Electronic Design

LM/MW converter for ham-only receivers

Before the advent of broadband communications receivers with synthesized oscillators, high-grade, hamband receivers used crystal oscillators and a variable IF to cover relatively narrow (500-600 kHz) frequency segments. The schematic shows a circuit that will adapt most of these receivers to cover the long-wave (150-400 kHz), standard (520-1600 kHz), and tropical (2.2-2.4 and 3.2-3.4 MHz) broadcast bands, plus all of the other interesting stations in the 0.1-1.6 and 2.0-3.6 MHz frequency ranges (Fig. 1).

The circuit consists of untuned bandpass filters, a MOSFET mixer, and a crystal oscillator. The only control is the band-selector switch. The actual tuning is done with the receiver itself (acting as a variable-frequency IF amplifier). Only three crystals are needed for six bands because the 80-meter ham band is used as a difference IF for the LW and MW bands, and the 40-meter ham band is used as a sum IF for the short-wave bands.

When one of the LW/MW bands is selected, a two-section low-pass filter with a 2.2-MHz corner is inserted to eliminate IF pickup, image frequencies, and to prevent overload by nearby shortwave transmitters. When one of the short-wave bands is selected, a two-section high-pass filter with a 1.8-MHz corner and a two-section low-pass filter with a 4.3-MHz corner are inserted for similar reasons. The design impedance for these filters is approximately 1200 Ω, so the component values are highly practical, and any mismatch in the driving source will not affect the passband shape. The mismatch at the output of the filters was found to improve the corners.

The frequency ranges mentioned above and in the chart (Fig. 1, again) are for a receiver with 600-kHz segments. If your receiver uses 500-kHz segments, coverage of the standard broadcast band is limited to 1500 kHz. The obvious remedy is to add a 5.5-MHz crystal. The capacitors in series with the crystals are a means of excitation control. In the unit shown, they ranged from 20 to 62 pF; the values used should produce approximately 3-Vrms oscillator output voltage. This voltage is applied to the mixer’s injection gate through a low-pass filter to eliminate oscillator harmonics. An emitter-follower with a voltage divider reduces both the mixer output level and its impedance to values suitable for a hot receiver. If higher output is needed, replace the resistive divider with a transformer output (Fig. 2).

The input impedance of the circuit is 75 Ω, so it must be driven by a lowimpedance signal source. A loop antenna with a 75-Ω output (see “High-Frequency Loop Antenna,” Electronic Design, July 22, 1996, p. 112) is the ideal signal source, since ferrite-rod “loops” can easily be constructed to provide coverage at the frequencies involved.

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