Introducing Superhet Receivers, Featuring Optional Add On Converter For The 2 Valve TRF Receiver
Welcome to this part of my site, Featuring a introduction to how The Superhet Receiver works and a one valve simple add on Converter for the two valve TRF Receiver, Featured as an existing project on this website by clicking on the following link Simple 2 Valve TRF Tuned Radio Frequency Receiver
The invention of the Superhet Receiver came about in the mid 1920s and has gained popularity ever since. It is used in all commercial AM/FM Valve and Transistor Radios, Cordless Telephones and WI FI adaptors we use for wireless networking our computers. Edwin Howard Armstrong, The inventor of the Regenerative Detector, Thought of a really ingenious idea. As you can see in the picture above, It is a working example of a typical Short Wave Communications Superhet Receiver. I have divided it in to 7 stages and we are going to work from the left. The first stage is the RF Radio Frequency Amplifier. Although it is not the essential element of a Superhet and many low cost receivers apart from FM don't incorporate them. Its purpose is to increase gain, Reducing second channel interference and Improve selectivity and prevent radiation of the Local Oscillator especially on VHF/FM Sets. The second stage is the very clever bit known as the Mixer or Frequency Changer. Suppose You want to tune to your favourite radio station? As an example it can be Virgin 1215 on the Medium Waveband. You first need to tune the mixer to 1215KHZ. Ignoring the third stage for now, We must decide what Intermittent Frequency to use which is dependent on the fourth stage we call the IF Amplifier and it is this stage that provides most of the gain. We will tune the IF Amplifier to 455KHZ, The UK standard IF Frequency in all AM Broadcast Receivers. The third stage is the Local Oscillator and this must be tuned to 1670KHZ to beat or as we call it, Heterodyne with the incoming frequency to make up the sum and as a result we are converting this to the low IF Frequency of 455KHZ. This is achieved by using a dual gang or separate tuning capacitor to tune both frequencies together at the same time. The Fifth stage is the BFO Beat Frequency Oscillator and is not needed when listening to normal AM Speech or Music. It is needed when listening to CW Morse or SSB Single Sideband reception on the Amateur Bands. It is just basically the same sort of component as the local oscillator and we tune it - 455KHZ For the reception Lower Sideband transmissions. We need to retune it + 455KHZ for the reception of Upper Sideband transmissions, That is about 20KHZ each side of the IF Frequency. As This frequency has to beat with the IF Frequency, SSB transmissions will sound more like Donald Duck or if you were talking down a drain pipe. The sixth stage is the Diode Detector which rectifies and converts the signal to audio frequency. The sixth stage is the Audio Amplifier and its purpose is to amplify the small AF signal big enough to drive a loudspeaker. As you may not be aware, Our Two Valve TRF Receiver can be converted into a Superhet Receiver by using it as a Intermittent Frequency IF Amplifier. The pictures below feature a circuit diagram of a suitable converter using the ECF82 Triode Pentode Valve and Instructions on how to proceed with this modification. Please read these words of warning before starting. The 6.3 Volt heater current must be increased to 2.5 Amps recommended minimum and may call for a new mains transformer. The local Oscillator works OK if I recall at HT voltages as low as 70 but may have to be increased to 100 Volts minimum for reliable operation which may call for a new HT Transformer. Although the converter should drive the original TRF Receiver satisfactory I would recommend building the optional RF Stage to give maximum gain and performance.
Please left click on selected pictures to enlarge image.
Constructing And Setting Up The Converter
The converter featured here has been chosen to form one of the simplest valve superhet receiver configuration possible, Using home made coils. Although it would have been nice to have chosen a 455 KHZ IF, I am afraid to say that the coils, Through not impossible are difficult to wind and valve IF Transformers are no longer available unless you are lucky enough to own an old none working valve receiver. For this reason a 1600KHZ IF has been chosen, Which is the gap between the top end of the medium wave band to the start of the 160 metre amateur Band.
