CS SO pF
circuit (IC), Although this device has limited dynamic range, it is sufficient for our purposes because it compensates with a better than average noise figure and sufficient conversion gain so that an RF amplifier is not needed in most projects.
Figure 3 shows a simple frequency converter circuit based on the NE-602 IC. It can afso be used as the front-end of a superheterodyne receiver, with the output (F2) being the desired IF frequency, The output impedances of the NE-602 are compatible with most crystal and mechanical filters used for IF selectivity-
The input side of the circuit shown in Figure 3 uses a tuned circuit consisting of the secondary winding of T1, and is resonated by the parallel combination of C2, C3, and C4. The tuned circuit must resonate at the desired RF frequency using an inductor that is not loaded too much when shunted by the 1,500 ohm input resistance of the DBM.
You can broadband the input
Fig. 4. Direct conversion output circuit for NE-602.
circuit by using an RF transformer built on a toroid powdered iron form, rather than the tuned circuit. The ratio L2/L1 is typically 10:1 to 12:1.; that is, there are also 10 to
12 turns on L2 for every turn on L1. Experiments and published data indicate that good starting numbers are 20 to 24 turns on L2, with 2 to 3 turns on L1 for frequencies in the upper shortwave region. As frequency is decreased, the number of turns is increased to about 34 to 40 turns on L2 at the AM broadcast band.
The capacitors and the 100-ohm resistor in the V+ circuit (connected to pin 8 of the NE-602) are used for isolation and decoupling. These components prevent RF in the NE-602 circuit from traveling to other stages in the radio via the DC power line or, alternatively, prevent signals from other stages from modulating the converter stage (or possibly causing oscillations).
The oscillator circuit in Figure 3 consists of the components attached to pins 6 and 7 of the NE-602 IC. In this case, a crystal oscillator is used, although a variable frequency oscillator could also be used. The operating frequency of the local oscillator section is set by the resonant frequency of crystal XTAL1 This frequency should be the RF frequency plus or minus the desired F2 output frequency, or
Was this article helpful?