Voltage Controlled Oscillators (VCO) for 23cm

One of my long term projects is to build a 23cm  FM transceiver. This post summarises a few experiments I conducted with a view to making a wideband VCO for testing purposes. The basic variations I tried were all varactor tuned:

1. Lumped LC (or perhaps more like parasitic LC at these frequencies),
2. conventional semi-rigid coax
3. DRO, or dielectric resonator
4. stripline

Version 1
Narrow tuning, layout needed more attention to get a higher frequency

Version 2

Despite my best efforts using a short piece of coax as a tuned circuit I had no real success with this approach. I was able to achieve oscillation but I didn't find this approach for me, a mere mortal, as being something I could reproduce when ever I wanted an oscillator at these frequencies.

Version 3
My DRO attempts were similarly mixed. I could achieve oscillation, but my junk box did run to a huge number of DRO's. I even tried modifying the DRO's found in old mobile phone filters.

I could get these oscillators to work. But just like crystals you never seem to have one on the frequency you want. I ended up achieving a maximum frequency of 1150MHz with this 1600MHz DRO. It seems from my testing of the DRO's I had that in general the maximum frequency when used in an oscillator is around 70% of the resonant frequency of the DRO alone.

What was interesting was how critical the board layout was. Putting a piece of copper foil under hte DRO, thereby extending the groundplane, lifted the frequency of this oscillator from 904MHz to 994MHz. That's about 10%. Placing a via next to this extension lifted the frequency still further to 1105MHz. I never managed any further significant increase in frequency. A raft of fiddling only achieved 1150MHz.

Version 4

Days had passed fiddling with these approaches when I tried a strip line version of something I had seen in "VHF Communications" once and at http://lea.hamradio.si/~s53mv/spectana/vco.html. Almost immediately I enjoyed success. I now had an oscillator I could  build over and over and I found that each time I built one it worked, albeit with some modest alteration to the stripline length if my guess was off the mark.

After my first attempt I decided to dispense with direct coupling the mmic to the transistor in the oscillator. Instead, I used a fourth stripline adjacent to the resonator striplines feeding a mmic to lift the output level. The circuit, shown below, has proved to be repeatable. The stripline length may need to be varied depending on your substrate and varactor properties. The stripline to change is the centre stripline which runs between the collector and base striplines. Shorten the stripline to raise the frequency, or lengthen the stripline to lower the frequency. Easy with a knife or a small piece of copper foil cut into a strip and soldered onto the existing stripline to lower the frequency. If you didn't have any copper foil, you could also use the shield from some thin coax or a piece of thick copper wire.

Apart from construction errors and defective parts this should always work. If you can't get it to oscillate try a transistor with a higher ft.

While I used some 6 pin mmics I recovered from a PCB, you could use just about any mmic that comes to hand if you modified the board layout.

Top layer of Stripline VCO, bottom layer is groundplane. Here the mmic is loosely coupled directly to the oscillator

I built one of these with a sweep generator on the pcb which I will post details of later. The pcb, which when compared to the direct coupled mmic version above, should help explain the principles of the layout.

Regards
Richard VK6TT