An oscillator that works in multisim but not in real life ?

[Colin hoof]

I have recently tried to build an rf oscillator. The circuit shown oscillates using multisim ,but when built does not. I cannot see why this is the case ,and would appreciate some help or suggestions for reasons why .

 

[JimB]

A strange looking circuit.
No obvious (to me) feedback paths, maybe the strange connection of Q1 and Q2 emulate a tunnel diode which exhibits a region of negative resistance.

That negative resistance region appears in simulation but not in real life.

My wild guess for what it is worth.

JimB

 

[crutschow]

What is the desired oscillation frequency.
High frequency oscillators are quite sensitive to layout and stray impedances.

 

[Diver300]

I agree with Jim and Crutshow and I would like to add a few points.

A 2.6 nH inductor might have a Q as high as 100. (https://www.farnell.com/datasheets/1681998.pdf). Has your simulation allowed for the imperfections of the components?

The inductance and capacitance imply a frequency of 200 MHz of higher. The transistor has an fT of 60 MHz. (https://www.onsemi.com/pub/Collateral/BF420-D.PDF) which is the frequency at which its gain drops to 1. The gain of 50 that the transistor has will be available up to about 1 MHz. At 200 MHz it will be useless.

2.6 nH is tiny. 4 mm of 0.2 mm diameter wire has more inductance (). From my experience of oscillators, the capacitance should be much smaller and the inductance much larger for it to oscillate. If you have 330 pF and 2.6 nH, they resonate at 167 MHz, when their impedances are both 2.7 Ohms. With a Q of 100, the best impedance you could hope for is around 100 * 2.7 = 270 Ohms, so with a supply resistor of 2.2 kOhms, the very best you could get is about 1/10th output swing compared to supply.

A larger inductor and a smaller capacitor will give more impedance at resonance.

Are you intending to get an oscillator to run at 200 MHz or so? What is the application? At those sorts of frequencies, layout is possibly the most important thing to look at, and the simulations generally only look at the circuit diagram.

 

[JimB]

As suggested by Diver300, your tuned circuit reactances are very low, I would aim for Xc = Xl = 150 Ohm at the desired frequency.

Also, as an experiment I would try this at a much lower frequency in order to get the feel of how the circuit behaves.
I would try at maybe 10MHz in the first instance, and go up in frequency from there when I had something which oscillates.

A quick calculation suggests that 2.3uH and 100pF would be OK for about 10MHz.

JimB

 

[Colin hoof]

Thank you Jim b,,diver 300 and crutschow,for all your feedback and suggestions. I have built so many types of oscillator and have never, managed to maintain oscillations above 30 mhz.I get the q and choice of active device, have to be considered.I need a grounding in rf and more exposure to circuits that actually work as intended !!

 

[Diver300]

Anywhere above about 10 MHz, layout becomes really important, and it is also not clear what the effect of the layout will be until you make one. Obviously as the frequency gets higher, the more difficult it gets.

 

[toggyuri]

Multisim! CD4060! spice model PLEASE

I can't find anything anywhere. No manufacturer gives ...

 

[eTech]

Multisim! CD4060! spice model PLEASE

I can't find anything anywhere. No manufacturer gives ...

CD4060 won't work at 10Mhz. Well...maybe at 15v VDD.

 

[toggyuri]

15 V is the current value.
It does not run in MULTISIM but datasheet contains this. :

• 14-bit Binary Counter/Divider
• Counting range: 0 to 16383 (In decimal)
• Operating Voltage: 3V to 18V
• Nominal Voltage: 5V, 10V, 15V
• Source current, sink current details are given in datasheet below.
• Maximum Clock Frequency: 30MHz at 15V
• Reset Propagation Delay: 25ns at 5V
• Available in 16-pin PDIP, CDIP,SOIC, TSSOP packages

Note: Complete Technical Details can be found at the CD4060 datasheet given at the end of this page.

 

[gophert]

The circuit shown oscillates using multisim ,but when built does not.

Some simulators will show some continued oscillation at a very low voltage (nV to 300mV) when an inductor and capacitor connected to a power source - with or without your NPN transistors. The idealized components with no internal resistance show continued oscillation.