circuit this time the radio frequency is amplified, not the audio frequency. This concept is more suitable for long distance reception.

Good sensitivity by rf amplification. The transistor required is a germanium rf type (an AF101 or 2SA175 or other rf type). Already a voltage of about 450 mV produces audible amplification. You will HAVE TO USE two separate antennas. Coils used have ferrite (powered iron) cores, variable capacitors are solid dielectric types with 500 pF. The rf chokes (RFC 1 and 2) may be made by winding about 200 turns of this enameled copper wire on a ferrite slig with a diameter of 10 and a length of 50 millimeters.

For detector receiver number two I used at night-time a loop antenna and for detector radio number one (left side of schematic) I use an out-side wire antenna of 20 meters length. This set-up produces a voltage of about 900 mV under load of the second detector set.

Adjust the 25 k-ohms trimmer for the loudest signal.

The 150 k-ohms resistor is not really necessary. Watch the polarity of the diode in the receiver number 1.

For 2000 ohms magnetic headphones

For 4-5 ohms earphone

For 2000 ohms magnetic headphones

SW-band crystal set with transistor AF amplifier powered by the transmitter energy.

An alternative use of a battery is possible. A second receiver for MW band delivers the voltage for the transistor amplifier. With S2 you can connect the headphones direct to the crystal set or to the output of the AF transistor amplifier. The 20 uA measuring instrument is for best tuning of the SW broadcast station. 2000 ohms headphones.
 

Another type of such radio that describes a vacuum tube Wheatstone Bridge 0-V-0 Regenerative Receiver with Passive hybrid reverse antenna isolation

All the crystal receivers described above are able to detect only AM (amplitude modulated) signals. The next circuits are definitely not crystal receivers. They use a combination of a passive and an active circuit. The reason why I present them to you is that they are capable of demodulating SSB (single side band) signals. If one could make the BFO taking power from the antenna, like the circuits I presented above, then these would be SSB crystal sets. This is a very good point of experimentation for someone that would like to try.

The Omega receiver

In searching for the superior passive receiver I decided to create the Omega project. The Greek letter Omega ( ) was used for some reasons, including my admiration for Ancient Greeks. I would lie to you if I said that I have already constructed the most efficient passive receiver already. This is a topic of much discussion and experimentation throughout the years and maybe decades. This is exactly what I am going to do in this page. But like every project it has to start somewhere. Your support and thoughts are greatly appreciated. Send me an email if you have anything to say about the topic, and who knows, we may discover other fascinating things related not only to this, but generally to electronics!
A receiver of this type must have the advantages of the topologies described above and minimize the disadvantages. It must be definitely a combination of some of the above circuits and it has to extend somehow to new ideas and approaches. But first let's just start from the basics.

Instead of starting from the RF selectivity or the antenna stage, I am going to start from the other way round, the detector. Every receiver must have some kind of detector. A good receiver must have an efficient detector. There are a few types of detectors but here I will present a few things about diode detectors. We have already seen these types of detectors but here I will give a betted approach based on their advantages and disadvantages. If a large off-balance output voltage (2-20V RMS) is available, the detector can be one of a variety of simple diode rectifier circuits. Although, as we will discuss shortly, consideration must be given to the type of diode used. Note that for any of the diode detector circuits shown below, the polarity of the DC output can be reversed by reversing all of the diodes.
 

Half-wave rectifier
The simplest detector of all is the half-wave rectifier circuit shown below:


 

Shunt-diode rectifier
A half-wave rectifier can also be constructed using the shunt diode configuration shown below.


 

Voltage-doubler rectifier
Given that RF chokes are bulky and potentially problematic, it is often sensible to try to avoid using them even if sensitivity is paramount (i.e., in situations where the choke cannot simply be replaced by a resistor). This can be done in various ways; the first example, shown below, being known as a 'voltage doubler'. This is a Villard voltage doubler. A Greinacher voltage doubles has been discussed in a previous section of the page.

Bridge rectifier
A detector which requires no RF chokes and does not care of there is a DC path through the network is the full-wave bridge, shown below:



 

Bi-phase rectifier
A full-wave detector which has only one diode forward voltage-drop in the path to the meter, and is completely free from grounding problems, is the bi-phase rectifier. This one of the best choices for crystal detectors.


 

For the Omega receiver I have decided to go for the OA47 diode, based on it's obvious advantages over the other diodes. The second generation of the receiver may use a FET detector, but I first have to make a better research on this type of detector. So, for the time being, and to satisfy more classical set readers I will end up to this type of diode.
 

Below I would like to mention two useful things about diodes. The detected voltage as function of the input voltage and the detector diode input impedance

I believe I gave you a feeling about the issues related to the detector. So let me summarize the construction of the Omega receiver at this point using the following schematic.

The input RF signal is connected to the center coil and the audio signal is coming out of the center tap and the positive of the two diodes. This one works ok if we are referring to a single frequency rectification like for example 50 Hz mains. In radio frequencies multiple signals appear simultaneously to the antenna and thus to the center coil and so we have to have a way of selecting the desired signal from the many unwanted ones. This can be done by creating tuned circuits with the use of a variable capacitor that is connected parallel to the output coil. Here we have a symmetrical configuration so we need two of these circuits and thus two capacitors. As I said earlier, care must be taken when constructing the coil. The symmetrical circuits must be as matched as possible for the circuit to work efficiently. This is the price you have to pay for the greater power gain. In the input center coil you must connect the antenna and the earth. Now the Omega receiver will look somehow like this:

Before I carry on I think it is time to present a few things about earthing. In crystal receivers earthing is as important as the antenna. The figures below show three good earthing techniques.

