"Michael A. Terrell" wrote:

Thanasis wrote:

In the book "Introduction to Radio Frequency Design" of W7ZOI and in
the section 7.9 "Voltage Controlled Oscillators" I read about how to
switch in and out a capacitor or an inductor in a VCO, using a diode
switch in order to alter the frequency of the oscillator.

Unfortunately this information is theoritical to me and I couldn't
make my oscillator change its frequency.

Does any one have a practical example of a Switched reactance VCO ?

Thanks
Thanasis Lazos

I worked on the Microdyne synthesizers using this method. It used strip
line inductors, with a heavy ground buss along the edge of the board.
The diodes were either forward or reverse biased to turn them on or off.
you can't just let them float. Use a capacitor at least 100 times the
highest value of the varicap to ground on the switching end of the
diode, and as short as possible connections for the both ends of the
diode. Connect a resistor to the diode and capacitor, and feed it a
positive or negative voltage for forward or reverse bias. They have to
be turned on hard, or they will rectify the RF and cause problems, but
you can't exceed the current ratings, or you will damage the diode. we
used + or - 12 volts for switching, and the diodes were used to switch
the range at about 10% of the operating frequency. Its simple when you
see it in front of you, but it is difficult to explain with text. This
design was used in their 1200, 1400, 700, and 2800 series receivers, so
you might find someone with a manual on one of these to see how they are
built.
--

Michael A. Terrell
Central Florida

On the Microdynes - did you use inductors on the DC lines
to the diodes to keep the RF off the DC ?