it doesn't oscillate...

[Othello]

I have this signal generator, as part of an aerial analyzer, which I built but it won't oscillate.
Maybe someone here can give me a pointer.
For one the voltage at S1 is not as indicated but about half of that.

 

[MrAl]

Hi,

Where does the emitter of Q9 go? It's shown unconnected on that one lead.

Also, what are the transistor part numbers?

 

[flat5]

I'm not proficient with oscillators but I don't see how Q7 is getting any base bias except perhaps through the base-collector of Q3.

 

[on1aag]

There's a dot missing, the base of Q7 should be
connected to the 2k7 resistor.

on1aag.

 

[audioguru]

The base of Q7 gets bias through a selected coil from the diodes, the same as Q4.

 

[on1aag]

The base of Q7 gets bias through a selected coil from the diodes, the same as Q4.

Yes Audioguru, you're right, I missed that one.

on1aag.

 

[Othello]

To answer some questions.

The emitter of O9 is connected to a frequency counter.
I built the circuit and when I did not get the 70 mA current (as per specs) when I switched it on, and no oscillation, I disconnected the base of Q9, so I am hopefully only looking at the operation of the signal generator.
As I mentioned earlier the voltage at S1 is not as indicated, it is only half of that.

Someone asked about the transistor types.
Q1, Q2 BC547

Q3 to Q8 PN 3563

Uwe

 

[Nigel Goodwin]

The base of Q7 gets bias through a selected coil from the diodes, the same as Q4.

Not much voltage, when it has to feed two BE junctions

But even worse, as the circuit is drawn, the coils directly short the base and collector of the first transistor.

 

[JimB]

There are many strange aspects to this circuit.
The DC connection from collector to base at Q3.
The resulting low votage at Q3 collector, and lack of bias for Q7 and Q6.
The ALC circuit around Q6 D3, the 100nF capacitor would short out any RF intended for rectification by the diode.

What is the origin of this circuit?
I guess that it either came from a commercial product or a magazine article.

JimB

 

[Othello]

The origin of this circuit is an Australian Ham who developed this Aerial Analyzer (easy to find under Aerial Analyzer, VK5JST, Jim Tregellas).
A radio club in Australia sells the circuit board as well as a bag full of parts for anyone who wants to build one and it has successfully been built thousands of times according to them. I try to paste a description here.

My problem in understanding these circuits is that we are not talking about a steady state, but a dynamic situation when you switch this thing on, when the current rushes in and the circuit hopefully settles into a working oscillator.

If it does not I lack the experience to see why.

Uwe

 

[Roff]

There are many strange aspects to this circuit.
The DC connection from collector to base at Q3.
The resulting low votage at Q3 collector, and lack of bias for Q7 and Q6.
The ALC circuit around Q6 D3, the 100nF capacitor would short out any RF intended for rectification by the diode.

What is the origin of this circuit?
I guess that it either came from a commercial product or a magazine article.

JimB

It's a little strange, maybe, but it appears to be a takeoff on the MC12148 MECL VCO, which replaced the MC1648. See the attached datasheets,

 

[Roff]

I simulated the oscillator portion (without the varicaps), using 100uH and 100pF, and 1uH and 10pF. It oscillates. and the tank has a nice sine wave, but unless you want some serious harmonics (which may be the case?), I would not take the output off the emitter of Q7.
You would need a separate emitter follower off the tank for a good sine wave. A direct-coupled PNP would work well with the voltages present in the circuit.
I just used 3904s and a 3906. Lower-capacitance transistors would be better, especially with the added capacitance of the PNP.
I am attaching the .ASC file, in case someone wants to run the sim in LTspice.

EDIT: I changed Q3,4,6, and 7 in Othello's schematic (Q1,2,4, and 5 in mine) to MPSH10, and the PNP follower to MPSH81. Much better at high frequency than with the 2N3904s!

 

[JimB]

OK, so it is a strange looking circuit, and there is a high probability that it will work ok, but it does not.

So, either there are faulty components, or a wiring error.

To simplify faultfinding, try disconnecting the 100nF capacitors from S1 and Q7 emitter.
Re-check the 1.17v.
If it is still "half", check the base-emitter voltage of Q3 Q4 Q6 Q7 and Q5, they should each be about 0.55v, bad check the voltage across each diode D1 D2 and D3, again expect about 0.55v.

JimB

 

[Roff]

If one of the diodes is shorted out or omitted (remaining diode cathode still grounded), the circuit wouldn't oscillate, and S1 will be about half the expected value.

 

[Othello]

Thank you JimB, now it works.

Two things.

D2 was bad. It is the one which gets soldered to the ground plane and I must have fried it in doing so, it just takes more heat.

The other mistake was the 4k7 resistor at the base of Q8. I was not getting the calculated base voltage and must have soldered in a 47k resistor instead. These resistors are 1% and I have the hardest time reading their color over their body color.
So now there is a strong nice HF sine wave.

Thanks also to the others but some of those discussions were over my head, I can't start thinking about redesigning something if I haven't even gotten the first unit going. JimB advice was just the right "down to earth" advice for my level.

When it's all done I can finally tell you the impedance of my antenna...

Uwe

 

[JimB]

Great!
A good result.

JimB

 

[MrAl]

To answer some questions.

The emitter of O9 is connected to a frequency counter.
I built the circuit and when I did not get the 70 mA current (as per specs) when I switched it on, and no oscillation, I disconnected the base of Q9, so I am hopefully only looking at the operation of the signal generator.
As I mentioned earlier the voltage at S1 is not as indicated, it is only half of that.

Someone asked about the transistor types.
Q1, Q2 BC547

Q3 to Q8 PN 3563

Uwe

Hi again,


What are the other transistors (Q9 and above) and the diodes?

 

[Mikebits]

I am a bit puzzled by Q1 and Q2. Are they functioning as a varacter?

 

[Nigel Goodwin]

I am a bit puzzled by Q1 and Q2. Are they functioning as a varacter?

Yes, using the reverse biased CB junctions, and both in series.

 

[Othello]

Hi again,


What are the other transistors (Q9 and above) and the diodes?

I guess you would also need the rest of the circuit diagram with this info.

Let me invite you to click on this link with has extensive infos on this particular device:
**broken link removed**

And good luck

Uwe