Frequency climbing in Colpitts oscillator? - Page 1
Forum section: electronics / beginners

author: frenky
author_rank: Unknown
timestamp: February 05, 2018, 09:04:32 am
content: I wanted to make a simple metal detector with this scheme:I made a coil with inductance approx 160uH, paired with 11nF should give 120kHz frequency.I get 125kHz which is close enough.Problem is that frequency keeps on rising.It starts to rise with 2Hz per second and after minute slows down to 1Hz per second. But it "newer" stops (at least in 20min it doesen't).What could be the problem?I have made another circuit on the breadboard and it behaves exactly the same?Please help.

author: BrianHG
author_rank: Super Contributor
timestamp: February 05, 2018, 09:32:48 am
content: Measure your 5v with at least a 4.5 digit DVM and verify the osculation peak voltage on pin 5 with a 4.5 digit DVM as well over the 20 minute time.

author: Twistedsnail
author_rank: Contributor
timestamp: February 05, 2018, 09:39:09 am
content: Are your 22nF caps supposed to be between C and E, and E and GND? Like this example:

author: amspire
author_rank: Super Contributor
timestamp: February 05, 2018, 09:43:42 am
content: What sort of capacitors are you using?Many types of ceramic capacitor are not stable. You need either NPO/COG ceramic or something like polyester capacitors.

author: Twistedsnail
author_rank: Contributor
timestamp: February 05, 2018, 09:47:12 am
content: Quote from: Twistedsnail on February 05, 2018, 09:39:09 amActually now that I think about it, both have the crucial feedback capacitor from the emitter resistor.Can someone explain the difference between having the second capacitor couple to V+ vs GND?

author: frenky
author_rank: Unknown
timestamp: February 05, 2018, 10:11:28 am
content: I am using this 22nF capacitors from a local electronic components store:MULTILAYERBrand: MilleniumMac Valtage: 50VIzolator X7RTolerance ±10%Type THTSome aditional info...I tried the circuit without arduino, so that I can rule that out.And tried with different voltage sources: 5V from arduino voltage regulator, 9V battery 20V battery...

author: amspire
author_rank: Super Contributor
timestamp: February 05, 2018, 11:07:22 am
content: XR7's are not the most stable of capacitors. You are getting a drift of just under 2 percent per second down to one percent after a while.The XR7 can have a temperature coefficient of over 2%/C, so it only takes a 1 degC change to cause the drift. You would be better with a more stable capacitor, but make sure the capacitors are nowhere near the transistor since the transistor may be warming the capacitors.

author: frenky
author_rank: Unknown
timestamp: February 05, 2018, 11:21:16 am
content: Thank you!I'll try:- replace caps with different type- move it away from transistor- thermally insulate capacitors

author: frenky
author_rank: Unknown
timestamp: February 06, 2018, 06:53:33 pm
content: So I replaced all caps with this one:https://www.digikey.co.uk/product-detail/en/epcos-tdk/B37987F1223K058/B37987F1223K058-ND/1596648After a few minutes frequncy becomes much more stable. Now it climbs at a rate 1Hz every 30sec which is A LOT better. And the climb slows down with time significantly.Tnx to all.

author: tecman
author_rank: Frequent Contributor
timestamp: February 06, 2018, 07:24:59 pm
content: Those are X7R.You really should put NPO caps in for stabilitypaul

author: frenky
author_rank: Unknown
timestamp: February 07, 2018, 07:18:04 am
content: I will try to get some NP0 caps, though it seems that that type mostly exist in pF range.

author: amspire
author_rank: Super Contributor
timestamp: February 07, 2018, 10:00:05 am
content: Film capacitors (polyester or polystyrene) are readily available and are much better then the XR7 ceramic capacitors. The film capacitor temperature coefficient is smaller then XR7 by at least an order of magnitude.

author: frenky
author_rank: Unknown
timestamp: February 07, 2018, 10:09:43 am
content: Tnx.It seems that I managed to find all the wrong capacitors for this project.

author: frenky
author_rank: Unknown
timestamp: February 08, 2018, 07:38:53 pm
content: So have found some 47nF NP0 localy. The frequency went down to 85kHz but it remains withing 1Hz for more than 10min.I could't be happier.Tnx to all.