uses hyperabrubt diodes which generally produce the
highest tuning ratio and best linearity, making them
well suited for the tank circuit’s capacitance in wideband
Starting with the MC1648, the frequency of oscillation
is determined by the inductor selected (band switch) and
the capacitance of the varactors junction as set by the
tuning voltage controls. The varactor tuning network of
VD1 and VD2 gives the optimum tuning range and
capacitance for a wide assortment of complimentary
inductors to cover a 0-180 MHz spectrum. Tuning voltage
is supplied by the sweep circuit through R12. The
resonating inductance is selected via S1A — a continuous
rotating 12-position wafer switch.
This switch requires a small modification which is the
installation of an RF ground plate to its rear end (see under
Construction). This is necessary due to the fact that pins 10
and 12 (A and B) have a +1.6 VDC bias on them. For this
reason, the low end of the tank circuit cannot return
directly to ground, but still must be maintained at RF
ground potential via C15, C16, and C17. This is a
“digitized” combination of capacitors (rather than a
traditional 0.1 mf cap) that provides better RF grounding
across a wide range of frequencies.
The RF ground plate also facilitates coil installation on
the band switch. The RF ground plate must be returned to
the MC1648 P10 by a lead connecting it to point B as
shown in the lower center area of Figure 1. A ferrite bead
(FB1) is added to this line to insure no RF is present on it.
Q1 provides a high impedance take-off point from the
tank circuit, and R2 (known as a stopper resistor) keeps Q1
stable and prevents the possibility of spurious oscillation.
Q2 and Q3 provide a generous amount of buffering and
(microwave monolithic integrated circuit) amplifier input.
MMICs are wonderful little devices used as gain blocks
with built-in 50 ohm input/output impedances. Their upside
is high gain, high output, and wide frequency response (DC
to > 4 GHz). The downside (at least for this application) is
they require a relatively high voltage supply and consume a
fair amount of power. I weighed these issues against a two-stage transistor amplifier fed from an eight volt supply, but
for pure simplicity elected to go with this device.
The extra added 24 volt supply (transformer, filter, and
regulator) takes up very little room and is quite cheap.
At this point and with all the circuit’s signal levels
adjusted properly, the RF output jack will display a clean
sine wave of + 10 dBm ( 2,000 mV P-P) with exceptional
flatness of ± 0.1 dB, all the way up to 80 MHz (bands 1-8).
Bands 9 and 10 (the highest bands) wander slightly from
that figure but are still at a respectable ± 0.7 dB. This is due
to the fact that we are pushing the limits here by outputting
a 100:1 frequency range while mandatorily using the same
values of the tuning cap in its tank circuit.
For any given frequency of oscillation, there is an
optimum L/C ratio. We are at the point of exceeding those
limits as the Q of the tank circuit drops to a low value at
the upper frequencies (i.e., we would desire less
capacitance and more inductance on the higher ranges to
maintain optimum Q, but the minimum capacitance is
limited by VD1, 2).
This circuit uses a range of 80 uH to 20 nH. A ratio of
4,000:1 and the tank Q varies considerably throughout the
total frequency range, which affects the tank circuit’s RF
amplitude — also putting a strain on the AGC to keep up
with it. Between the MMIC amp and RF output jack, an
attenuator is desired to gain some amount of output signal
level control. However, commercial step attenuators are
extremely expensive ($150-$1,200) and continuously
30 December 2013
■ FIGURE 2.Attenuator and demod probe.