Hello, I built an upconverter based on this schematic -

I'm using the SA612A mixer, here's the datasheet - http://www.nxp.com/docs/en/data-sheet/SA612A.pdf?

I'm trying to upconvert 0-30MHz into the VHF range, where my receiver can pick it up, I'm using a 80 MHz oscillator, because 100MHz ones are not available locally.

The only thing I'm getting is a massive signal on 80 MHz and nothing else. There is FM broadcast bleeding through (probably after the mixer)

I'm fairly new to electronics, I'm looking for any ideas on how to troubleshoot the board.

 

Hello,

The capacitor C14 and coil L4 make a tuned circuit of 100 Mhz, to suppress the 100 Mhz.
When you use 80 Mhz, there will be no suppression of that frequency.

Bertus

 

Thanks, bertus, that makes sense.

I understand this board was designed for 50 Ohms, yet both my coax and my receiver are 75 Ohms, is this fatal in terms of signal loss?

I'm now seeing only some of the stronger FM stations upconverted (obviously bleeding through after the filter?), but not much else.

Any ideas on what else I could try?

 

Hello,

The 75 Ohm coax will have some influence, but it will not kill all the signal.

How did you construct the converter?
Perhaps you can get some ideas from the PDF in this post:
https://forum.allaboutcircuits.com/threads/rf-related-links.22011/#post-623642

This is one post of the thread I posted some time ago:
RF related links

Bertus

 

It's constructed on a protoboard, similar to this - http://www.elecfreaks.com/store/images/PDBM_12.jpg
http://www.elecfreaks.com/store/images/PDBM_12.jpg
I've changed L4 and C14, which did decrease the signal strength on 80 MHz and its harmonics, but it's still there.

Is it possible the strength of the LO leaking though is a problem?

I've also tried bypassing the Cauer filter and connecting J2 SMA directly to C2. I noticed FM broadcast and other VHF signals get upconverted quite nicely, but nothing is being upconverted from the 0-30MHz band.

I removed L5 and made a separate bias-tee to power my PA0RDT. I've tested the antenna and its bias-tee on another HF receiver and they work quite nicely.

 

Hello,

Did you see the remark on the webpage, you likely got the schematic from?
"Applying both points from above will result in full double-balanced operation (this is what NE612 is intended for) and will lead to significant LO feedthrough reduction, so no more crappy and hard to tune L4-C14."

http://mightydevices.com/?p=494

Perhaps you could also try an upconverter using a double balanced ring mixer like the SBL-1:
https://www.george-smart.co.uk/projects/funcube_upconverter/

Bertus

 

Hello,

Did you ever try to listen to the HF bands using WEBSDR?
Here is a dutch site that allows you to listen from 0 to 29200 kHz:
http://websdr.ewi.utwente.nl:8901/

Bertus

 

You might not have realized that when bertus said:

Hello,

The capacitor C14 and coil L4 make a tuned circuit of 100 Mhz, to suppress the 100 Mhz.
When you use 80 Mhz, there will be no suppression of that frequency.

Bertus

He was hinting (very strongly) that if you adjust the value of C14 or L4 so that they resonate at 80 MHz that will reduce the amount of 80 MHz in the output.

While you are at it you might want to adjust the value of C17 or L6 so they resonate at 80 Mhz.

 

Thanks for the help, I indeed understood that, but the LO signal was not suppressed sufficiently. I guess it's the finicky air wound coils I used.

In the end I got frustrated with the NE612 and made an upconverter with a diode mixer along similar lines to what bertus suggested. I also used a significantly lower frequency oscillator to avoid the FM broadcast band.

After some trial and error, weeding out noisy power supplies and shielding, it does work, although I still do get some noise and IF images from the FM band.

 

http://mightydevices.com/?p=494