27 MHz Amplification Query

[RMMK]

and if i just add a pre-scaler IC before the transistor like SAB6456 with the same .hex file loaded on the uController.... will it work for higher frequencies as well?? Till this moment what i have understood is that the above said prescaler IC will divide the input frequency by 256 and will show the same in the divided form on the LCD!
I am just asking this for information purposes........ and probably for the other 433MHz version i will work on afterwards!

 

[betwixt]

Dividing by 256 will make it measure up to 1GHz although the reading on the LCD at that frequency will be 1000/256 = 3.90625MHz. You could make it switchable so it can divide by 64 instead which will make it show 15.625MHz and be slightly more accurate. If you want to be really clever you can arrange to multiply the result back up again in software or better still, adjust the measurement period to compensate for the divider and get the correct reading at full accuracy.

Brian.

 

[E-design]

Another 60 MHz budget frequency counter.
**broken link removed**

 

[RMMK]

Pheww! After a long bad mouthing session i was able to work on an oscilloscope, signal generator and a spectrum analyser! The Oscillator stage is oscillating at 12-14MHz and not on 27MHz and weirdly the 2nd stage Q2 (as shown in schematic - 2N2222A) is acting as a frequency doubler with the signal at the collector at around 20MHz but after that the signal gets lost........ The whole waveform is distorted a lot and the output from the oscillator stage is not even close to sinusoidal wave.......

 

[chuckey]

Your oscillator, 1µH @ 27 MHZ ~ 300 Ω, 27 PF @27 MHZ ~ 300 Ω. Unfortunately, there is also a 150 PF cap feeding the next base plus a 60 PF feedback cap plus some stray capacity. So the tuned circuit will be resonating closer to 1/2 of 27 MHZ, and so this is the oscillator frequency. Put the 150 PF in series with the Vcc end of the 27 PF and take the junction to the base of the next transistor. This will raise the resonance frequency of the LC circuit and will reduce the damping on it.
All your circuits suffer from this problem, too much capacitance, so they tune low. Or would tune low if you would only add some variable component. If your LC are 20% off frequency, the gain of that stage will be 1/10 of its maximum - hence no output.
Frank

 

[E-design]

Here is a proven design that you may adapt the first two stages for your design.
http://www.talkingelectronics.com/p...tters/27MHzLinks-2.html#4 CHANNEL TRANSMITTER

 

[RMMK]

Here is a proven design that you may adapt the first two stages for your design.
http://www.talkingelectronics.com/p...tters/27MHzLinks-2.html#4 CHANNEL TRANSMITTER

Sir its actually the same design i am using...... apart from the PA stages, the placement of 27MHz crystal and the tuned amplifier; everything other than these is more or less the same!
I have on advice of a member (Sir Brian) in this forum placed the crystal such that its one leg is on the base of the transistor and the other on the ground!

 

[chuckey]

"more or less the same" is not good enough (as explained) for RF LC circuits, in the link there is no 27 PF cap across the oscillator inductance and the 47 PF coupling capacitor is much lower then your one, hence the circuit is tuned to 27 MHZ and not 12 MHZ. Get the oscillator working on frequency first, the rest will follow.
Frank

 

[RMMK]

Alright sir! I am in process of building the circuit in a pcb! I will report results in due course!

 

[BradtheRad]

In case this can help...

I played with the simulation so that oscillator output swings almost to the supply rails.

A single voltage follower should be sufficient to provide current gain, 1W or more as you require.

This should be less cumbersome than several amplification stages.



I found I needed to add some amount of resistance at the supply. It isolates LC oscillations to some extent. Output can be adjusted up and down between the supply rails.

It seems impossible to get this topology to maintain oscillations on its own. It still needs some encouragement (such as from a crystal). The intensity does not have to be strong. Mine shows 1V going through 300k ohms.

 

[E-design]

With the correct bias conditions it will sustain oscillations.

 

[BradtheRad]

With the correct bias conditions it will sustain oscillations.

I tried it with your component values. Oscillations begin at the start, but they fade slowly. Eventually it goes into stagnation. Current flows through the supply, coil, transistor, and emitter resistor. The ampere level is unchanging DC.

Looking at your scope trace, it does have healthy oscillations. However the net amplitude appears to be decreasing.

 

[E-design]

If you look at the simulation over a longer period, it shows that the closed-loop gain stabilized and the amplitude remains constant.

 

[BradtheRad]

If you look at the simulation over a longer period, it shows that the closed-loop gain stabilized and the amplitude remains constant.

Very interesting. Evidently something changes after 20,000 cycles, to start the oscillations going.

I monitor the coil and capacitors, to watch where anything might change.

The only think I can figure, is that the bias capacitor charges to a stable voltage. Then your oscillations take off like a rocket.

I have tried to 'speed things up' by reducing that capacitor's value. Then I increase it when it stabilizes. I still cannot obtain sustained oscillations.

I try changing transistor gain. 100, 200, 300. No effect.

Gradually my simulation stagnates to a condition of unchanging DC current flow.

 

[RMMK]

thanks everyone...... I will try the mods and will come up with the results! Sorry for the delay ...... i had some hourly exams to take care of! I also havent been able to build the frequency counter as of yet!

 

[E-design]

Gradually my simulation stagnates to a condition of unchanging DC current flow.

It must be something with your simulator. I took the trouble of actually testing the circuit on the bench, and the measurements agree closely with my previous simulator results.

The last plot clearly shows the initial start-up transient response.

 

[BradtheRad]

It must be something with your simulator. I took the trouble of actually testing the circuit on the bench, and the measurements agree closely with my previous simulator results.

The last plot clearly shows the initial start-up transient response.

Yes, you confirmed it's true. I salute you.

Now I wonder how to fix it in Falstad's simulator. I would have thought a similar algorithm is at work. Both need to reach convergence in each frame.

The lower capacitor is supposed to create voltage swings across the transistor. A drop in emitter voltage causes an increase in bias current. The two effects reinforce one another, leading to oscillations.

It is helpful to see your startup response. Mine also shows some ringing effect for the first few cycles. But then it fades. Not enough gain, maybe.

 

[E-design]

Did you use my circuit with the base connected capacitor? Without this capacitor, it will not work.

 

[RMMK]

Ahh its so frustrating! I tried building the frequency meter but i am not getting any display on LCD's....... I double checked my connections but there seems no problem! Can anyone plz help me troubleshoot it?? I was really looking forward to this frequency meter for onward applications........... i build the circuit on a veroboard and used a 2N2222A transistor in place of BFR91........ Also I used LMB162AFC LCD display...... I tried power it up using a 9 Volt Battery and also with a 5V battery and they both failed to make it show any display! I tried connecting the positive and negative terminals of the battery to the Pin 15 and 16 of the LCD with the 9 V supply connected to the input of the circuit; with this the backlight of the LCD started glowing! Plz help.......

- - - Updated - - -

https://www.circuitvalley.com/2011/07/60-mhz-frequency-meter-counter.html

This is from where i followed the project! Its getting hellish now that one after another nothing is working..............!!!!!

 

[BradtheRad]

Did you use my circuit with the base connected capacitor? Without this capacitor, it will not work.

Yes, I added it. I realize it stabilizes voltage at the base terminal. It is often added when we operate a transistor as a common-base amplifier.

Yet I could not sustain oscillations. I can't figure it. I've varied the timestep, resistances, transistor gain, etc. Perhaps something is missing in Falstad's algorithm, perhaps in the transistor model.

So I transferred the capacitor to the emitter leg. Now it's cooking. Oscillations build and continue.



The above design may resemble one which appeared earlier in this thread.