A question about Function Generators

[Gazucha]

Hi chaps,

I was recently given a 1978 HP / Agilent 2213A Function Generator.

 This thing supposedly gives up to 13Mhz, AM, FM and has 2 separate channels. It is a long way from my basic capacitor-based frequency playthings.

 So the question is this...

  "What is the source of the frequencies in a Function Gen' such as this?"



 I can't seem to find any clear details.   
 
 Here's a link to the manual, if needed.

http://circuitslab.case.edu/manuals/Hewlett_Packard_3312A_Function_Generator_Operating_and_Service_Manual.pdf

[alm]

See the theory of operations. A constant current source alternatingly charges and discharge a capacitor. Driving a constant current in a cap produces a ramp, so this produces a triangle wave. Changing frequency range changes the capacitance, the vernier changes the current.

[BiOzZ]

A highly controlled VCO run by a highly regulated voltage and current reference that is compared to most likely a calibrated OCXO
as ALM says it makes a triangle wave thats than shot threw a comparator than thats converted in to the frequency types you need

at high frequencies (late MHZ to late GHZ) its fed threw one or more PLLs to multiply the frequency after the first stage ... look over at https://www.youtube.com/user/TheSignalPathBlog one of there videos discribed it very well but i dont remember what one ... maybe this one

[Gazucha]

See the theory of operations. A constant current source alternatingly charges and discharge a capacitor. Driving a constant current in a cap produces a ramp, so this produces a triangle wave. Changing frequency range changes the capacitance, the vernier changes the current.

 So, when there is no frequency at all exiting Vp-p, yet still a D.C. voltage, variable by the Vernier, the problem should be one or more faulty capacitors or lack of constant current source?
 Then, what component would be this current source? Is this a VCO? Although I didn't see one in the schematic.

Thanks for the input.