faster oscillator turn-on time

[biff44]

I have a saw resonator oscillator that is taking too long (~ 150 uS) to turn on and start oscillating. Does anyone have any suggestions on how to speed up the turn on time? The saw resonator in a hartley single transistor oscillator circuit. Current draw is throttled down to ~ 1 mA since it is a battery powered application. I turn on/off the oscillator by controlling the base current.

 

[radiohead]

It is normal that the settling time is long. SAW resonators have high Q-factor so it takes a long time for them to get to swing in and get into steady state! There's no way around it unless a simple PLL-locked VCO has adequate phase noise performance (which is the case for most applications).

 

[biff44]

Yes, there is the narrow band = group delay. There is also probably some time delay due to the acoustic, rather than light wave, propagation. But, given all that, I was wondering if there was a speed up method to employ. Inject RF noise at start up? Capacitively couple in bigger base current at start up? etc. Anyone ever try and succeed?

 

[PaulHolland]

Hi, There is a way our of this problem but its more expensive. Leave the OSC on all the time and put one additional transistor behind the OSC before the Antenna and modulate the collector current by ON/OFF this way you have high switching speed and still low power consumption. if you need you can power down your OSC after you send the data !.

 

[biff44]

Good idea, but I would have some trouble with that. It is an unshielded keyfob, AM modulated, and the leakage RF would activate the nearby receiver. I just have to speed it up a little.

 

[PaulHolland]

Hi, Use a transistor with a higher fT than the one your using now.

 

[Abhishekabs]

Hi biff44.
I am trying to make an oscillator whos turn on time is 150pS.
According to my information For switching transistor on & off , Q of the circuit must be low. ( Thats why i am using microstrip circuits).
You said that you control the base current to turn on & off, i suggest that you should use make turn on & off circuit externally(say circuit which provide pulse of 10nS), Swithch ground of an oscillator on &off insted of controlling base.
Which transistor you have used for oscillator & what is frequency of oscillations?

 

[biff44]

bfs17a transistor and 315 MHz.

Added after 1 minutes:

Hi, Use a transistor with a higher fT than the one your using now.

Trouble with that one is that the transistor I have now, BFS17A, is trying real hard to oscillate up at 700MHz and ignore the saw resonator frequency of 315. If I give it more high frequency gain, I will be in real trouble!

 

[E-design]

I wonder if it will be possible to switch in another loading element for a very short period to lower the overall Q of the resonator to speed up starting. I never tried this myself. It should be interesting to investigate with the aid of a simulator. Also oscillations can only start when the active biased region for the transistor is established. The time delay caused by the bias network can be much longer than the oscillation start up time (td) of the resonator self.

td (resonator) ≈ (2 Ql)/ω where Ql is the loaded Q of the resonator

 

[BigBoss]

You can add a switchable resistor by aid of a fast transistor to lower the Q factor of the resonator and then you turn off this transistor...

 

[E-design]

In this case it appears that most of the delay comes from the bias network