Electronics > RF, Microwave, Ham Radio

Up/Downconverter issues

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bryan.mitchell:
Hi all,

I am having some issues with my up and downconverter. What I'm trying to do is to receive a 125 kHz signal, upconvert it to 433.125 MHz, transmit it, receive that signal, and downconvert it back to 125 kHz. I have 2 circuits which are shown in the attached images. I have verified that the mixers are working properly but there's something wrong going on and I can't seem to know what it is. Somewhere along the way the signal either isn't being transmitted or received properly, and I'm in the process of figuring out where. I would like to get some pointers and also to see if there is something wrong with the design of the circuit that I have missed.

Thank you

edavid:
The FS1000A doesn't have the frequency accuracy or stability to be used this way.  I don't know if that's the only problem.  You can try using the same FS1000A as the LO for both mixers, and see if the rest of the circuit works.

PA0PBZ:
"There is something going wrong" is not exactly a problem description we can work with.
What exactly are you trying to do? I can see RFID in the schematics, are you trying to extend a RFID reader? You know that you need a duplex link for that?
I can also see some problems with your mixers, for instance you are producing 2 signals, LO+IN and LO-IN without filtering, and filtering 100KHz @ 433 MHz is almost impossible.

uncle_bob:
Hi

What are the values for the components used? For instance, are L1 and C1 taken from the Linear app note?

What sort of level are you getting out of the "RFID Loop". If it is -100 dbm, the system you have will not transmit much of a signal.

Bob

lostengineer:
hello everyone, i'm working with the OP on this project and I have some more info to post. Thank you for any help! We are both novice engineering students and are pushing the limits of our RF knowledge with this project.

The goal of the project is to take a 125kHz signal (same format as RFID but NOT directly RFID) and upconvert it to a 433.125MHz signal. This means that the LO is 433MHz. The up-converted signal will be transmitted about a hundred meters and down converted back into the same 125kHz signal in order to excite a 125kHz receiver.

As I said, it is not directly RFID. What I mean is that we do not need to get a response back from the end receiver - so a duplex system is not needed. We chose 433 as the LO because it seemed to have good range and was easy to work with (I don't think we need to do too much impedance matching or special PCB traces at this frequency - but I may be wrong). Otherwise, the LO freq. choice was arbitrary.

We have loop antennas for 125kHz built and they appear to be working fine. We are trying to use the FS1000A simply as a signal generator / LO source because it was cheap and easy. It's just a SAW stabilized Colpitts oscillator I believe. After testing we came to the same conclusion as edavid did. Multiple tested FS1000A's all gave us different frequencies within about 1MHz as shown on a spectrum analyzer. With this knowledge we did connect one FS1000A to both the up and the down converter. This also did not work. Can anyone recommend a better circuit or pre-built module to generate a LO @ 433MHz?

I believe the up-converter circuit is working. When powered on we get a nice spike on the spectrum analyzer around 433. As soon as we move our 125kHz loop antenna next to an RFID reader module we see two new spikes pop up on the spectrum analyzer - one at 433.125 and one at 432.875. I will post a screenshot of this shortly. As PA0PBZ said, this is the correct response - a signal at LO+IN and LO-IN. Also, it is true that we are not filtering anything at the moment.

I was hoping the down converter would be able to receive the LO+IN signal while the LO-IN signal was just lost / unused power. We're not seeing the original 125kHz signal at the output of the down-converter mixer. This is our problem.

My best guess at the moment is that the down-converter mixer needs a more powerful input signal - possibly a LNA inline with the antenna instead of just a wire antenna at 433MHz?

My next best guess is that the op-amp circuit on the down-converter output is not working properly. The purpose is to make the down-converter mixer output from differential to ground referenced as well as amplify. This circuit was copied directly from the LT5560 datasheet under the low frequency application example. 99% of component values used in the design were directly from the LT5560 datasheet.

I'm working on getting real measurements (I will post scope & spectrum analyzer screenshots in a bit). For now, is there anything fundamentally wrong with our RF design?

Thanks again!

- lostengineer (KJ4AWM)

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