# How to increase operational frequency to 70MHz in this Colpitts Oscillator?

#### oviniciuslara

Joined Nov 7, 2020
13
Hello everyone,

I'm building a radio transmitter with a Colpitts oscillator schema and everything works fine. I built the circuit and tested in the Oscilloscope and with L=470uH, C1=100pF, C2=100nF I created a 549KHz 6Vpp senoidal (with 5v as source), as we can see bellow:

Theoretically, the formula tells us that the frequency with these values should be around 734KHz, so I think we're close as I'm using a breadboard and it has parasite capacitance and others interferences.

But I want to increase the operational frequency because I can't built an antenna for this wavelength (546 meters), even with 1/4 wavelength (136 meters) it still not viable. So I'm trying to increase the frequency up to 70MHz because it's wavelength is 4.3 meter and it's 1/4 wavelength approximately 1 meter and it's feasible to build.

So my first try was to change the inductor to a tinnier one, I started to decrease the inductor value one by one to see if the frequency was increasing. But the circuit stops to resonate when L is bellow 13uH (5MHz), even though in the simulator works.

So my question is, what can I do to increase the operational frequency up to 70MHz with this schematic?

PS: This is a project I'm doing to adquire knowledge in RF for university. I don't intend to transmit data above the 100mW allowed by BR regulations.

#### Papabravo

Joined Feb 24, 2006
18,783
Building a stable VHF circuit with lumped components a bit more difficult. You have to keep the leads very short and watch out for parasitic effects from the components that you do use. I think you should learn to crawl and walk before trying to run a marathon. Acquiring a nanoVNA wouldn't hurt so you can at least characterize and measure what you are working with.

https://www.amazon.com/NanoVNA-H4-A...hvlocphy=&hvtargid=pla-4584070147654603&psc=1

#### oviniciuslara

Joined Nov 7, 2020
13
@Papabravo
Thanks for the quick response. I have in hand this NanoVNA and the TinySA to help me with the measurements. What do you suggest me to achieve this goal?

#### Papabravo

Joined Feb 24, 2006
18,783
@Papabravo
Thanks for the quick response. I have in hand this NanoVNA and the TinySA to help me with the measurements. What do you suggest me to achieve this goal?
Learn the techniques of "Manhattan style" and "dead bug" construction. Don't try to go all the way in one leap. Experiment with some intermediate steps. The Radio Amateurs Handbook is a great place to start.

This guy "devttys0" makes pretty good videos about stuff you may be interested in. 28 MHz. (Amateur 10M Band) would be an intermediate step.

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#### oviniciuslara

Joined Nov 7, 2020
13
Thanks! I have worked with simple oscillators like Hartley & Colpitts, now it's time to go further. Anyway, do you have anything to point in the schematic?

#### Papabravo

Joined Feb 24, 2006
18,783
Thanks! I have worked with simple oscillators like Hartley & Colpitts, now it's time to go further. Anyway, do you have anything to point in the schematic?
I've never seen the tank circuit for an oscillator connected the way you have done it.

#### upand_at_them

Joined May 15, 2010
905
Where are you probing for the signal? Without a buffer amp following the oscillator it's possible that you're loading down the circuit and preventing oscillation at the higher frequencies. The base biasing resistors seem high to me. Did you experiment with dividing them by 2, or 5, or 10?

Don't jump right from 549KHz to 70MHz. Move up in frequency in steps and see when things stop working properly and what you can do to fix it.

#### oviniciuslara

Joined Nov 7, 2020
13
@Papabravo It's a Common Emitter Colpitts configuration.

#### oviniciuslara

Joined Nov 7, 2020
13
@upand_at_them I'm probing it in the out label. I didn't tried loading down the bias resistor however.

I did make it work in 5MHz just adjusting the inductor to 13uH, but the Vpp was low, maybe you're right and I need an amplifier?

#### oviniciuslara

Joined Nov 7, 2020
13
@MrSalts Thanks for the link. You achieved 120MHz with the exact components from the list or did you modified the circuit? Also can you tell me if the BC547 or BF494 is equivalent of 2N3904?

