"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 ?

In article ,
(Thanasis) writes:

"Michael A. Terrell" wrote in message
...

They have to be turned on hard, or they will rectify the RF
and cause problems ...

I was feeding the diode with 1mA. I couldn't imagine that should feed it
with ~20mA in order to be stable my oscillator.

Mike Terrell had a very good suggestion on switching diodes'
current. Lots of folks have found out that higher current levels
are absolutely needed. [me for one...:-) ]

I might suggest an alternative: The CMOS switch ICs such as Dan
Tayloe uses in his excellent Mixer for DC receivers. That CMOS
structure is designed for higher signal levels with relative isolation
of internal driving circuit to the switch portion.

I haven't tried that yet but am planning on that for next year.

Len Anderson
retired (from regular hours) electronic engineer person

In article ,
(Thanasis) writes:

"Michael A. Terrell" wrote in message
...

They have to be turned on hard, or they will rectify the RF
and cause problems ...

I was feeding the diode with 1mA. I couldn't imagine that should feed it
with ~20mA in order to be stable my oscillator.

Mike Terrell had a very good suggestion on switching diodes'
current. Lots of folks have found out that higher current levels
are absolutely needed. [me for one...:-) ]

I might suggest an alternative: The CMOS switch ICs such as Dan
Tayloe uses in his excellent Mixer for DC receivers. That CMOS
structure is designed for higher signal levels with relative isolation
of internal driving circuit to the switch portion.

I haven't tried that yet but am planning on that for next year.

Len Anderson
retired (from regular hours) electronic engineer person

On Sat, 08 Nov 2003 06:08:28 GMT, wrote:


Thanks. I'll have to experiment with resistors vs inductors.
I guess I always assumed you *had* to use inductors to keep
the RF off the supply line.

There is only one problem that I can think about when using only
resistors to isolate the diode control voltage is the thermal noise
voltage generated by any resistor. Especially in VCO control voltage
lines, the thermal noise voltage generated by the resistor will add up
to the control voltage, changing the capacitance and hence generate
phase noise in the oscillator.

However, a reverse biased switching diode at 12 V should have a quite
low absolute capacitance and thus, any switching voltage noise would
have a very minimal effect on the capacitance.

Paul OH3LWR

On Sat, 08 Nov 2003 06:08:28 GMT, wrote:


Thanks. I'll have to experiment with resistors vs inductors.
I guess I always assumed you *had* to use inductors to keep
the RF off the supply line.

There is only one problem that I can think about when using only
resistors to isolate the diode control voltage is the thermal noise
voltage generated by any resistor. Especially in VCO control voltage
lines, the thermal noise voltage generated by the resistor will add up
to the control voltage, changing the capacitance and hence generate
phase noise in the oscillator.

However, a reverse biased switching diode at 12 V should have a quite
low absolute capacitance and thus, any switching voltage noise would
have a very minimal effect on the capacitance.

Paul OH3LWR

Paul Keinanen wrote:

On Sat, 08 Nov 2003 06:08:28 GMT, wrote:


Thanks. I'll have to experiment with resistors vs inductors.
I guess I always assumed you *had* to use inductors to keep
the RF off the supply line.

There is only one problem that I can think about when using only
resistors to isolate the diode control voltage is the thermal noise
voltage generated by any resistor. Especially in VCO control voltage
lines, the thermal noise voltage generated by the resistor will add up
to the control voltage, changing the capacitance and hence generate
phase noise in the oscillator.

However, a reverse biased switching diode at 12 V should have a quite
low absolute capacitance and thus, any switching voltage noise would
have a very minimal effect on the capacitance.

Paul OH3LWR


You also have a capacitor to ground at both ends of the resistor.
--


Michael A. Terrell
Central Florida

Paul Keinanen wrote:

On Sat, 08 Nov 2003 06:08:28 GMT, wrote:


Thanks. I'll have to experiment with resistors vs inductors.
I guess I always assumed you *had* to use inductors to keep
the RF off the supply line.

There is only one problem that I can think about when using only
resistors to isolate the diode control voltage is the thermal noise
voltage generated by any resistor. Especially in VCO control voltage
lines, the thermal noise voltage generated by the resistor will add up
to the control voltage, changing the capacitance and hence generate
phase noise in the oscillator.

However, a reverse biased switching diode at 12 V should have a quite
low absolute capacitance and thus, any switching voltage noise would
have a very minimal effect on the capacitance.

Paul OH3LWR


You also have a capacitor to ground at both ends of the resistor.
--


Michael A. Terrell
Central Florida

In article , Paul Keinanen
writes:

On Sat, 08 Nov 2003 06:08:28 GMT, wrote:


Thanks. I'll have to experiment with resistors vs inductors.
I guess I always assumed you *had* to use inductors to keep
the RF off the supply line.

There is only one problem that I can think about when using only
resistors to isolate the diode control voltage is the thermal noise
voltage generated by any resistor. Especially in VCO control voltage
lines, the thermal noise voltage generated by the resistor will add up
to the control voltage, changing the capacitance and hence generate
phase noise in the oscillator.

That should not normally be a problem. The basic formula for
RMS noise voltage is SQRT(4 k T Bw R) with k = 1.38 x 10^-23,
T is temperature in Kelvin, Bw is bandwidth of noise, R is Ohms
of the resistor.

Assuming a VCO control loop bandwidth of 5 KHz, temperature on
the warm side at 320 K, and a resistor value of 10 KOhms, the
RMS noise voltage would be about 0.94 microVolts.

If the VCO control voltage range is 4 VDC and the tuning range is
approximately linear over 4 MHz, that 0.94 uV will produce a phase
noise of 0.94 Hz RMS. Not a great deal...:-)

It's more than likely that stray voltage garbage in the circuit from
other sources (such as inadequately bypassed supply rails) would
be a potential problem. [pun intended]*

Len Anderson
retired (from regular hours) electronic engineer person













* when puns are outlawed, only outlaws will have puns...