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Hi Badai,
Why do you want a cutoff frequency as low as 60Hz with this very high frequency RF circuit? A circuit's cutoff frequency is defined as the frequency it begins its cutoff, where its output is down to 0.707 of its voltage at higher frequencies, nearly the same.
As the project's notes describe, it already has 60Hz rejection since its C4 and R7 have a cutoff frequency of 727KHz plus the high cutoff frequency of its C5 for which its load is difficult to determine.

The cutoff frequency for a series capacitor that is driven from an extremely low impedance, and the capacitor feeds a resistor to ground is:
One, divided by 2 times pi times R times C.

I round the calculation off slightly and make it much easier:
0.16, divided by R times C. ;D

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Hi Badai,
A 2N2222 won't detect 250MHz very well. :'(
Its minimum Ft rating of 250MHz only guarantees that it will have a tiny amount of gain at 100MHz, just enough gain to work as an oscillator or mixer at 100MHz.
The Ft rating is the frequency at which the gain is only one, like a piece of wire.

Additionally, the Ft rating uses a transistor voltage of 20V, where its capacitance is low. With the project's 9V battery which drops to 6V over its life, the 2N2222 will have a voltage of about 3V and an Ft much lower than 250MHz.

You might be lucky and get a 2N2222 with an actual Ft of 300MHz at 20V. Then it would have a gain of 10dB up to maybe 80MHz in this project when its battery is 6V.

The project was designed for a microwave transistor with an Ft of 6GHz, and the circuit has a gain of 10dB up to 1GHz with it.

post-1706-14279141843426_thumb.gif

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  • 2 weeks later...

Hi Badai,
I couldn't find out anything about your AA215 diode. Your 2N2222 transistor will have gain up to at least about 60MHz, no gain at about 180MHz and lose more signal at higher frequencies.

The VCO is an audio oscillator with no output at a very low detected signal level and a higher audio pitch with higher detected signal levels. 

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AA215 diode I used for the design circuit only. The real diode is 1N34A. How to calculate the gain defined by C8,R6 and R12? What does it means, VCO that operates out of frequency range? What is the purpose of TR3, is it used as amplifier too? Why it need used 4.7k

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Hi Badai,
1) The project specifies using a microwave transistor and diode, you are using a general purpose 250MHz transistor and high frequency diode.
2) The DC gain of opamp A1 in the project is usually defined as the ratio of resistors R6 (1M) divided by R12 (220) if the circuit is driven from a very low impedance. However the driving source impedance is not low in this circuit and adds to the R12 resistance reducing the gain of the opamp. At maximum sensitivity setting, R5 (560) and R2 (470) are in series with R12 so the DC gain is 800.
3) C8 (10nF) across R6 (1M) cuts-off frequencies above 16Hz, so A1 will have a maximum gain of 800 at about 3.2Hz, 566 at 16Hz, 400 at 32 Hz and 200 at 64Hz etc.
4) The article should say, "The VCO operates through-out the audio-frequency range".
5) TR3 is used as a current amplifier and drives the speaker through R4, since the power output of A3 is not nearly enough to drive a speaker.
6) The 4.7K pot is the project's sensitivity control and is adjusted so that the circuit ignores external electromagnetic fields (from mains wiring and local AM, FM and TV stations).
7) The 8 ohm speaker produces the tone from the VCO. The higher the pitch, then the higher is the signal level that the project is picking-up. This project is not a radio and will not reproduce voices.

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