#### ApacheKid

Joined Jan 12, 2015
721
Hello everyone,

I'm building a radio transmitter with a Colpitts oscillator schema and everything works fine. I built the circuit and tested in the Oscilloscope and with L=470uH, C1=100pF, C2=100nF I created a 549KHz 6Vpp senoidal (with 5v as source), as we can see bellow:

View attachment 268591

Theoretically, the formula tells us that the frequency with these values should be around 734KHz, so I think we're close as I'm using a breadboard and it has parasite capacitance and others interferences.

But I want to increase the operational frequency because I can't built an antenna for this wavelength (546 meters), even with 1/4 wavelength (136 meters) it still not viable. So I'm trying to increase the frequency up to 70MHz because it's wavelength is 4.3 meter and it's 1/4 wavelength approximately 1 meter and it's feasible to build.

So my first try was to change the inductor to a tinnier one, I started to decrease the inductor value one by one to see if the frequency was increasing. But the circuit stops to resonate when L is bellow 13uH (5MHz), even though in the simulator works.

So my question is, what can I do to increase the operational frequency up to 70MHz with this schematic?

View attachment 268593

PS: This is a project I'm doing to adquire knowledge in RF for university. I don't intend to transmit data above the 100mW allowed by BR regulations.
See: https://electronics.stackexchange.com/a/605007

#### Danko

Joined Nov 22, 2017
1,358
This circuit easily oscillates at 70 MHz:

#### Attachments

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#### Papabravo

Joined Feb 24, 2006
18,783
@Papabravo It's a Common Emitter Colpitts configuration.
Maybe it is, but the tank circuit appears to be in the wrong place.

#### Ya’akov

Joined Jan 27, 2019
5,997
Radiating from an antenna at 70MHz is going to contravene government regulations almost anywhere on earth.

#### oviniciuslara

Joined Nov 7, 2020
13
@Danko
Thanks, I will try to build that soon. But a question, what changed to make it work? Also the formula for the frequency with those values are not even close to 70MHz so what make it to increase? What am I missing?

#### Danko

Joined Nov 22, 2017
1,358
@Danko
Thanks, I will try to build that soon. But a question, what changed to make it work?
Nothing changed, it is designed from zero.
Components, placed between collector and base of transistor, should shift phase of signal on 180° at frequency 70 MHz.
Also the formula for the frequency with those values are not even close to 70MHz so what make it to increase?
Did you use this formula: f = 1 / (2π √L C) ?
Try online calculator: https://goodcalculators.com/resonant-frequency-calculator/

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#### MrSalts

Joined Apr 2, 2020
1,832
Radiating from an antenna at 70MHz is going to contravene government regulations almost anywhere on earth.
How big of a violation is it really? The FCC doesn't even think its important enough to staff the team to monitor and enforce the regs.

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#### michael8

Joined Jan 11, 2015
295
Read the datasheets? The BC874a has a minimum Ft of 100 Mhz. The 2N3904 has an Ft of 300 Mhz.

The Ft is were the common emitter beta extrapolates to 1 (no gain). I'd usually want the transistor I was
using to have an Ft way more than the frequency I was using it at, possibly 10 times higher. For
70 Mhz, try an Ft of 700 Mhz. It can be done with less but as above the 2N3904 will be easier than
the BC874a.

Try out something like the MPSH10:

https://www.digikey.com/en/products/detail/nte-electronics-inc/MPSH10/11651499

#### oviniciuslara

Joined Nov 7, 2020
13
@michael8 Thanks for the heads up, I will try to put hands in one of these soon, but in the meanwhile I only have BC547, BF494, F450, F245C, F423.

Based on this information the best suitable transistor would be the BF245C because its cut off frequency it's about 700MHz, just 10 times what I want. But it is a JFET, would be necessary to rearrange the schematics, right?

https://www.nxp.com/docs/en/data-sheet/BF245A-B-C.pdf

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