Electronics > RF, Microwave, Ham Radio

Up/Downconverter issues

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uncle_bob:
Hi

Ok, how to design a radio:

1) Take a look at the basic requirements for board layout and component selection at the frequency you have decided to use. Price out the parts and re-evaluate if this is the right band for you.

2) Evaluate the frequency references needed in the design. What frequencies do you need? How stable do they need to be? What power levels do you need? What kind of phase noise do you require? What sort of tuning range do you need? How will they be tuned?

3) Look at the link budget for the radio. You have a modulation approach, at minimum modulation, you have a signal. You put that signal out of your power amplifier at some level (10 watts maybe). That goes into your feed line and you loose something (2 db maybe). It then goes into your antenna and you do or do not get gain depending on the antenna design. It flows through space and you have loss. On the other end you have an antenna with or without gain, a feed line and an amplifier. That amplifier has a noise figure. Your receiver may or may not be limited by the noise figure of the input amp. If it is, the level of the signal at that amp gives you a signal to noise ratio (yes it's a bit more complex than that). That SNR should be something you can work with (say 40 db).  After you add up all the various losses and modulation issues --- do you get 40 db SNR (your goal) or not?

4) Look at your modulation scheme. There are a *lot* of ways to modulate a signal. Each has it's benefits and drawbacks. Some are easy to transmit and hard to receive. Others are hard to transmit, but easy to receive. Some are quite complex on both ends. Does the modulation you are using make sense for the application?

5) Given the design parameters so far, take a look at the power available for the radio. If it is going to run on batteries, will it run long enough with a practical battery pack? If it plugs into the wall, can the wall socket supply enough power to do the job?

6) Take a look at the possible antennas for the radio. Are they of a size that makes sense? If they are directional (directivity = gain) how will they be steered? Can they be designed with the tools you have? Will you use an off the shelf antenna?

Once you have that stuff worked out, take a look at the parts you can use. Do a trial design and see if it meets your cost goals. If not, look some more or change the goals. Design and evaluate each section of the radio. Compare it against the system requirements. Once you have a fully compliant design, look into the rules and process for RF or microwave pc board layout and design. PC boards are about the only practical way to get the job done with modern parts at the frequencies you are considering.

You are now out of the basic design phase and into the "build and test" phase. You likely will loop back through the design phase a few times before the whole system is up and running.

(Yes, the above is far from complete, it does hit enough of the points that you can get an idea of what is involved).

Bob

vk6zgo:

--- Quote from: lostengineer on July 14, 2016, 04:31:32 pm ---ok wow. first off thanks for the help everyone! A bunch of stuff is starting to click.

So, I believe most of you understand what we're trying to do, but I'll clarify again so no assumptions are made. Please let me know if anything isn't clear. We're not trying to build a whole radio or receive and decode a signal at all - but maybe we must to make this work. We already have a system that we are trying to extend which generates a 125kHz data signal much like an RFID reader would. This system comes with a good receiver for this 125kHz signal which works within a few feet of the reader/transmitter as it is a magnetically coupled signal - not yet RF I think. No response to the 125kHz signal is produced by the end receiver....at least at 125kHz.

We also do not know or care what the modulation or data is in the originating 125kHz signal. This is a design goal. We want to transmit it as is and have it appear on an identical 125kHz antenna at the receiving side - simply increasing range of the signal. This receiving side antenna - the 125kHz loop antenna on the output of the down converter - will then excite the true receiver which came with the system.

Tomorrow is the next time I'll be working with the hardware so further testing will have to wait until then. We did try to connect the output of the up-converter to the input of the down-converter and the whole system appears to be working (albeit barely)! We even detected a proper output signal with the up and down converters about a foot away from each other with only antennas! I'll post screenshots of the spectrum analyzer tomorrow.

As for improvements:

1: We know the LO is a huge issue. We will be switching to a TXCO based design as recommended by uncle_bob. I don't think we need ridiculous precision but i'd like to be within a few kHz maybe?  Can you guys recommend a TXCO circuit which would work well with our mixers? I'm doing research on this now...

2: Power - bryan did not post our entire circuit design, but I figured transmit power would need a boost on the up-converter 433 side. We have already designed a RF power amplifier for this based on a reference design for the RF2126 amplifier IC. I have a bandpass filter on the output of this amplifier. This has not been prototyped yet. I'm posting a more thorough schematic.

   On the down-converter side, we'll add a matching band pass filter on the input followed by a LNA before the input to the mixer.

3: Impedance Matching - Quang says we need it. I think we can make all signal traces on our PCB less than 19mm as he recommends. The whole PCB is less than a business card in size and we have not finished their design. I mean this stuff is small... the LT5560 mixers are 3mmx3mm DFN's - that was a bi**h to solder. 

All the passive parts & their values were taken from reference designs for 433MHz or 125kHz respectively from the LT5560 datasheet / RF2126 datasheet. As you can see from the newly posted schematic, I added a bunch of extra attenuators, solder bridges, and connectors for aiding in testing. I'm not sure we need all of that, but it doesn't cost anything extra to add pads to the PCBs so why not?

I'll update tomorrow with more testing screenshots. Thanks!

--- End quote ---

You are reading,but not absorbing !

Firstly, even though you insist you are making "up" & " down" conversions,that is really not what you are doing.
You are taking a "baseband" signal (your 125kHz) & modulating a 433MHz carrier,amplifying the resultant AM signal & transmitting it 100m to a Receiver,where it is demodulated,so you get your 125kHz back.

In concept,this is the same as "up" & "down" conversion,but in practice,it presents different problems.

Normal "up converters" mix a signal which differs in frequency to the LO,but not by such a huge factor.
For instance,a PAL TV transmitter vision IF signal will have the Vision Carrier (Vc) at 33.4MHz.

In order to present a "onair"  Vision Carrier at,say  189.0MHz,the LO will be at 222.4MHz (189.0+33.4MHz)

OOPS!,I messed that up in my attempt to keep it simple! :-[

The IF carrier at  33.4MHz is,in fact the Sound  Carrier ( fs),the vision carrier (fv) is at 38.9MHz,so for an "on air" Vision Carrier at 189MHz,the LO will be at 227.9 MHz.

I stuffed up the next bit,too,but the overall argument is still correct.

The LO & the other  conversion product (189.0-33.4MHz=144.6MHz) are far enough away in frequency that they may be easily filtered out with normal LC tuned circuits.
The other product from this conversion will,in fact,be 266.8MHz.

A conversion product (LSB)at 432.875MHz,or (USB)at 433.125MHz cannot be simply filtered,if at all.
For this reason,when the baseband signal is very much lower in frequency than the LO,the process is regarded as that of modulation,rather than frequency conversion.

By the way,if you decide to stick with AM,your LO stability requirement can be less strict .
The Receiver can be a simple diode detector.
Using such a detector with an AM signal no Receiver LO is needed,as the difference frequencies between the sidebands & the carrier are at the original baseband frequencies.
If your Transmit LO drifts,the sidebands drift with it,maintaining this relationship.

Please do some real reading on Radio Transmission & Reception----App Notes simply do not hack it!
Your Uni will have a Library,& in that Library will be books written by Radio Engineers,& probably some by Radio "Hams"
 too--very often the same people.
If you can find an ARRL Handbook or an RSGB Manual,you will pick up a lot of useful concepts that a few paragraphs on this thread cannot convey.

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