Hartley VCO for radio frequencies - additional products in spectrum

Eres_89 said:

BigBoss,
My wrong - of course you have right. I just quickly compare in mind HEMT to FET.

frankrose,
I used simple "needle" probe which you can use to measuring ICs (pads are quite small) with shielded coax. I connected capacitors as closer as possible to the pad. I also try with 1k resistor as you wrote. Strange is fact, that distortion are visible only when I touch Vref pad... example: I touch it only bu using simple small screwdriver, thin wire, needle, tweezers (all without connection to power supply! ) and modulation just occurs...

Click to expand...

The coils are printed on a substrate and a metallic proximity may create such problem( because the frequency is too high). I hope the oscillator is well screened.
I guess there is a screening error and if the coils are close to this Vref pad, some EM coupling may occur and this force the oscillator to change the frequency.Thus, the oscillator starts to swing..
I lived similar problem years ago, an electrolytic capacitor was pretty close to oscillator's coil and when I click the PCB the oscillator was swinging to left and right.A mechanical coupling between coil and
cap. was creating small shift on the frequency.
Can you post the layout and the enclosure if it doesn't harm you ??

frankrose,

ground loop can be good explanation, but last time I use battery as a main power supply and connect all grounds into one point. I didn't noticed any improvement - I can say that the amplitude of those products are even bigger. I also observed new behaviour - when needle touching the VREF pad, and it's not powered - VCO spectrum is chaotic - products appears randomly and it's looking like oscillation conditions have been unsettled... When I turn on supply (lab power supply or just mentioned battery) - VCO is generate carrier signal on 21 GHz and those additional product from beginning.

BigBoss,
I don't have layout on my actual computer. I think it is not a matter of coupling, because the appropriate distances have been kept between the elements.

My guess is that when the pin is just touched by the needle and supply is OFF, so there is no vref voltage the diode is not biased well, maybe wrongly biased by noise, floating or negative supply output, and this screws the oscillation. I don't know.
You mentioned you tried LC filtering, but actually inductors can't filter out high frequencies because of the coil capacitance between the turns. Did you try to add simple RC filter on vref? With different type of capacitors (foil and ceramic) connected in parallel and higher series resistance values than 1.5k. I would try with 10x and 100x bigger resistance values. It could prevent accidental ground resistance's effect maybe.
And another thing which got into my mind, does your circuit is earth or common grounded? This screwdriver thing makes me suspicious, that when you touch the vref it will be connected through you to earth. I would try to put a capacitor between the common ground point of your circuit and the analyser earth ground point.