Wide Range VCO
Wayne Maxwell KD4YGU
This project was designed to give
'The Sweeper' an HF to UHF sweep range. Based on a Mini-Circuits 'designer kit'
of DIP VCO's, the goal was to keep the project as simple as possible. The kit contains
seven VCO's that cover a range from below 25mhz to over 1025mhz with 50 ohm
ouput impedance. The VCO's operate with +12 volts Vcc and use a tuning voltage
of 0 to +16 volts.
The VCO's are wired in parallel and
+12 volts is applied selectively through the front panel switch, to activate
one VCO at a time. Unpowered VCO's present very little load on the active VCO,
although I have not measured the exact value. A good usable signal was
displayed on all of the circuits I swept.
The VCO's require a tuning voltage
from 0 to +16 volts for full range, and the Sweeper provides only +5 and +/-12
volts. I used a mini DC to DC converter from Power One, which generates an
unregulated +19 volts from the +5 volt supply. The 19 volts is regulated to +17
volts by an LM317LZ adjustable voltage regulator, to provide Vcc to an LM6132
rail to rail opamp. This opamp is configured as a non-inverting amplifier with
a gain of slightly more than three. The 0 to 5 volt VCO OUT signal of the
Sweeper is fed to the input of the LM6132, which produces the .25 to 16 volt
tuning voltage for the VCO's. A rail to rail opamp was used instead of the more
inexpensive LM358 to allow the the tuning voltage to swing to 16 volts with a
+17 volt supply. An LM358 would only swing to about 14.5 volts. I didn't want
to use a higher Vcc on the opamp as the absolute maximum tuning voltage
specified for the VCO's is 18 volts. In the event of an opamp failure, I didn't
want to fry $80 worth of VCO's.
The VCO outputs are capacitively
coupled to a Type N output connector labled 'LOW OUTPUT' and also to the input
of a type MWA320 Wideband Hybrid Amplifier, that provides a frequency counter
ouput at the Type N connector labled 'HIGH OUTPUT'.
Output level control is provided by
using an inline attenuator of the style published in any ARRL Amateur Radio
Handbook. The simplist is one with switch selectable attenuation steps and 50 ohm input and output impedance.
The project cost is about $100,
which is higher than what I had really wanted, but the reality is, designing
discrete wide range high frequency VCO's rather than using the pre-packaged
DIPs probably would have brought the cost to nearly that level anyway, and
sacrificed simplicity. Although I haven't compared prices, I would dare say
that $100 is probably a pretty good price for a sweep generator with this kind
of range. Good luck building and enjoy!
References:
http://www.minicircuits.com/ VCO kit (K-POS2) datasheets
http://www.power-one.com/ DC to DC converter (DSP1N5S17)
datasheet
All
parts except K-POS2 and MWA320 are available from Digi-Key. MWA320 is available
from Newark Electronics and K-POS2 is available directly from Mini-Circuits.