Electronics > RF, Microwave, Ham Radio

VCO coil implementation-Advice needed.


I want to try building this VCO using Manhattan type construction. I am wondering about how I can construct the oscillator coils.
L3 and L4 are likely to be made from a few turns of stiff copper wire that is centre tapped. However, there is another coil that needs to be placed next to this coil that couple the RF into the RF buffer. Does this coil need to be separate or can all 3 coils be made from the same winding? I have attached a photo from the service manual showing the VCO section of the RE101(VCO LEVEL ADJ label). The photo shows the coil mounted on some kind of former and the coil appears to have 6 turns. There appears to be some provision for adjustment.
How would I implement this coil arrangement now using available parts? Any help appreciated. Thanks.

To try to duplicate a circuit, normally one would need to know the coil dimensions, and in the case of a slug tuned coil such as this, the characteristics of the tuning slug.  Without this information, you will have a lot of trial and error to reach the frequencies you desire.

I don't know what "VDR" on L3/L4 and the pickup link stands for, but could mean 5 turns and 1 turn. At any rate, the pickup link should be on the same coil form, and should NOT be electrically connected to L3/L4. If it were connected to the end of L4, the RF voltage at that point would be in series with the voltage magnetically coupled into the pickup link, as well as changing the symmetry of the oscillator circuit.

I'm sorry, but I don't understand what, "How would I implement this coil arrange now using available parts?";  what does “implement” mean?

By "available parts" do you mean off-the-shelf inductors from a commercial source? That could well be well be very difficult. But I'm sure these coils can be made, at these frequencies, it's so easy to do. If you go back some time, there was virtually no commercially equipment available for the 2m amateur band and pretty much everything was homebrew.

The photograph is of no help unless you id the coils.

Suggest the starting point is L3, so the first question is what is the capacitance of the varicaps?

I believe the oscillator works at around 60MHz.Because 5 Turns over 8mm Diameter makes approx. 200nH.
Oscillator may work 60MHz.. ( just a guess ).Choke Coil ( 15uH ) gives me a hint.

The BB106 is 4-20 pF, with 4 connected in that style you will have 4-20 pF total across the whole coil.
Let's suppose we arrive at 22 pF with stray capacitance at the maximum capacitance. We want a minimum frequency of 86 MHz, so we'll need approx 156 nH. With 5 turns, a suitable combination is 6 mm length, 8 mm diameter. You'll have to choose a wire around 0.6-0.8 mm diameter and then adjust the length to tune the circuit.
As regards the secondary, in theory the best choice would be a slightly larger turn on the center of the other coil, to symmetrically load the push-pull. But it will work also close to one side.
Definitely in air and with no ferrite core at those frequencies. Better if silver coated copper.


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