A
Apparatus
- Jan 1, 1970
- 0
Hi,
I am working on building a MURS band (154.600 MHz, 20 KHz) FSK
transmitter. The issue that I'm running into is oscillator stability.
My current idea is to use a crystal based colpitts oscillator with a
JFET for the feedback element, adding a varactor in parallel for a
small amount of tuning. However, crystals themselves have typical
precision ratings of +/- 50ppm. At 150 MHz, that's +/- 7.5 kHz
tolerance, nearly the size of my band. Then the additional issues are:
the precision of the varactor, stray JFET capacitance, etc.
I have thought about also using a mixer, with two oscillators as above,
one of which is tunable via a varactor, both at around 75 MHz.
Probobalistically, the tolerances should somewhat negate (assuming much
higher mixer precision).
Without a manual tuning element, how can I hit 154.600 MHz and keep the
FSK tuning within my 20 kHz bandwidth? I feel as if I am trying to get
a hole in one from a mile away. Any insights would be much appreciated.
Cheers,
Chris
I am working on building a MURS band (154.600 MHz, 20 KHz) FSK
transmitter. The issue that I'm running into is oscillator stability.
My current idea is to use a crystal based colpitts oscillator with a
JFET for the feedback element, adding a varactor in parallel for a
small amount of tuning. However, crystals themselves have typical
precision ratings of +/- 50ppm. At 150 MHz, that's +/- 7.5 kHz
tolerance, nearly the size of my band. Then the additional issues are:
the precision of the varactor, stray JFET capacitance, etc.
I have thought about also using a mixer, with two oscillators as above,
one of which is tunable via a varactor, both at around 75 MHz.
Probobalistically, the tolerances should somewhat negate (assuming much
higher mixer precision).
Without a manual tuning element, how can I hit 154.600 MHz and keep the
FSK tuning within my 20 kHz bandwidth? I feel as if I am trying to get
a hole in one from a mile away. Any insights would be much appreciated.
Cheers,
Chris