Configuring The 2 Valve TRF Receiver
There are two simple important modifications that must be carried out on the receiver before proceeding. The VC1 and VC2 Variable capacitors must be replaced with 65PF or 365 PF Trimmers mounted on matrix board together with the IF Coil. If using 65PF trimmers, The medium wave coil in the receiver must be modified or rewound with a coupling winding of about 10 turns and may call for experimentation to achieve best results. Picture 3 shows a simple diagram explaining how to do this modification. If the 365PF trimmers are used, Which I think is a better option and should give better IF performance, Then the short wave SW1 Coil must be wound with a coupling winding of about 20 turns in the same manner as the medium wave Coil. The Receiver will now be our Intermittent Frequency Amplifier and must be tuned to 1.6 MHZ. If you are lucky enough to own a RF Signal generator this should be not to difficult to achieve and as the IF does not have to be spot on, Tuning to the top end of the medium wave band, Clear of any strong stations should give satisfactory results without the aid of a signal generator.
Configuring The Receiver When The RF Amplifier Is Used
If you have built the receiver with the optional RF Amplifier added on, You need to refer to picture 4 which is a diagram explaining how to proceed. As you will see, It is done in the same manner as the receiver and it calls for replacing the RFC Choke with a new RF Coil. There are two special notes regarding the receiver modification when the following configuration is used that you must be aware of. The RF Coupling and coil modification is not required and must be left as it. You only have to replace VC1/2 with 65 or 365PF Trimmers. Tuning the receiver IF is slightly more difficult when using this later configuration but I will give you the following details. You need to temporarily remove the EF91 Valve from the RF Stage. To avoid high voltage HT damage to the signal generator, You need to couple it to pin 6 anode of the valve holder, with the junction of R2 and C10 through a high voltage capacitor of about 10PF 500V. The receiver must now be connected and tuned to 1.6 MHZ, Using the new 65PF Trimmer. The next step is to disconnect the receiver and reinsert the EF 91 RF Valve. The capacitor and signal probe is now reconnected to ether end of the converter coupling coil L5. Reconnect the receiver and adjust TR1, RF trimmer for a maximum tone signal. Note that you may experience HF Oscillation when both trimmers are peaked to there exact frequency, In which case. You may have to detune slightly or reduce the IF gain with the existing RF Gain Control. As this is now a high gain IF amplifier there should be not to much loss regarding performance.
Constructing The Converter
If all the above tasks in modifying the receiver have gone OK, You can now go ahead with the construction of the converter. You need to refer to pictures 1and 2 for the circuit diagram and component layout. Referring to picture 2, This is the best recommended layout of the converter and is constructed basically the same way as the receiver. Please note that the tuning capacitors VC1/2 are shown as dual gang component and if this method is used, it will be necessary to wire 65PF trimmers across each gang to obtain correct tracking and frequency coverage. If this is your first attempt at building a superhet receiver, It would be better to go for 2 separate tuning capacitors for the mixer and oscillator circuits as this makes the alignment much more easier. Points in the layout diagram marked E are chassis earth points which make the construction easy and avoids having all these silly long wires that can lead to instability and backlash. Band switching may also be used with this converter and is done similar as in the TRF Receiver, Although please bear the following points. As you will need to switch the aerial, mixer and oscillator, Adding up to 4 windings, The risk of instability and loss of performance is very high compared to the TRF Receiver. I would at least to start with, Build it as a single band set using SW2. There are also other important points to remember regarding the coils and tuned circuits. Try to keep the IF, mixer and oscillator coils well separated and if possible use non magnetic tin plate to form a shield, Particularly when band switching is incorporated. The tuning capacitors should be mounted about 70mm away from the front panel to allow room for using a spindle coupler and insulated nylon coupling cut from an old potentiometer to link the tuning dial, As this will prevent detuning effects caused by hand capacity. If all these points are correctly followed, allowing plenty of time and patients you should have a powerful working and stable superhet design.
Constructing And Winding The RF Coils
Construction of the RF Coils for a superhet receiver is more tricky then for a straight TRF Receiver, Particularly medium and long wave coils and for this reason I have not included them. Although not impossible, The performance I experienced using the old Denco Plug in type coils was very poor with severe whistling and IF Breakthrough, Particularly on the high end of the medium waves during the evening. Picture 5 refers to the physical construction of the coils and the tables below show the winding instructions for each individual band. The coil formers are 1 inch in diameter for the suggested band coverage listed in the tables below. The 15mm Coil Formers used in the TRF receiver would be OK, Although there will be slightly extended high frequency coverage.
