the compact layout. With only the emitter of the first stage bypassed, the amplifier is stable with about 23 dB gain.
The second mixer is Mini-Circuits SRA-1A and again, a similar doubly balanced mixer could be made from discrete diodes and transformers.
The VCO is copied from a VFO in the 1989 ARRL Handbookand modified for varactor tuning. The tuned circuits were scaled from their original values for 5 MHz to tune to 7.8135 MHz. The varactor is a small one of unknown type that was on hand. Coarsely adjust frequency by adding or removing turns from the inductor. Make fine frequency adjustments by changing the value of the fixed 24 pF capacitor across the inductor. A small 5 pF trimmer sets the final frequency after the VCO shield is soldered in place. The trimmer has about a 150 kHz tuning range. After the coarse frequency was set, the VCO output frequency was measured as a function of tuning voltage to find the mostlinear part of the tuning curve. In my prototype, linearity is best for tuning voltages between 3 and 13 V. Final frequency adjustment was done with the tune voltage set at 8 V. I then set values in the scan generator to provide the optimum tune voltage.
The 455 kHz IF amplifier is shown in Figure 4. It provides enough gain to bring the output of the second converter up to a suitable level for the detector. The three stages are syn chronously tuned to give good selectivity. The IF transformers are from a line of adjustable coils available at many parts houses in Japan, but other 455 kHz IF transformers should work satisfactorily. (It might also be possible to substitute a salvaged 455 kHz IF strip from an old transistor radio.)
Gain control is accomplished by diode switching additional resistors in parallel with the emitter resistors of each stage. A single-pole, multiposition rotary switch and a simple diode matrix increases the gain of each stage sequentially. I found that any noise introduced into the emitters from the switching voltage is amplified and contaminates the output. This problem was cured by filtering the control voltage used to switch the diodes.
Fig 4—Schematic of 455 kHz IF amplifier.
The log detector is based on the NE604 and is shown in Figure 5. The circuit is based on the applications circuits shown for the NE604 in the Signetics data book.7 The NE604 is a high-gain device, so take care to use short leads on bypass capacitors and minimize feedback paths. A 455 kHz ceramic filter between the IF stages of the NE604 helps with selectivity. The inputs to each stage are loaded with 510 U to improve stability, as recommended by the Signetics data book.
The detector output is taken from the RSSI output (pin 5) of the NE604. This is a current source, terminated in a 100 k!,2 resistor to give a voltage output of about 0.5 V per 10 dB. Signetics warns that an RSSI output greater than 250 mV with no signal input indicates possible regeneration or oscillation. After buildingthedetector, ground the wiper of the 500 £2 vertical-offset potentiometer and measure the RSSI voltage at pin 5 of the NE604 with a high-impedance voltmeter while the detector input is disconnected. A voltage of more than 250 mV indicates possible stability problems in the detector.
An LM10 op amp in a voltage-follower configuration buffers the NE604 RSSI signal and provides a low-impedance output to drive the display. I chose the LM10 because it works well at input and output voltages down to its negative supply. This allows grounding the IC's negative supply, which eliminates the need for a negative power-supply voltage. A small, adjustable positive voltage summed into the LM10 input provides a vertical-offset adjustment to align the trace on the display with a convenient graticule.
A 6.2 V Zener diode supplies power to the NE604.
The scan generator, shown in Figure 6, provides sweep voltage for the display and tune voltage for the VCO in the second converter. The sweep generator is based on a 555 timer and a current source consisting of a pair of PNP transistors. The 555 timer is connected in the usual manner for an astable multivibrator,s except that the timing capacitor is charged from the current source instead of a resistor. The 0.47 |iF timing capacitor is charged until its voltage trips the threshold comparator in the 555 at -A Vcc. When that happens, the timer trips and discharges the timing capacitor through the 470 ii resistor until its
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