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PLL VCO Temperature Instability

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fuseless

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I recently constructed a PLL synthesized weather band receiver. When I apply heat using a hair drier, the reference oscillator crystal remains on frequency. The VCO LO only remains on frequency for a short time until the VCO control voltage reaches its limit 7V, then the PLL drops out of lock.

Why does the VCO control voltage reach its limit so quickly at only 90 degrees F?
 
The VCO has to be made using NPO or negative tempco capacitors.
 
Hi Mike,

Thanks for the quick response. All the capacitors are of the COG type. The VCO tank circuit coil is a coilcraft series 164(unshielded) variable inductor with an aluminum core. The data sheet has an ambient temperature of -40 degrees C to +85 degrees C but do you think the coil is to blame?

Thanks......fuseless
 
Show your schematic. Bad temp performance can be due to a lot of things.

Coil, caps, transistor parameter change, control voltage driver, load change with temp, mechanics of shielding.

What is tuning range of varactor control voltage? Is is centered.

If you are using an aluminum core then I assume it is a UHF oscillator.
 
Since the caps are COG types, my best guess would be to look at the varactor and the tuning voltage (similar to RCinLFA's comments).

In the example you provided, what is the tuning voltage nominally before you apply heat?

Another thing you maybe seeing is that the second (or third) harmonic of the VCO is gaining amplitude and the PLL is now locking to it. Although if that was the case, you would normally see the VCO tuning voltage jump from max voltage (or somewhere close) to min voltage as it is now locked to the VCO harmonic (thinking is the fundamental) and it wants to bring it down.
 
I would bet that the major contributors will be, in this order:

- coil inductance drift
- capacitor value drift (even NPO drift can be significant, but even if small, they all add to drift)
- transistor and varactor diode reactance drift
- DC power supply voltage drift

The coil may be spec'd over a broad temperature range, but that doesn't mean it's value is rock stable.

You can compensate to some degree by having a larger lock range for your VCO, but that means increasing the Hz/Volt gain of the VCO which has, as you probably know, many other ramifications.
 
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