Harros said:Well, I have finished the RF amplifier design, it seems working well (with a gain of approximately 30dB). It seems the tank filter greatly improve the gain of the amplifier... What do you think about this amplifier?
However, there is a problem where the output of the amplifier for the signal with nano volt range is not desirable as the output waveform (the waveform is perfect) is shifted up and down in simulation. It seems its due to input coupling capacitor as I try varying the value of that capacitor and investigate the result waveform for the 1uV-range input, the waveform shifted up and down (similar to the above-mentioned situation) when i adjust the capacitor with the value smaller than it supposed to be... What do you think?
Harros said:I have redesigned the circuitry to lower the resistance of the voltage divider. I tried to tap the 50 ohm load about 10% across the tank circuit ( i mean i tap it with the tapped value in the previous circuitry up side down), the gain was however greatly decreased (to 0.18dB...), and the waveform is shifted up and down... I wonder why this happens...
By the way, i am planning to add a power amplifier to the output of the crystal oscillator to boost the transmitting power of the transmitter to 20dBm (100mW) if possible. The oscillator output swings around 4 volts... Do you have any suggestion on the type of amplifier that i can build?
RadioRon said:Oh, I see why that happens. You have the 50 ohm load DC coupled to the tank circuit which messes up the bias very badly. You need to put a 100 nF cap in series with the output load to block DC.
There are many types of amps you can use. I will consider and repost
Harros said:There is no gain improvement and output improvement even the 100 nF cap is put in series with the output load, unless the load is tapped at 90% across the tank circuit...
RadioRon said:Your bias design is the type that insures that HFE variation does not significantly affect the operating point, so you don't need to check each transistor.
It is common practice to use the application circuit suggested in the data sheet or application notes from the manufacturer. This is especially true for ICs with complex functions such as this one.
I find it a bit humorous that you say the amplifier works well. Of course I understand that you mean in simulation only. In my experience, something that works well in simulation does not necessarily work well in practice, especially at RF frequencies. The simulation does not account for stray coupling, non-ideal characteristics of components, stray inductance and capacitance, high impedance grounds, magnetic 120Hz induced noise, power supply noise, AM broadcast radio interference, changes in component values due to heating, bad soldering, undesired feedback, and a few other things. The simulation is extremely useful to get the basics worked out though, so it is still a valuable step in design.
Harros said:Well, what should I do next? Should I try implementing this amplifier circuitry on the PCB to see the result? By the way, it seems so complicated to have transistor in amplifier design. Is there other option for me to build this amplifier? Besides, I have limited budget on this project...
So, should I implement the BJT transistor amplifier circuitry on the PCB for result and troubleshooting? What if we implement the op-amp in the amplifier design? Do you have any suggestion on the op-amp that we may use in the design? By the way, I am running out of time... (5 weeks to go before presentation)RadioRon said:This is the kind of amplifier design that gives you the most design flexibility. You could compare this level of design with programming a computer in assembler. You have to deal with many details that a higher level language shields you from, but on the other hand you can do anything imaginable and with much higher execution speed.
There are easier alternatives. You can use a pre-designed amplifer like an RF gain block IC. Or you can use a CMOS inverter with added negative feedback.
There are many RF gain block ICs available. These are ICs in which they have done all the work for you and you only need to make A simple pcb with a few components and it will work. Here are some examples:
http://www.minicircuits.com/products/amplifiers_monolithic.html
http://media.digikey.com/pdf/Data%20Sheets/Avago%20PDFs/MSA-3111,%20MSA-3186.pdf
**broken link removed**
These kinds of amplifiers have much much more bandwidth than you need which can be a danger. Because the bandwidth is so large, you have to design your circuit with a layout and parts suitable for 1 GHz operation so that the amplifier doesn't oscillate.
I have seen people make amplifiers successfully at 1 MHz using simple CMOS (HCMOS) logic gates. If you put a feedback resistor from output to input, these gates will operate in a linear way and make an easy amplifier. You still would have the problem that the gain might be too high and the amp will oscillate. The lower the feedback resistor value the better I think.
Harros said:Thank you for your advices, I know what should I do now.
For the rx ferrite loop, I am thinking of making it to be a single block, where terminals from primary loop are connected to the tuning capacitor, and the terminals from secondary loop are connected to the BJC connector. Is there any thing that should i pay attention to?
My rx rod antenna is about 1 meter high, is it too big for the buffer amp?
Harros said:Well, I am thinking of providing the power to the Phase Detector using a power supply, thus I have made changes on the diagram... Do I need to add extra capacitors to the power supply part as what have been done in previous design (as shown in the picture below)?
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