VHF oscillator precision

First, you don't have 20 kHz.

I don't perceive a problem. The maximum channel bandwidth is, per the FCC,

11.25 kHz and permissible modulation deviation is +/- 2.5 kHz. If you build the oscillator using insight from the crystal supplier and achieve the (very doable) +/- 7.5 kHz stability, The signal will always be contained within the 11.25 kHz limit. A decent receiver will have no problem capturing the signal at it's frequency tolerance boundaries.

Don

you either have to pay for a better osciallor i.e. 5 ppm or you have to include a trimmer and pay for someone in the factory to tune it.

Mark

Temperature compensate the oscillator. And if that doesn't work, put the thing in a crystal oven.

Michael

Hello Chris,

Michael's suggestion would work. If that costs too much you will need tuning. Either by hand or, if it is a mass product, by an auto-cal procedure at final QC where the setting is stored in flash (assuming you have a uC with flash on there).

Regards, Joerg

I don't see a sound basis for the "probabilistic solution". Surely some units would be as bad as some single crystals ?

Once your straight crystal spec is unsuitable, the next step is TXCO - packaged oscillator units with a varactor and thermistor setup inside to tune out the shift. These are stable enought for your application, however, note the aging spec. These do have a trim pin for fine frequ shift, though it is probably not for FM. If you can't FM such an oscillator, you can use it as a reference for a PLL and modulate the oscillator control voltage. At that stage you might want to save money by choosing a low frequency oscillator at sub 20 MHz and using the PLL to multiply it up.

There are chips around which make all this this terribly easy. Analog Devices RF have some parts for FSK transmitters. Quite a few others.

Possibly you can bypass the whole thing by buying complete modules with an data input pin !

Roger Lascelles

And for a Really Cheap(TM) crystal oven, you mount an appropriate value resistor (180 ohm 1 watt for a 12 volt system) next to the crystal on long leads so that the resistor can touch the crystal case on one side and a cheap (1N4148) diode on the other side of the case as a temperature sensor. Shrink sleeve both the resistor and diode so that they are in intimate contact with the crystal case. Use an op amp comparator to turn on a saturated driver transistor and adjust the bias on the opamp so that it switches off when the temperature of the diode gets to 50°C or so. Use a bit of hysteresis on the comparator so that it doesn't chatter during the switching period.

Jim

????

The 11.25kHz channel bandwidth has to contain 2*(highest mod frequency before roll-off) + 2* (Max deviation). That means audio roll-off would need to start at (11.25 - 5)/2 = 3.125kHz. That's assuming the carrier is on frequency. As soon as the carrier is off-frequency by (say) 1kHz the sum of (max dev + mod frequency) has to drop by that amount to remain strictly within that allowed bandwidth.

Having said that, the O/P appears to have misunderstood the +/- 50ppm crystal tolerance. This doesn't mean it wanders around by that much in operation. Rather it is the tolerance band for the crystal out-of-the-box and before any frequency trimming circuit is applied. Depending on the resonant mode chosen, a simple trim-cap or coil will normally suffice. Let's face it, commercial two-way radios have existed in such narrowband environments for decades using - in many cases - no temperature compensation other than a carefully chosen ceramic cap with a known tempco.

If the O/P wants to reinvent the wheel, that's fine. For my money, I'd employ an existing two-way off ebay and save my design time for a project where there isn't an easy cheap off-the-shelf solution.

In message , Apparatus writes

The biggest problem will be to pull the crystal frequency to nominal,

20kHZ in 200MHz is 200ppm so 50ppm at roomtemp will be within this. Always place the varicap in series with the crystal at this frequency and tune out external reactance to zero by a an inductance.

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dd