[SOLVED] Problems when trying to prototype RF circuits

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Thank you, I am going to look into them. We use Tektronix at work so I am familiar with them.
 

Okay, I have got one working finally! Though it is not very clear and there is a lot of static ( I have not hooked up the varicaps to test yet, only a variable inductor ) it is the step in the right direction. I have implemented all the changes you made and it seemed to work out. Though, like I said, I am not using 2N2222. I didn't even know you can use that transistor for RF, thought it had to be an RF transistor to work. I'll try and connect PLL later and see if I can get that working. I'll let you know. You've been a big help and I thank you! Attached is the semi working prototype on a cleaned PCB.
 

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electrojosh

you really cannot play with RF circuits without a frequency counter. It should be one of the first things on your "to get" list
you need to be able to determine that the oscillator and final transmitter freq are what they are supposed to be

Dave
 

The nice thing about these old Tek 465, 475 series scopes is that they have a channel 1 or 2 output on the back to feed a frequency counter. So whenever you scope around you can instantly see the frequency as well. They somehow decided to drop this feature on the 350MHz 485 models.

It will really help if you can see the output level on the buffer to select the right coupling cap value and resistor if needed. Try and use a 10pF coupling cap without the resistor for now until you can measure the actual output level.

The RF transistor you mentioned should work without any problems. Make sure you don't have the emitter and collector switched around.

Can you measure your DC values for both conditions like in my example?
 
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I did measure the voltages at the points that you measured them and they were very close to your voltages for both conditions. I did remember I had a little handheld freq counter and rf power meter that was given to me by one of the electrical engineers i work with gave me, might do the trick.
 

Okay, took the new circuit that I built and hooked it into the PLL, works beautifully! Very clear and very stable. Don't know the range yet but that's okay. Thank you E-design for redesigning and giving me the new schematic with your results.
 

When you get a scope, play around with the value of the coupling capacitor to give you more output, but keep it from clipping or getting peaky as that will increase the harmonics that may interfere with other frequencies or services.
 

I cannot tell, but do you know the power output in this oscillator. I plan on adding in an amplifier but just want to know for reference.
 

It would be low, under 10mW when connected to a antenna load.

One more thing, the current circuit will suffer from modulation sensitivity. Basically, the received audio will be louder at the bottom end of the transmit frequency band and get softer as the transmit frequency increases. You could add another set of varicaps to make sure the modulation sensitivity stayed more constant over the band. If you are planning to keep it only at one frequency, then it will not really affect you.
 
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Well, right now I can not get it higher than 100 Mhz but that is fine. Got a circuit that can boost it up to a few hundred mW? Where would I add the varicaps?
 

You could modify the circuit as below. You can make the 1nF much smaller. That will enable you to get to the higher frequencies.
 

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Okay, One last question and I think I got this thing all built up. If I took just the VHF Amplifier off this oscillator, https://electronics-diy.com/tx200.php, would that boot it to at least 200mW? Granted not using the BF199 but the same transistor I have been using.
 

I am not so sure it actually makes 200mW into a 50Ω or 75Ω antenna. Maybe between 20 to 50mW? You don't need a lot of power when using a good antenna to cover your property. You can get out quite far on just 10mW. The FCC has determined that you can transmit about 200' with acceptable noise ratio, on just 12nW using a simple 1/4 wave antenna.
 

Nope, just using a 28" telescopic antenna from radioshack.

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Also, how would I measure the output impedance of this oscillator so I could better match it to an antenna or an amplifier. I am mainly just building this thing for fun. Or what is the impedance of this thing. Sorry, not an engineer. Though I work with them to build what they design so I have a general theory of how things should work but when it comes to RF, whole 'nother ball game.
 

Also, how would I measure the output impedance of this oscillator so I could better match it to an antenna or an amplifier.
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I presume you don't have the means to measure the impedance directly, and even if, it probably won't work. Because varying the extracted output power also changes the impedance.

The usual way is to estimate the output impedance from circuit parameters and transistor data, design a matching network (which turns out as CLC pi filter in most cases) and tune it for maximum output power. The procedure is called load-pulling in RF engineering.

Another problem is to know the antenna impedance. Tuning the matching network for a nominal load, e.g. 50 ohms is almost easy, tuning with an empirial antenna is involving additonal issues. A telescopic antenna rod isn't a well defined antenna because it misses a sufficient ground plane. So everything you connect to the circuit or even things placed in it's vicinity (e.g. the operator) become part of the antenna and modify it's impedance.
 

Okay, how would I find the impedance or at least close to it of this circuit? I have already found some stuff online about the LC pi filter and even some quick calculators for them. I would like to better match this oscillator to an amplifier or antenna. Also, I am using the SS9018HBU RF tansistor for both sections
 

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As a first estimation, the output impedance is 470 ohm || some pF capacitance, at least if the transistors fT is sufficient above working frequency. The capcitance can be absorbed in the matching network.

You can measure the output impedance in an AC simulation by feeding an AC current to the output, by the way.
 

Another question is if the amplifier is dimensioned for the intended output power. 470 ohm collector resistance limits it to a few mW from the start.
 

You talking the 470 ohm resistor that is at the collector of Q2? Could I drop that for a better output power and decrease the impedance of the output. Just trying to raise this little oscillator to at least 200 mW and would like to keep everything matched nicely. https://electronics-diy.com/tx200.php I'm just trying to use the 200 mW output section around Q2 in this circuit and build it for this oscillator
 

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