max2706

Hi Harshkumar,

at first glance, I do not see that you are doing anything wrong. Did the component values that you are using come from the evaluation board schematic? It seems that the VCO is lots of  current, more current than the LDO can deliver. Here are some experiments you could try. 

1. Apply a voltage from an external power supplpy to the VCC pin (leave VTUNE open). Set a current limit on the power supply of around 10mA. If this current limit triggers, there is a possibility that your part is damaged.
2. If you do not get excessive current, apply a second voltage to the Tune Pin and set the limit on this current to, say, 1mA.  Turn on your two voltages and see what current is flowing from each source. If you are using two separate power supplies, connect the grounds together.
3. During this testing, connect the output to a spectrum analyzer to see if you are getting any outupt signal. 

Please try these experiments and let me know if you make any progress. 

Best Regards

Eamon Nash

Hi again Hi Harshkumar,

your circuit seems to match closely with the default evaluation board schematic. So it's possible that the device is blown. Do you have a second device or an evaluation board to test?

Eamon

Hi Harshkumar,

regarding your question about using a voltage divider to generate the Vtune voltage, the current into the Vtune pin is very low. So I don't see a problem with your approach. As a test, you could try feeding a low impedance voltage (e.g. from a power supply) directly on to the Vtune pin to see if this makes a difference. I don't expect that it will but worth an experiment. - Eamon

Hi Harshkumar,

let me address the questions in your last two posts.

1. Will the VCO circuit function properly if assembled on a general-purpose board or zero PCB? 

Eamon: The output frequency traces should have a 50 ohm characteristic impedance. If they do not, the part will still work but the power transfer to the next stage (that is being driven by the MAX2607) will not be optimal. But right now, it appears that the circuit is not oscillating at all. In that case, the probelm is probably not the PCB. 

1. Can I still expect an output in the 100 MHz to 300 MHz range, which is the specified range for the VCO?

Eamon: Yes, if the circuit is operating, you should get a frequency in that range. 

1. Also, do I need to use a spectrum analyzer for accurate measurement, or will a DSO with bandwidth greater than 300 MHz suffice? 

Eamon: You can use either but the setup is going to be slightly different. If you want to observe the frequency on a spectrum analyzer, connect OUT- to a series capacitor, followed by a 50 ohm resistor to ground. And Connect the OUT+ output to the spectrum analyzer (whose input impedance is 50 ohms) through an ac-coupling capacitor. You can also connect an oscilloscope in this mode but make sure the set the input resistance of the oscilloscope to 50 ohms. 

1. I'm also concerned about the impedance of the probe — could that be affecting the output? Right now, I’m only seeing a 50 Hz signal, which is far off from the expected range.

Eamon: If you want to observe the output frequency on an oscilloscope, Connect a series resistor followed by a 50 ohm resistor to both outputs, just like the main block diagram on the front page. Then connect a high impedance probe to the top of one of the 50 ohm resistors. The capacitance of the probe tip will have a minor affect on things but it's fine for debugging. In this mode, set the input impedance of the scope to High Impedance. 

1. Even if there’s a mismatch in the inductor or capacitor values, shouldn’t the output still fall somewhere within the VCO’s operating frequency range? Please advise on this.

Eamon: Yes, you should get a frequency that is roughly in the range. 

1. If there’s an alternative to the VCO that can generate the required frequency and transmit at around -10 dBi, please suggest it.

Eamon: What is your target output frequency?

1. I followed the steps you provided and measured the currents successfully:

• VCC is drawing between 2.3 mA to 2.65 mA

• VTUNE is drawing around 0.09 mA to 0.10 mA

8. What exactly should I connect at the OUT+ and OUT- pins to observe the VCO output?

Eamon: See previous answer. 

8. I'm not planning to use the VCO in a differential (bi-directional) configuration, so could you please advise the correct setup for single-ended output measurement?

Eamon: Connect OUT- to a series capacitor, followed by a 50 ohm resistor to ground. 

8. Should I connect OUT+ through a coupling capacitor directly to a spectrum analyzer or high-bandwidth oscilloscope?

Eamon: Yes but make sure that the input resistance of the oscilloscope is set to 50 ohms. 

8. Do I need to leave OUT– floating, grounded, or terminated in some way when not using it?

Eamon: RC to ground as described earlier.