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Circuit appears to be working but very easily breaks into oscillation - sometimes just getting up from my desk triggers it, or while tuning, and sometimes after a few seconds with no disturbance that i'm aware of.

circuit is built on a tin plate from a flattened 9V battery case... because i'm keeping leads as short as i can inductors are probably too near each other?

I'm not confident yet with s-paramenters so i left it out of my calculations...

enter image description here enter image description here enter image description here enter image description here

EDIT: as requested, image of circuit and scope traces enter image description here when transmitter is off or when not tuned in enter image description here tuned in, 400 hz mod clearly visible :) but it's hard to get this, typically it is self oscillating.. enter image description here and suddenly.... the frequency of the oscillation is totally dependent on the tuned circuit, it is fully tunable accross the full range of the trimmer 5 - 50 pf. enter image description here

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  • $\begingroup$ Please report your observations that show: 1) "it seems to be working"; 2) it "...breaks into oscillation". These may help the community to diagnose the circuit. $\endgroup$ – Brian K1LI Sep 17 '18 at 11:28
  • $\begingroup$ A picture of the circuit would be useful also. $\endgroup$ – Phil Frost - W8II Sep 17 '18 at 13:12
  • $\begingroup$ Im away from home for a few days but ill get back to you with images . In the meantime 1) it appears to be working. I can tune into a 87.5M modulated signal. While tuning i have 0mv on scope, then suddenly it peaks to 100mv pp with the modulation clearly visible. But 2) it breaks into oscillation, this can happen at any time, and I suddenly get 7v pp sine wave. Frequency adjustable by the cap. Nice oscillator except thats not what i wanted to build :/. Once oscillating needs power cycle. Scope on collector of pnp. $\endgroup$ – user13171 Sep 17 '18 at 21:42
  • $\begingroup$ If the question has turned to rhe physical implementation... Does this imply my design values on first impression appear to be correct?? For example im claiming 57000 gain from an LC and two transistors... Because i know nothing yet i dont know what to make of this value... Does it sound too ambitious? Typical? Or does an experienced ham expect more gain thn that? I.e. does it ring any alarm bells? Same goes for all calculations and assumptions i made.. $\endgroup$ – user13171 Sep 17 '18 at 21:52
  • $\begingroup$ Very helpful observations. When you return to the bench, please tell us the frequency of the "7v pp sine wave." $\endgroup$ – Brian K1LI Sep 20 '18 at 0:21
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When designing circuits for the VHF region, careful design and construction practices must be utilized in order to obtain stable and repeatable gain.

Here are a few thoughts that may improve the reliability of your circuit:

1.) Increase the drain resistance. This will reduce the tendency for UHF oscillations (and reduce the gain).

2.) Use toroids for your inductors if possible. Toroid forms help to provide self shielding of the inductor. If you cannot do this, orient the axis of the inductor such that it points towards the amplification stage in order to minimize stray coupling.

3.) Keep all wire lengths as short as possible but in particular, minimize the gate lead length and its associated connections.

4.) You may get some improvement by installing a vertical shield over the FET to isolate the input from the output. Notch the shield to allow the shield to fit tightly over the FET.

5.) Consider using a common gate topology with sufficient drain resistance to combat UHF oscillations. This will have lower gain which can be made up with a second stage of RF amplification if needed.

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I see two coils in close proximity in the "as built" photo. One coil is connected to the detector; the other to the antenna -- and their axes are colinear. Whether this was the intent or not, your receiver is acting as a regenerative type -- detected signal in one coil is coupling to the other. Without a control to limit current in the coil that's acting as a "tickler" or an adjustment to limit coupling (one classic method was to rotate the tickler -- one way it was maximum, rotated 180 degrees it was out of phase and minimum), the circuit can enter oscillation and there'll be no easy way to stop it.

Tiny disturbances, such as signal strength fluctuations, body capacitance, or stray inductance (from a wedding ring, for instance) can change the conditions enough to trigger this. Once it starts, it's generally self sustaining, in that the regenerated signal will be enough stronger than the un-augmented to keep oscillating.

Adding a variable resistance in series between the detector and its coil will allow you to reduce the coupling and control oscillation.

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There is much about the topology of your first stage amplifier that looks like an oscillator. It's reasonable to try a design that is more likely to be stable: the common-gate amplifier. This topology sports low input impedance and high output impedance, which seem to be a good fit to the rest of your design:

Here is an LTSpice model of a possible circuit using the MPF102:

enter image description here

Note that the J310 or similar is often preferred for VHF applications because the reverse-transfer capacitance is about 10X smaller.

The 3-dB passband of the input filter is about 68MHz to 88MHz:

enter image description here

and the voltage gain is about 20dB:

enter image description here

Here's a plot of the frequency response:

enter image description here

Be sure to follow all of the recommendations in the earlier post, particularly shielding the input from the output as well as possible. Also, don't assume that your toroidal inductors are correct: the magnetic materials have large permeability variations, so measure them to ensure you get the expected results.

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  • $\begingroup$ ok this makes sense.. i'll start a new CG circuit. but CG is low input impedance, the antnna is low impedance, the LC is high impedance... should i weakly couple the antenna to the tank and a weak couple again to the S? or have antenna on the S, and tank on the D ...? $\endgroup$ – user13171 Sep 28 '18 at 8:22
  • $\begingroup$ You can connect the coax from a center-fed half-wave dipole directly to C5, in place of V1. The total length of such a dipole is 6-feet. Antennas for the FM radio band should also work reasonably well. $\endgroup$ – Brian K1LI Sep 28 '18 at 13:58
  • $\begingroup$ @Ryan: What do you mean by, "... the LC is high impedance"? $\endgroup$ – Brian K1LI Sep 28 '18 at 13:59
  • $\begingroup$ I thought that at the frequency of interest, the LC at resonance should in theory be an open circuit. but because of losses it's more like a parallel RLC... R is quite high typically upper double digit k ohms. then, if you put a low impedance like say a common base... this lowers the Q of the tank to the point theres no selectivity and very low voltage gain $\endgroup$ – user13171 Sep 29 '18 at 0:06
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    $\begingroup$ Back-burner addition: it's good practice to bypass the junctions of R3,L3 and R4,L4 with parallel .001uF and .01uF caps at each. This ensures the assumption that these junctions are DC, without AC modulations. $\endgroup$ – Brian K1LI Oct 1 '18 at 17:43

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