Multi-band Receiver
Band | Turns | PF | ||||
L1 | L2 | C3 | C4 | C1 | C2 | |
80M | 64 | 5 | 100 | - | 470 | - |
40M | 26 | 2 | 100 | 15 | 100 | 82 |
20M | 16 | 2 | 47 | 12 | 39 | 47 |
15M | 9 | 2 | 33 | 12 | 33 | 39 |
10M | 8 | 2 | 15 | 18 | 33 | 47 |
C12/1 Coils Winding details - Table | ||||||
In Fig. C-12/1, the circuit of a multi-band FET regenerative receiver is given. Even though 2N4416 is recommended for Q1, it can be substituted with BFW 11. This gadget can be attached to a low power transmitter and thus used as transceiver. Since Q1 works as a Detector also, only limited changes are advised on the 2.2K resistor at Q1 Source and 2.7K voltage dropping resistor on the positive power supply line. If the operation curve of a transistor is different it is the value of the voltage dropping resistor that is to be adjusted first. Some times it may have to be reduced upto 820Ω. Instead of the 25 PF variable capacitor at S1- D one section o f a 2 J gang reassembled with only 4 plates can be substituted.
The actual value of C1 in the circuit might vary from the values shown in the chart at different bands. The value of C1 may be concluded through a trial and error experiments using capacitors of near by values. If the value of C1 is reduced highly it may affect the sensitivity of the receiver while if it is higher it could lead to a loading effect in the receiver. The L1 part of the Antenna coil is intended for regeneration while L2 is part of tuning. Please note that on 80 Meters neither C2 nor C4 is required. When this gadget is attached to a transceiver, it is because of over voltage limiting is necessary that two 1N4148 diodes are connected to the ground. Q4 can be used to mute the receiver every time the transmitter is switched. When positive voltage is applied to the base of Q4, it conducts and thus grounds the audio signals.
Here, a 3 stage RF Filter is needed before the Volume Control. Close S2 while receiving CW signals. The 100μH RFC is intended to avoid all AF oscillations and RF interferences at Q3. An appropriate substitute Audio Amplifier also can be used here.
In Chapter 8, the working arrangements of a BC receiver when attached to a transmitter was detailed (Ref.C-8/1). If it is an SSB transmitter that is attached to the BC Receiver, the arrangement discipline is slightly different. If the 9 MHz Crystal used for the SSB is to be simultaneously used for Receiver also, first the 7 MHz signal should be mixed with 1.9 - 2 MHz VFO so that at the output a 9 MHz is possible for the Crystal Filter. The 9 MHz signal having a bandwidth of approximately 2-3 MHz is to be fed either into a 9 MHz IF stage or converted again to generate an IF of 455 KHz. Since Crystal Filter suppresses all images and harmonics a 9 MHz IF is enough or rather better. In the model circuit of the IF stage shown below in fig. C-12/3, additional facilities BFO injection, AGC control and 'S' Meter circuit are attached.
In this circuit almost all noises that bipolar transistors generate are suppressed here. The LA4510 IC is enough to give a pretty good audio output. This circuit works in 3 ordinary cells. IC-1 is to be fixed on 16 PIN IC socket. For BFO signal injection first fix a 1" long SWG 24 wire vertically, close to Pin no. 8. The BFO signal may be coupled using 2 coil winding on 24 SWG wire piece with 40 SWG wire. The signal can also be fed into Pin No. 8 using a capacitor. Transformers T1, T2 and T3 are made reassembling Can type Philips Short Wave Antenna coil. It is because ordinary IFTs cannot handle 9 MHz amicably. Perhaps assembling all the RF coils are the only thing that should be done with utmost care. First wind 7-0-7 primary using 40 SWG wire. The secondary also is wound using the same gauge of wire. Unless carefully done there could be primary - secondary shorting and coil to can leakages. Also make sure that the winding is not open.
The coil should be fixed on the board only after tuning it to 9 mHz. We need a9 MHz Crystal Oscillator and a RF voltmeter to check the coil. The side band crystal oscillator used here is enough for the Crystal Oscillator. First ground one lead each from primary and secondary (not centre tap of the primary). Further connect a 22 pF capacitor in parallel to the primary and feed the 9 MHz signal at the active lead of the Primary. Further check the RF Voltage at the active terminal of the secondary. Tune the IF coil for peaking. If peaking is not available at the tuning range of the coil core change the primary capacitor value in steps of ten PF. Once you could notice clear peaking, fix the core at that point. There shall only be one peak and that need to be sharp as well. Fix the same capacitor on the board also. A drop of wax dropped on the core is enough to keep the core unmoved.
If you are using an ordinary VU meter instead of a custom 'S ' Meter, the the needle depletion need not be exactly in proportion to the db wise change in the signal strength. To avoid quick deletion of the meter needle use a 47μF capacitor in between the base and ground of BC 548. The presents in S meter circuit decides the minimum and maximum depletion levels of the 'S' Meter. Always avoid wires to the 'S' Meter coming closer to components in the IF Board. Before connecting the 'S' Meter set it for low depletion.
Both the ICs in this circuit are easily available in the market. Special circuits for mixing, detection and AGC are already incorporated in the ICs. Unless we use an external AGC regulator, use a fixed resistor there.
An excellent Communication Receiver is the dream of any home brewer. The same circuit is enough for a multi band communication receiver with 455 KHz IF. Only T1 T2 and T3 and its' tuning components that are to be changed. Ceramic filters of 455 KHz also is usable in this circuit. We will continue our discussion on the disciplines to be observed while the same IF board is used for different different frequencies.
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