PLL Oscillator Issues

I need some help with a crystal oscillator which is being very stubbornand not oscillating. It is part of a PLL and has a crystal with 63.9151MHz written on it. The manual says the crystal is a 3rd overtone which is not surprising at this frequency. The circuit seems to be a Colpitts oscillator with a few capacitors switched in and out in order to pull the frequency slightly. The transistor tests ok in a transistor tester so I removed the crystal itself and built an oscillator on breadboard. This works fine with the 63MHz crystal but oscillates around 21.8MHz which is probably the fundamental. However back in the transceiver it's back to being dead again, I'm guessing the circuit is dampening oscillation at the fundamental hence why it's not working.

My question is can a crystal be faulty in such a way that it stops oscillating on the overtone yet will still work at the fundamental? It seems unlikely a passive component in the circuit is at fault.

I've never come across a crystal that behaves that way.

It appears from the circuit that the frequency can be pulled slightly by applying binary low levels to the four lower lines so D03 - D06 are forward biased. Is it possible a bad combination is pulling it too far and stopping oscillation? Also please confirm you are measuring the frequency at TP11 or the junction of L12/C56 as connecting a probe anywhere else may stop it oscillating anyway!

Brian.

I didn't think a crystal would behave like that but it was worth asking the question. The binary combination goes from 0000 to 1001 so I'll have to check this is working correctly and not pulling it to far out as you say. I can confirm I'm using TP11 to test the output. Looks like I may have to remove each passive and check they've not gone out of spec.

Brian (as well).

The circuit comes from the Yaesu FT-480R 2m transceiver so would have worked as designed. I certainly agree that the oscillator on the bread board wasn't tuned to create the 3rd overtone, I was simply testing the crystal to see if it worked at all. To see it oscillate at the fundamental seems to show the crystal is ok so I'm looking at why it doesn't it work in the PLL unit. A thought I had was that maybe a crystal can be harder to get to oscillate at the overtone rather than the fundamental.

3rd OT crystal operation depend on a circuit suppressing the fundamental wave. My first problem with the schematic is that L10/C51 doesn't seem to be tuned for 63 MHz which would be expected for an overtone crystal oscillator.

It might be that the original circuit oscillates at 63 MHz though, possibly with only a small margin, but fails on a breadboard due to parasitic wiring inductance.



Usually a special crystal cut prefers the overtone. Suppression of the fundamental respectively promotion of the overtone by a tank circuit may be required nevertheless. It looks like L10/C51 has this purpose.

The crystal spec is 3rd Overtone so I assume it would prefer the overtone then, however it is over 30 years old.

Question is will replacing the crystal with a new one get it oscillating again? Other than the crystal and the transistors there are only passives which tend to be very reliable.