Circuit : Ramon Vargas Patron - Lima Peru
Email :ramonvargas15353@gmail.com
Description
This is a new Regenerative receiver for the Short Wave bands. It uses readily available parts and covers 3Mhz
to around 12Mhz. The full design notes are available in two PDF files, the notes below are an extract for
this web page.
NotesC 1 and the paralleled bandspread capacitor C2 are effectively connected in series with the 2.2-nF Vackar capacitor used for regeneration enhancement throughout the band. They should permit tuning the receiver down to 3MHz when the stray capacitance of the circuit and L1 are taken into account. For this matter, 7.6uH of inductance will be needed for L1. However, with C 1 and C2 at minimum capacitance we will be just below the 25m band and missing a lot of interesting SW listening. For this reason L1 was left with the number of turns and inductance mentioned in the following paragraph.
Coil Winding Details
The RF transformer consists of windings L1, L2 , L3 and L4, wound on a black-plastic 35- mm film container, as a 3.15-cm diameter coil former. Coils were wound using AWG #25~27 plastic-insulated solid-copper hook-up wire with an equivalent AWG #20 gauge for close-wound solenoids. Design techniques recommend that L2 should have 1/3 the number of turns of L1 and L 4 1/6 of that number of turns. L1 +L2 is a single coil having 17 close-wound turns, with a tap at the 4th turn. L1 is then 13 turns and L2 is 4 turns. L3 is a close-wound 4-turn coil with a 3-mm separation from L2, measured between wire centers. L4 is the antenna coil and consists of 2 close-wound turns and has a 6-mm separation from L 1. Using a digital BK Precision 875A LCR Meter values for the coils were obtained as L1 = 6.3uH, L2 = L3 = 0.6uH. L4 was not measured. L 1 was later confirmed to be 6.72uH by a more precise measurement method. Minimum tuned frequency was 3.19MHz. A trade-off had to be made if we wished to tune the 25m band. L3 permits a better match between the low base-emitter impedance of the 2N3904 transistor from the RF stage and the L1-C1 tuning tank, thus preserving selectivity. L2 is the feedback coil, known also as the tickler, and is the mains for returning a controlled amount of RF energy from the RF stage output back to L1 in phase with its own magnetic field. The relative phase of the windings is as indicated by the dots in the drawing. All coils are wound in the same direction, L4 on top, followed by L1 +L2, then L3.
As receiving SSB signals is in our plans, we need very stable operation of the detector- amplifier stage in the oscillating mode. A good biasing scheme for this effect is that which uses a two-resistor voltage-divider network for the transistor’s base bias, and an emitter resistor for DC feedback. The 16k-ohm, 7.5k-ohm and 1.5k-ohm resistors fulfill the requirements for the stage bias. The 10-nF capacitors connected in parallel with the 7.5k-ohm and 1.5k-ohm resistors perform RF decoupling.
RF current components at the RF stage output must be kept from flowing into the AF amplifying stages. This is the reason for including the 3.3-mH RF choke (which should be placed at right angles with the input RF transformer in order to avoid unwanted magnetic coupling) and the 4.7nF capacitor in the circuit. They act as a low-pass filter in conjunction with the 6.8k-ohm collector-bias resistor and input resistance of the first AF stage. The AM modulation is recovered across this 6.8k-ohm resistor. The following two AF stages amplify the detected currents to comfortable earphone/headphone listening levels. The 22-nF capacitor connected from the collector of the 2N3904 output transistor to ground is used for tone correction purposes. If the reader is able to purchase from a surplus radio shop a vintage Rochelle-salt piezoelectric high-impedance earphone (these are units having impedances in the 100k-ohm range) or the easier to find low-cost ceramic-piezoelectric 10k-ohm earphone, these may be connected directly across the primary of the output audio transformer. The low-cost ceramic types have been found to fail quite easily if you connect them to a point of the circuit where a DC voltage exists. So, it is much safer to connect them across the primary as said. The vintage Rochelle-salt types don’t seem to present this issue. By the way, the Rochelle types are also known as crystal earphones.
Full Design NotesRegenerative Shortwave Receiver Part 1
Regenerative Shortwave Receiver Part 2
The Finished Receiver
The set of four photos below give a general view of the author's prototype. In the first picture, the brown wires connecting to the receiver at the left are, the antenna lead-in wire, rear most, and the ground lead, foremost. The external ground is a 30cm x 30cm piece of aluminum sheet on the floor next to the desk where the receiver is. This is known as a capacitive ground connection and works very well for SW frequencies.
Ramon's Youtube Channel
Ramon also has a youtube channel and has videos of some of his receivers in action. Below are a couple of links to Ramon's Colpitts Receiver in action.
