Oscillators

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From: Richard Urmonas (richard@dnd.icp.nec.com.au)
Date: Wed May 25 1994 - 01:59:14 EDT


> Clark Fishman wrote:

>For those who are homebrewing some radios you might want to try
>this: I you are building a VFO use a hunk of coax cable...
>I use RG144 because it is small and easy to roll up ...
>as the inductor of the oscillator...I built a test oscillator
>from an article in Aug 1980 QST ..it is a Clapp operating
>at about 45 MHz....it is very stable..I rolled the coax
>on an empty plastic wire spool(pretty clever uh) and held
>it in place with plastic tape and a tie wrap. Measure
>the output frequency with my counter almost no shift was measured
>when I put a large pair of pliers on the spool...the coax is a
>quarter wave transmission line and is self shielding.....
>I checked the output spectrum on my spectrum analyser
>and it is as good or better then the analyser. I will
>bring it in to work and put it on a $70000 HP analyser.
>
(stuff deleted)
>Anybody out there play with these guys ?????

Actually the oscillator you describe is using the coax as a parallel
resonsnt circuit. The shorted end is transformed to on open circuit
by the quarter wave line. For frequencies slightly off that which
is quarter wavelength the apparent impedence will be either
capacitive or inductive (depending if the frequency is above or
below "resonance"). So the coax behaves as a parallel resonant circuit.
(This is most easily seen on a Smith Chart). The resonant point
can be shifted by paralleling capacitance (consult your Smith Chart!)
which results in the misconception that the coax is an inductor.

Transmission line oscillators are commonly used in commercial equipment
(e.g. cellular phones). In such applications typically the transmission line
is a "ceramic resonator". This is a coax line made from a high dielectric
ceramic, so the propogation constant is extreme and the line becomes very
short. Another advantage is that the ceramic is mechanically very rigid
hence microphonic effects in the oscillator are minimal.

As well as the shorted quarter wave there are two other configurations
I have come across. One involves the quarter wave line having a capacitor
at the end. This is transformed to an inductance by the quarter wave line.
Hence the inductance is simmulated by a more mechanically stable
capacitor.

The third type uses a half wave section in the feedback loop. This is
used with an inverting amplifier satisfying the requirements for oscillation.
This is obviously less popular due to the requirement for a half wave line.

73 Richard Urmonas VK3DRU


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