svensl
Joined: 20 Oct 2006
Posts: 1
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 Posted:
Fri Oct 20, 2006 1:51 pm Post subject:
puzzeld by oscillation frequency value |
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When implementing a Hartley oscillator on a PCB board, using a BF199 TO-92 transistor and the rest 0805 inductors, resistors and capacitors the frequency of oscillation is only 1/5th of the expected value.
The same circuit simulated in Spectre using a model for the BF199 yields accurate results. Why is the frequency of oscillation so far off? Any ideas?
The picture shows the oscillator. The frequency is supposed to be around 200MHz with 6.8nH and 2.7nH for the inductors and 68pF for the capacitor. But as a PCB circuit it just under 50MHz.
BTW, the capacitor Cf, what should its value be? So far I always choose it such that its reactance is small compared to the reactance of L1.
Could it be the 0805 inductors? The data sheet gives a Q factor of 40 and a resistance of 0.2 Ohms.
Cheers
picture at:
http://userweb.elec.gla.ac.uk/s/sven/osc1.jpg
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