[SOLVED] Tuning an oscillator (tunable by voltage)

[LandLack]

I'm ashamed to ask these things, but I'm very inexperienced. I want to use JTOS-200 as a reference (**broken link removed**). There are 4 ports: RF out, Vcc, Vtune, ground. The output and the ground are simple, but I would like to know something about the two other ports:

1) on the "Electrical Specification" it's written that the DC operating power has: Vcc= 12V and Max_current=20mA. So I've to connect Vcc port with 12V, right? But what about the current limit, do I need to put a resistor with more than 600ohms in series with the 12V source, or something else?

2) How can I tune the voltage if I don't know about the Vtune port input resistance? I thought that I could use a simple voltage divider, but I don't know how to connect the circuit, since without the input resistance I can't determine the Vout.

Thank you for your response,

Regards,
LandLack

 

[FvM]

Use stabilized 12V, no series resistor. 20 mA is a current consumption specification, not demanding for current limit

Vtune is driving a varactor diode, the input resistance is probably high, may be infinity. But without a manufacturer specification, that's still a guess.

 

[audiomik]

There is plenty of info in the second page of the linked pdf relating to the control voltage and output frequency.

Control Voltage range is between 1 and 16 Volts DC, with a superimposed modulation voltage required for FM operation.

Suspect with the one octave control range at 100 to 200 MHz that this is to facilitate a form of control loop derived from a reference, such as used for frequency synthesis.

Would expect you will achieve best results with the Frequency control by sourcing your Voltage from a low impedance source relative to the tuning frequency?

hope this assists
Mik

 

[LandLack]

Hi, I would like to know something else about the same device:
I've connected the tuning voltage pad with a trimmer potentiometer with a maximum value of 100Kohm. But the datasheet (that you can see on the first message) says: " all specifi cations: 50 ohm system" (under "Maximum ratings, in the first page"). Actually, I've purchased an oscillator and tested that pad's resistivity with an ohmmeter, and it showed 7.8Mohm; so I don't know what to believe, if it's 50 ohm, or 7.8Mohm.

 

[keith1200rs]

The tuning input will probably be connected to a reverse biased varactor diode - it won't be 50 ohms. A 100k pot should be fine.

Keith

 

[LandLack]

Thank you very much

 

[spudboy488]

The 50 ohms system (usually) means the RF output load impedance. So for all the charts and limits in the data sheet, the RF output of the device was being sent to a 50 ohm resistive load or RF measurement device.