29 Mhz DAC tuned crystal oscillator

[hafrse]

Hello,

I have a problem in my tek spectrum analyzer where the DAC tuned 29Mhz Local Oscillator is not working, no 29Mhz signal measured at c12.

Picture (you need to log in to see the attached picture):

x-tal Oscillator is in the red rectangle, DAC tune signal is the LOTUNE input to the left side, checked the active components all ok (I think),
when powering up the SA, the SA runs test routines, DAC goes up to 10v, down to -3V and up to 3.72 where it stays after the power up sequence (10 sec.).

The DAC is used to fine tune the oscillator from the processor.

The main question here, does this oscillator circuit in the red rectangle always run or it is enabled/disabled by the voltage from the DAC also? I disconnect the DAC input , still nothing at c12.

Many thanks in advacne for any help!

George

 

[hafrse]

Update:

Injected a fixed 29Mhz signal from asignal generator, analyzer works good but without fine tuning of the 29Mhz.
Also tested the DAC by running cal rutines, it reacts and gives different voltage outputs.

 

[WimRFP]

Left of the varactor there is an inductor. Left of the inductor there is vertical 1 kOhm resistor. If the bias voltage is -5V (I can't read it well from the image), the oscillator should oscillate over the voltage range mentioned. Sure it should oscillate when LOTUNE = 3.72V. It will very likely stop oscillating when the varactor diode is forward biased, but this doesn't happen.

If you checked the DC bias voltages and everything seems OK, the problem may be in the 29 MHz crystal.

It will also stop oscillating when C107, C108 and C110 are broken. When these capacitors are in an "open" condition, they don't change the DC conditions of the circuit.

A broken capacitor, or bad soldering, in the tuning circuit in the collector of the left transistor will also stop the oscillation, without affecting DC bias.

 

[hafrse]

Hello,

Thanks for your explenation, just what I wanted to hear that the LOTUNE does not stop the osccillator and this oscillator should run whithout LOTUNE as well, which means I do not have problems with the DAC, calibration data or software based problems.

Bias is ok, Regarding the caps and the soldering , I checked very carefully , no soldering problems and no visible damage to the capacitors.

Regarding the crystal, I found a 26.660Mhz crystal in my possesion, stoped it in but no luck (not sure if there are different types of crystals or the frequency was far away t) Can x-tals be bad in time/use where they stop to oscillate ?

I will try to replace the caps and see if it helps, the caps are smd:s and the schematic is littel confusing rrgarding the caps, there is no unit specified... It says .01 , is that .01 nf or .01 uf ?

Thanks

 

[WimRFP]

In my life I experienced maybe 4 times that a crystal broke down, not including mechanical misuse.

Regarding LOTUNE, when testing, I would put some voltage in it to make sure the capacitance diode is reverse biased.

I would expect that this circuit can also oscillate at 26 MHz, as the Q factor of the LC circuit in the collector of the left transistor is low. It all depends on the resonating impedance (resistance) of the crystal.

If you have a stable generator and spectrum analyser or oscilloscope, you could figure out whether the crystal is still active.

 

[hafrse]

I have both, How can I test the crystal with sa and sg ?
Thanks