My 1296 Transverter

As the HF bands seem to have gone a bit flat of recent, I decided to have a go at 1296 MHz. I had the opportunity to buy a Microwave modules MMC1296 RX converter as well as a DF8QK TX mixer board.

I was thinking that I had the bare bones for a transverter if I could pick off enough local oscillator level to drive the DF8QK as well as the existing RX section. Mentioning these ideas to Kevin ZL1UJG, he mentioned that the LO from the MMC1296 is pretty dirty with spuries etc. He was saying that was better to use a clean LO like VK5EME EME65 kit and double it.

The Local Oscillator
OK – I needed the output from the LO / Doubler to be at 1152MHz. With a 96MHz crystal from the MMC1296 and a Butler oscillator design as in EME65 and a doubler board from Kevin, I preceded to get this bit running first.

The Doubler Board with SMD BFR92 and ancillary componenets at left..

I etched up a board for the LO and it didn’t take long to get it running. The coil I used was a Toko unit available from a surplus shop here in Auckland. As I understand it – they came from a Plessey product. The ferrite slug in it seemed fine at this frequency and no mods to resonance were needed. 

As an aside, one can check for “free air” resonance in this Butler design by swapping the crystal for a 47 ohm resistor and the osc will run at its own frequency. This I a good thing to do as I found that the reliability to start was an issue if off the chosen resonance. The unit seemed to “snap” in and out of oscillation if too far off.

One thing I cannot stress enough when making SHF gear is the need for  “shielding   shielding  shielding!” 

When the LO is multiplied up by a large factor, any small shift in the fundamental frequency will be magnified greatly. I found still even touching the case of the LO / doubler was enough to cause a shift that was noticeable in SSB.  Earthing the crystal case to the PCB top groundplane is well worth doing.

On the specan I could get about +11dBm out (12mW). I used a Wilkinson divider to do a power split. Essentially this is 2 quarter wave sections of 75 ohm coax (RG-179) that terminates in twice the characteristic impedance of the loads. It is a Y junction with 100 ohm chip resistors joining the two end of the Y together.  The Power Splitter

RX Mixer
After the power divider, I was able to get +4-5dBm for the RX mixer. This MMC unit has a diode ring mixer that then has an IF amp with a dual gate FET. My unit had been butchered around a bit and had the IF amp bypassed.
First thing was to put it back to original and start from there.

The LO was fed in to the existing LO input on the top side of the board as I just had to pull out the wire link feeding from the bottom side. This easily disabled the feed from the old LO.

The MMC1296 RX Converter with LO feed at mid left..

Feeding the generator in the Diode Ring mixer, gave me an RX sensitivity of  about   –100dBm at 1296.2MHz, it was a start but still a bit deaf for my liking.. After having sorted out my frequency stability issues  with the LO as mentioned above, I then thought about a preamp. The simplest way and quickest for me was to use an ERA-2 MMIC  for some gain and post filter it with another useful little filter board ex ZL1UJG.  Fifteen minutes later after rounding up some parts in the shack, I now  have   –135dBM input for a signal out with about 4 S points on the little FT817.
This is a quick and dirty way of getting some more RX gain as the MMIC has about 6dB noise floor. Later a GaAs FET maybe introduced. Anyway – it got me on the right track.

The DF8QK TX Mixer.
After building up this board and setting up the correct DC Bias conditions, I then set about following the procedure to align it.

I found that I didn’t have enough LO drive for the TX mixer  so another ERA-2 device  has  been inserted in the line here to get up to about +17dBm.

The generator was handy here to dial up the correct output at 1152MHz  and then get the correct current flowing  through the balanced mixer. I think I really needed +15dBm to get what I want but I am getting +17, which should be OK.

After tweaking and so on, I can get about 200mW (+23dBm) out of the BFR34 that is running on 12v instead of its recommended 24v.  From this, it drives a BLV93 and so far is putting out about 800mW of clean 1296. I had trouble with the PA taking off and  had to load the input down for it to be stable. This has reduced the output power somewhat. 

 Click here to see a larger Pic
In addition to this I found that the spurie level was high due to the fact that the 2m IF drive was too high. I need only 1mW and the 817 was on its lowest power. The input attenuator is at its lowest setting and all is good.

Currently I think my output is down due to the output trimmer having lossy dielectric.

TX/RX Switching…
This is done by a little micro called a 68HC908QT1, which has 3 outputs, and each is delayed by 100mS. The output order is  1,2,3 on PTT Low and  Releasing PTT the outputs go back the other way 3,2,1.

This project in  itself was  kindly done by Keith ZL1BQE  and I am sure this little chip may find its way into other people’s projects.  Please see The Sequencer  Project in my Projects Page.

The output relay is a Disk smith PCB mount which is mounted right on the rear N connector  by means of a small piece of  double sided PCB  and  PC standoffs. The input relay is off a PABX  POTS line card. It is DPDT and one side is used for RF switching the 2m IF and the other side for the DC supply to the TX and RX sections. The LO runs all the time for obvious reasons.

Switching the DC as needed to the relevant sections just  takes caution against noise or  potential instability issues, not that there should be any of course!

So far , that is it but more will come as the project nears completion.

See the Pictures of the unit top and bottom...

Top Side                  Bottom Side                    Completed Unit

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