Now that I have shown you the earthing bit let's go back in our Omega receiver. For the tuned circuits to be matched in a symmetrical circuit the tuning must be made equal to each of them. An easy way to achieve this is to use a dual gang variable capacitor. Each gang is connected to one of the two secondary coils, so when you rotate the capacitor you tune the symmetrical circuits equally. This has the advantage that the user does not need to tune two capacitors each time but just one double. This is fine in theory but in practice there is no perfectly matched capacitor. Even the dual gang types have small differences in the capacitance of each of their sections. To overcome this problem we can add a small variable capacitor in parallel with each main tuning capacitor. The total capacity of each gang will then be the capacity of the main tuning plus the capacity of the small variable capacitor. By tuning the small variables respectively, and measuring the total capacity of each gang of the big variable capacitor, you can create a matched dual gang capacitor.
For this circuit, measuring and matching your capacitor is not actually needed. This is because it is not only the capacitor that affects the frequency but also the coil. If the coil of the one section has a bit different inductance than the coil of the other section then the capacitor of the first section must have a bit different capacitance than the capacitor in the second section in order for the two tuned circuits to be equal. The ideal case would be to have perfectly matched coils. In that case we would need a perfectly matched capacitor. But the fact is that homemade coils are difficult to be made perfectly matched. Thus, the addition of a small parallel variable capacitor offers the advantage of perfectly matching the two sections of the circuit. The schematic of the Omega receiver up to this point, can be seen below.

Tuning such a circuit is not as difficult as it seems. First turn the two small variables at about a half of their capacity. Then tune to a frequency stable loud station using the dual gang variable. Then re-tune the two small variables one after the other, in order to achieve the loudest output signal. The less the bandwidth of the incoming signal, the more accurate you can match the two sections.

The heart of the omega receiver circuit is fine up to this point. It should be a very sensitive classical crystal receiver for all the reasons described in the previous sections. But the selectivity is not great yet. As I said earlier, the two main performance issues in crystal receivers are sensitivity and selectivity. For the Omega receiver I do not want to degrade sensitivity for the benefit of selectivity and vice versa. A form of circuit to improve selectivity must be used. There are several circuits that can do that, each one can do it better or worst than the others and each one has advantages and disadvantages. At the beginning of the page I have shown many circuits that are using tuned circuits of coils and capacitors to improve selectivity. Some of them are using band pass filters. To my point of view band pass filters are the most suitable for good selectivity. They perform well and that is why they are extensively used in microwaves where the applications are critical. They are also very straight forward. They allow only a certain part of the band to pass into the next stage. I have read that band pass filters have some insertion loss, but I think this can be compensated with the detector type that I have used. I am telling that because almost all the band pass circuits that I have shown at the start of the page use just a single diode detector, yet they claim that they perform very well. A band pass filter is composed of two parts. The low pass filter, which cuts off the higher frequencies and the high pass filter, which cuts off the lower frequencies. The two parts are coupled together incuctively or capacitively. By using this combination of low pass and high pass filter, a band pass filter can be made. The next picture shows such a band pass filter, in the red square.

Note, there must be an error in the above circuit, although I haven't test it at the time I am writing these lines. All the band pass filters that I have seen connect their bottom side to the ground. Based on this, I would then connect the L2, C1A, C1B and L3 to the ground, at their bottom end, but this is something I have to test first. The circuit is called double tuned band pass filter because it uses two tune capacitors. It is also capacitively coubled using another variable capacitor.
The circuit below comes from an old miller set. It does not use capacitive coupling but it connects the two parts of the filter using a third coil. I have found that this coil provides negative mutual inductance between the two tuned circuits to maintain a constant bandwidth across the entire tuning range. however I have also found that in the capacitor coupling circuit above, the variable capacitor C2 should be adjusted for the best selectivity and volume over the entire broadcast band. So I think that these two circuits have about the same performance.

The capacitive coupling circuit is extensively used today by radio amateurs in modern sets because it's performance is good and it is easy to construct. Besides, I cannot find more information about winding homebrew coils for the second circuit, so I have decided to go for the first approach. However, since it uses a dual gang capacitor to tune the two filter parts, the addition of a small variable capacitor in parallel with each main tuning capacitor is mandatory for the reasons described previously. The filter will be inductively coupled to the detector section and to the antenna. Based on the above considerations, the final circuit up to this point for the Omega receiver can be summarized below.

Now, do not be scared about the many variable capacitors. The actual tuning capacitors (knobs) are three. This circuit seems quite selective and complete, but there is a glitch. Generally, the more components you have in such a passive circuit, the more losses you have. If you connect just a diode at the antenna using no coils and capacitors, these losses are minimum and they are determined by the losses of the diode, the antenna cable and the losses of the headphones. But the selectivity in this case is very poor and it is only determined by the resonance of the antenna itself. For a successive receiver you have to find a compromise between selectivity and losses or as usually mentioned, selectivity and sensitivity. This is an interesting topic that needs a lot of experimenting and constructing the actual circuits, rather than investigate them in theory.