"Paul Burridge" wrote in message
...
On Tue, 24 Feb 2004 12:30:12 -0700, Jim Thompson
wrote:

Aha! I just noted from Paul's post that "my" oscillator is called a
two-transistor Butler. Wonder when that was conceived? I've been
using the my direct-coupled version for 40 years.

Interesting. This is the type I've decided to build as the basis for
my next sweep generator attempt, since it's claimed the config is more
amenable to being pulled than most others (although I won't be
attempting to pull it anything like as much as my first abortive
attempt using a Cmos-based inverter). Do you find it's a reliable and
stable design, Jim? I assume if it wasn't you wouldn't still be using
it!

Why not use a VCO built from an LC oscillator? Its really easy to build one
with a single transistor that sweeps your band.

I built a simple one that looked like this:


+------+--------+-------+
| | | | /
| .-. C|L1 --- C1
--- | |R1 C| ---
C3 --- | | C| / |
| '-' | |
| | +-------+------o antenna
| | | |
___ | | |c --- C2
Vin o-|___|-+------+-----b| Q1 ---
R4 | | |e |
| | | |
| | | |
| | +-------+
| | |
| .-. .-.
--- | |R2 | | R3
C4 --- | | | |
| '-' '-'
| | |
+------+--------+

Q1 = 2N3904

L1 = 0.5uH
C1 = 20pF variable from 10pF to 80pF
C2 = 4.7pF
C3 = .22uF
C4 = 1uF
R1 = 47k
R2 = 10k
R3 = 1k
R4 = 100

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

I was able to sweep the band from 40MHz to 42MHz by first adjusting C1, then
changing Vin (with a pot) to vary the output frequency. I built it on a
solderless breadboard (!), and had no problems getting it to work. With your
newfound PCB skills, should be a snap to get something like this working. I
tested it by counting waveforms on my oscilloscope, so your mileage may
vary.

The resonant frequency is near

f0 = 1/2.pi.sqrt(L1Ct)

where Ct = C1 + C2||Cbe + Ca + Ccb

Cbe is the capacitance between base and emitter of Q1, which is varied with
base voltage; Ccb is the 'fixed' capacitance between emitter and base of Q1;
and, Ca is the capacitance of the wire antenna. I'm unsure about how to
calculate the power output of the thing, but I'm guessing its tiny. I was
able to sweep the FM band (using a 0.1uH inductor) pretty reliably, and it
knocked out radios in other parts of my house when I swept past them.

The Q of the inductor should be as good as you can make. You can buy highQ
inductors. At this value, an aircore is probably pretty good. Otherwise,
your resonator won't stay on frequency. It'll drift with temperature, and
probably the phase of the moon, as well.

Regards,
Bob Monsen