Troubleshooting a VCO 145MHz

[Navajo]

vco 145mhz

Hello everybody

I am working in a VCO that can be seen here **broken link removed**
The oscillator works good, good frequency good shape but when I introduce the varactor I dont get what I would like to have, that is a 2 pick signal one for the 0's and other for 1's in frequency domain.

I have look for the problem, everywhere and I cant find it.

Any idea will be wellcome.

 

[vco96]

The ft of 2N2222 is about 200 MHz, try using a different transistor like 2N5179. The emitter follower has a low impedance output so terminate it lets say at 100 ohms. Replace C7 with a .01 uF cap so it acts like a short. The 5V input signal may be too large so try a voltage divider to bring the amplitude down to 50% or more.

good luck.

 

[E-design]

The C1/C2 ratio should be bigger than 1 for the circuit to have enough loop gain to start easy. Yours work somehow by the capacitance on the collector reflected back into the base.

Also your output waveform looks like the osc is "squegging", a condition where because of a wrong feedback ratio involving C1,2 and Re will cause the the transistor to be driven into cutoff at regular intervals. This causes a type of amplitude modulation with a lot of sidebands.

Try altering the capacitor feedback ratio and/or adding some inductance in series with Re

 

[Navajo]

I have tried some of your advices and I have got some improvements. Still I have a quite dirty spectrum. I will reread some documents and I will post new questions soon.

Than you very much.

 

[E-design]

Reduce your input switching freq to say 10k or 100k and see if there are any more improvements. Start with a low switching freq and work your way up. This way you will be able to identify where your circuit limitations are.

Remember you are FM modulating the signal. By definition FM have a infinite number of sidebands although sidebands having levels less than 1% of the carrier level are ignored. Depending on the amount of deviation from the carrier (modulation index = freq deviation/mod freq) 1,2,3 etc. you can have (6,8,12 X mod freq) the required RF bandwidth

For example: If you have a mod freq of 5kHz with a freq deviation of 75kHz, the mod index will be 15. From Bessel functions table the required bandwidth will be 2 X 19(pairs of sidebands) = 38 X mod freq = 190kHz.


Pulse Modulation is used together with the "Vernitel" method to reduce overall bandwidth. In this method pulses AM modulate a subcarrier which then FM modulate the main carrier. The channel data is split into 2 signals high and low, each signal is transmitted over a seperate channel having a smaller total-bandwidth than the original data channel.

 

[vco96]

If you are sending digital data or pulses, you don't need the input low pass filter. Keep R3 and get rid of C10, make C7 a .01 uF. By applying 5V at the varactor, capitance will change a lot, so you must reduce the inut amplitude. How much frequency deviation you need for 0 or 1?Checking out FSK modulation (frequency shift keying) in google or yahoo.

good luck.