SW1 2-5 MHZ 26 SWG
L1 10 turns.
L2/ L3 50 Turns.
L4 15 Turns.
SW2 4-10 MHZ 26SWG Recommended Coil, To get the set up and running
L1 8 Turns.
L2/ L3 24 Turns.
L4 10 Turns.
SW3 8-20 MHZ 20SWG
L1 5 Turns.
L2/3 12 Turns.
L4 6 Turns.
SW4 16-35 MHZ 20SWG
L1 2 Turns
L2/3 5 Turns
L4 3 Turns
Component List For Superhet Converter
Resistors 0.5 Watt
R1 47R. R2 1k2. R3 100K. R4 47K R5 47K R6 47K or 22k for better oscillator performance when using 70-100V HT Line.
Capacitors Disk Ceramic Or Silver Mica 350Volt Minimum
VC1/2 365PF. C1/2 0.01uf. C3 0.01uf C4 1000PF. C5 100PF. C6 10PF. 65 Or 350PF Trimmers, Minimum of 4 And depending if you are going to use band switching.
L1, L2, L3, L4, See Text
Valves. 1 ECF 82 HF Triode Pentode
Valve Holder B9A Chassis mounting type
The component list above is the minimum required to get the converter up and running.
Testing And Setting Up The Converter
Before reconnecting the receiver, Very carefully check the wiring for any simple mistakes, Such as short circuits, wrong connections and dry solder joints. Reconnect the receiver and allow about 1 minute for all the valves to warm up. Connect to the L1 Aerial terminal about 5 metres of wire. Try rocking VC2, The oscillator and main tuning capacitor around midway to the high frequency end of the band and you should be able to tune a few stations. If this is so try rocking VC1 The mixer and aerial trimmer for maximum signal strength. If all is well it is now time to align the frequency coverage. This can be done with the aid of a broadcast receiver tuned to the frequency you are looking for, By listening for identical signals but takes longer. Connect a RF signal generator to the L1 Aerial terminal. Using tone modulation, Tune the signal generator around the range of about 4 to 10MHZ and when you hear a tone, Make a note of the frequency. If the frequency for example is about 7 MHZ, Tune VC2, Very slowly to the low frequency end of the band and keep retuning the signal generator, Maintaining VC2 for maximum signal strength at the same time. When you get to the extreme limit of the low frequency end of the band, That is with the capacitor vanes fully open, Make a note of the Frequency. If for example the frequency is now 5MHZ, This needs to be corrected. Retune the signal generator to 4MHZ and adjust the oscillator trimmer very slowly until you hear a tone. Now peak the mixer capacitor, VC2 for maximum signal strength. Repeat this step several times by doing the same procedure at both the high and low frequency end of the band, As one adjustment affects the other.
Using The Receiver
You may notice two things different compared to when you had the set configured as a TRF receiver. You will now find that you don't have to keep readjusting the regeneration control every time you tune from one end of the band to the other, Because the TRF Receiver is now the IF Amplifier and the converter takes care of it all. Also you will not find stations as cluttered together, Making tuning more easier and being able to resolve the more weak distant stations. This is due to better selectivity, By having more then one tuned circuit. The regeneration control is still used when listening to CW or SSB transmissions by adjusting it in the same way, Until the set just oscillates and when the RF Stage is not used, It acts more like an RF Gain control. When using The RF Amplifier you will have to be careful not to have the RF gain to high, Because this may cause overloading and cross modulation resulting in distortion. I hope visitors enjoy reading this page and if you are interested in VHF, Why not have a go at my transistor design by clicking on the following link Transistor FM Superhet Receiver Also this converter circuit is used as the front end in my new featured 7 Valve HF Receiver and with slight modification regarding the coils, It should be suitable. Please click the following link Severn Valve HF Superhet Receiver Designed For Advanced Constructors for more information on this design
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