Electronics > RF, Microwave, Ham Radio

Cheap stable VFO design

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MrSlack:
So I started with grand plans, to build a nice stable but cheap varactor controlled VFO about a month ago that tuned ~7.000-7.040 MHz. Cheap instantly excludes large air variable capacitors, rare FETs, 10 turn pots and other such things.  This went through several iterations and periods of instability as well as about 4 different varactor types. Turns out varactors are expensive as well.

Basic idea is a simple discrete regulator providing an 8.5v reference voltage for the tuning voltage. This is then used to generate a tuning voltage with coarse/fine controls. The voltage is summed and then there is a simple emitter follower. This provides a voltage to the front end of the oscillator. This is a simple colpitts oscillator which uses a 1n4002's reverse biased capacitance vs voltage curve to generate a voltage controlled capacitance. This is used to trim the oscillator.

Drift is about 20Hz/min and is designed to be relatively stable based on battery voltage. Not wonderful but for the general cheapness, adequate.



Course should read coarse above. It was about 2AM when I scribbled that out...

nugglix:
Hi!

That really looks simple and straightforward.

Will steal a lot ...  ::)

I just discovered that (LTspice) simulation and reality are not necessarily share the same traits.
To my surprise the used varactor (1SV149) was much better in reality than in the simulation.

My current problem is that the transistor distorts the waveform, so I need a way to limit the gain
when the oscillation has started.
I tried 2 anti-par BAT-85 diodes but that introduced a lot of jitter.
I'll do some more experiments tonight - aka build a new oscillator.  :D

How does the waveform of your circuit look like?
Iow, do you care about that at all?

Cheers
  Guido

MrSlack:
Yeah I started this design in LTspice. Reality wasn't anywhere near the simulation output.

1n4002 is surprisingly linear for a 0.8-8.5v tuning range. I tried a few BB series varactors and discarded them in the end.

The output waveform is not very clean as you'd expect.. Peaks look fine but troughs are flat, therefore harmonics. I'm adding a buffer amp to it and a low pass with a 3dB point of about 7.2MHz matched to a 50 ohm output to start with. The biasing of the oscillator transistor could be improved as well to keep it in the linear region. In fact version two of this may have a JFET as they are somewhat less tetchy about temperature as well.

Not a bad idea with the diodes. Might have a play with that.

I'll post a picture of the breadboard when I get a few mins. The very definition of fugly.

nugglix:
My goal is a LO which allows me to cover AM to 160m ham band.
My first IF is 10.7MHz, so I need an oscillator from 11.2MHz to 14.5MHz.
This turns out to be quite a thing -- at least for me.

I went the JFET route today.
See attached files.

The waveform (fft_osc) is nice, I think.
But the bastard quits oscillation on varactor voltages over 3.6V.  :--
Something the simulation didn't show.
Will add the buffer tomorrow and see if it helps -- but I doubt it.

The clean spectrum amazed me. I'll try this diode limiter again on my
BJT (BF 959) oscillator. Which has a wide tuning range, but also shows
distortion and amplitude issues.

The 2 blue parts at the lower left are 10┬ÁH inductors in parallel.
Ahhh... might be that the Q of the inductors is to low.   |O
Thanks for helping me already :)
Will try a T68-2 with 30 windings tomorrow.

T3sl4co1l:
The diode is shorted by the inductor, and the diode is shorting the inductor during half the cycle...

Tim

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