CB-to-SW down converter

This converter enables long distance (DX) reception of AM or SSB stations in the 27-MHz citizens’ band (CB) on a short-wave or medium-Wave radio (note, though, that AM or SSB modulation in the 27-MHz CB band is no longer allowed in a number of countries).

CB-to-SW down converter Schematic diagram

The converter consists of a prestage, T1, and a mixer/oscillator, IC 1. The antenna signal is coupled inductively to the gate of dual-gate MOSFET via tuned circuit L1-C1, which acts as a 27 MHz input filter. The operating point of the MOSFET is determined by resistors R1-R2 connected to the gate-2 terminal. The amplified signal is fed to the mixer amplifier via a coupling capacitor and a second tuned circuit, L3- C2. The oscillator on board the SO42P IC (manufacturer: Siemens) works with a quartz crystal, XL The quartz frequency is selected such that the difference frequency produced by the mixer (also on board the SO42P) falls within the tuning range of the radio Connected to the output of the converter. For instance, if an inexpensive 26.800 MHz crystal is used, the frequency of the received 27-MHz station, ƒin, is
ƒin = 26,800
where ƒdial is the frequency reading on the radio’s tuning scale (in this case, the received stations will appear in the medium-wave band). Other crystal frequencies may be used, e.g., 10 MHz, to move the CB band into the SW range (1.7 MHz).Construction of the converter is fairly easy on the printed circuit board shown here. Parts shown with a dashed outline are fitted at the solder side of the board. The converter is shielded all around to prevent spurious radiation. The antenna and the IF output is best connected via coax sockets (S0239 or BNC). The two inductors in the converter are simply adjusted for best reception. The converter is powered either by a regulated 9-V adaptor or by the radio it is connected to if this is capable of furnishing 9 V at a few tens of mA.
Parts list

CB-to-SW down converter Schematic diagram
Resistors:
Ri, R2 = 100 k Ω
R3 = 39  Ω
R4:R5:R6 = 330  Ω
Capacitors:
C 1; C2 = 56 pF ceramic
C3 = 2.2 nF ceramic
C4 = 10 pF ceramic
C5 = 1 nF ceramic
C6 = 47 uF 16V radial
C7 = 100 nF ceramic
C8:C9 = 10 pF ceramic
C10 = 27 pF ceramic
C11 = 220 pF ceramic
C12 = 3.3 nF ceramic
Semiconductors:
T1 = BF982
Integrated circuits:
IC1 = SO42P
Inductors:
Li:L3 = 113CN2K50989189ADZ (Toko)
L2= 1mH choke
Miscellaneous:
K1; K2 = BNC socket
X1 = see text

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