QO-100 Experiments – New QRP Transmit Up-converter

New Up-converter and SynthShield LO

I started out on QO-100 using a small home made transmit up-converter built around a number of modules available from eBay. Apart from some failed mixer boards this setup worked well for a period of 18 months or so. I have to confess that the first time the mixer failed it was me, pure operator error… I forgot to reduce the drive level after a 144MHz QSO!! Having learnt that lesson the hard way, things worked fine for the best part of a year, then another mixer bit the dust!! This happened shortly after I decided to change from using the K3 + Anglian transverter to using the TS-2000. I’ve owned the TS-2000 for a very long time but had never used it in Satellite Mode. I took great care to set the power level to the minimum 5W and also added a large 10dB inline attenuator, and as the TS-2000 was dedicated to this new QO-100 role I knew that it wouldn’t be operator error a second time. The new setup performed extremely well for just over a week before another mixer problem occurred. Further investigation and tests highlighted the infamous TS-2000 ALC-overshoot was most likely the cause of the early failure. As I didn’t have a spare mixer board I changed my setup to use the Adalm Pluto with SDR-Console for both receive and transmit. This has worked extremely well for the last few months and makes a very nice working setup.

Even though the Adalm Pluto was working so well I still had a desire to use the K3 or TS-2000 as the prime mover on QO-100 and to keep the Pluto for terrestrial operation / experimentation or for measurement purposes using the excellent SATSAGEN software. This along with my interest in KiCAD and experimentation got me researching ideas for a new transmit up-converter.

The design criteria for this up-converter was nothing unusual, IF input of either 144 or 432MHz. LO based on the ADF4351 SynthShield and the frequency switchable to suit either IF. RF output on 2400MHz with power in the region of +10dBm (that’s more than adequate to drive my existing transmit setup). To be built on a 100mm x 100mm PCB to stack on top of my existing QO-100 downconverter.

New Up-converter (top) and down-converter (bottom)

 

Yesterday the build reached a point where I was able to test the up-converter. I built it one section at a time. First the IF section which allowed me to validate the level of 144MHz input at the mixer pads. Then the LO section and again to ensure that the level was correct for the type of mixer it was measured at the mixer pads. The mixer was then added and finally the RF section completed.

The final output level was lower than what I’d expected but it’s perfectly acceptable and far more than the output from the Adalm Pluto and what is needed to drive my remote driver / PA stage. I suspect the lower output is due to my use of a two-layer FR4 board and use of standard SMD capacitors rather than high-Q capacitors in the final RF section. Further experimentation with capacitor values and positioning could possibly improve things.

I used the Adalm Pluto and SATSAGEN software to look at the output and was pleased to see the LO at 2256MHz was approximately -55dBC. With the additional pipe-cap filters in the driver stages to my PA I’m confident that any unwanted mixer products will be better than -60dBC.

New Up-converter output being checked using Adam Pluto and SATSAGEN

 

Once these tests were completed I connected the up-converter into my system and ran a few live on-air tests. I was quickly called by Ron, PP2RON who confirmed the signal was sounding okay, this QSO was then quickly followed by two more both confirming all was well.

The next step is to house the new up-converter and down-converter in a suitable enclosure, this will mean more metal bashing, the part of a project I least enjoy.  🙂

 

Lessons learnt:

  • I need to investigate using KiCAD to design four-layer boards. Moving to a four-layer board should improve things and bring the RF lines much closer to the desired 50 ohm impedance.
  • When ordering parts I quickly realised just how real the current global components shortage is. Several of what I would consider to be common items were not available and on backorder with very long lead times.
  • I was amazed at the quick service provided by JLCPCB and Mouser. It took 12 days from the time I uploaded the PCB Gerber files to having the completed boards in my hand (and this was using their economy air service) and 48 hours from placing an order for components on Mouser UK/US and having them in my hand. I should add that I have no relationship with either of these companies other than being a satisfied customer.

 

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