Hi,

I am starting to explore the RF world and superhet topologies. I want to build or at least design a tunable, (lets say 400 MHz-4 GHz) RF receiver. A major problem that I face is how do you make an LO suitable for such project? Internet is full of ,,sine wave generator" circuits using opamps, discrete component VCOs etc. All those circuits (as I imagine) are capable of 10s of MHz, but then distortion, pin inductivity and other nasty non linear effects start to kick in. On the other hand, I could not find anything real by typing ,,local oscillator" into the component dealer`s search bar... What would be the professional approach, how do you build or design a proper LO by yourself, are there any alternatives? Thank you in advance.

 

That's a tough one. Today you might use something like this:
https://www.analog.com/en/products/adf4351.html#product-overview
https://www.analog.com/media/en/technical-documentation/data-sheets/ADF4351.pdf

 

I am starting to explore the RF world

Then starting with lower frequencies would be approriate.

 

That's a tough one. Today you might use something like this:
https://www.analog.com/en/products/adf4351.html#product-overview
https://www.analog.com/media/en/technical-documentation/data-sheets/ADF4351.pdf

That chip looks really promising... I am just not sure about the waveform, as far as I know proper LO should output pure sine and from the description of the mentioned chip, the output is generated from a VCO (most likely, the waveform would look something like a sawtooth...) Is that a consideration? Or such frequencies are not affected by the actual LO waveform?

What are the alternatives if I decided to stick to a fixed frequency LO, what frequency should I choose (keeping in mind 400 MHz-4 GHz RF signal). What approach should I take: crazy expensive OCXO, some discrete element VCO, or anything else? Thank you again.

 

I think the recommendation to start with lower frequencies is a good one. Learn the circuits and techniques at much lower frequencies and then move up, perhaps a decade at a time, and get a handle on the new demons that pop there head up at each stage.

 

I think the recommendation to start with lower frequencies is a good one. Learn the circuits and techniques at much lower frequencies and then move up, perhaps a decade at a time, and get a handle on the new demons that pop there head up at each stage.

I understand what you are saying. I am not asking for a complete RF transceiver circuit, but I still need at least a hint on what I asked previously...

 

Start by making up a lower frequency receiver as has been recommended a few times above. Then, add down converters for the higher bands.
What are you actually trying to receive? The answer to that question can have a significant role in the base band receiver design.

 

Also note that a frequency range of 10:1 for a single receiver is outrageously difficult. Note that the AM band is about 3:1 and FM is just 1.2:1. There is a reason for that.

 

Start by making up a lower frequency receiver as has been recommended a few times above. Then, add down converters for the higher bands.
What are you actually trying to receive? The answer to that question can have a significant role in the base band receiver design.

My goal is to make a wide band audio receiver. I know that 400 MHz-4 GHz covers a lot of communication channels, and I will not hear anything audible in most cases, but I suggest you leave all the other questions and advices to me, Ill find it out by myself. For now, I need advice exactly on LO selection and implementation.

 

but I suggest you leave all the other questions and advices to me, Ill find it out by myself

Well, why do you ask for advice?
There are many folk on this forum with vast experience that are willing to help answer your questions. Then, I suppose it is up to you to ignore the advice if you want.
As has been stated, to make a 400Mhz to 4GHz receiver is no easy task. It is not something I would attempt.
And

My goal is to make a wide band audio receiver

does not answer the question. There is no "audio" in that band. There are various modulation techniques that may carry an audio signal on the RF carrier.
What electronics design have you done? If this is your first electronics, it is way too complex for a start.
Stability will be a major problem, amongst others.

 

First, work out a block diagram of your radio receiver.
What IF frequency are you going to use for instance.
Will it be a double conversion superhet? (or more)
What de-modulators will you require?

 

My goal is to make a wide band audio receiver. I know that 400 MHz-4 GHz covers a lot of communication channels, and I will not hear anything audible in most cases, but I suggest you leave all the other questions and advices to me, Ill find it out by myself. For now, I need advice exactly on LO selection and implementation.

https://www.poetryfoundation.org/articles/69379/an-essay-on-criticism

Fools rush in where angels fear to tread is a proverb that means inexperienced people jump into situations that wiser, more experienced people avoid.