Saturday, March 6, 2010

Progress Made on 1296 Rig

Well, we figured out the problem with the 1296 rig, but I'm unsure how to fix the problem.  Apparently there has been no oscillation problem all along, the local oscillator chain is producing harmonics that are causing some strange mixing products.  We had a strong output on around 1250 mhz that was always at the output anytime the transverter was keyed.  This signal goes away when the oscillator is detuned so that it stops oscillating, but is always there anytime the oscillator is going.  The signal levels are equal to the desired 1296 signal when driven by the IF rig.  So when the rig is running, I had an output on 1296 and a carrier at 1250 (or thereabouts).  We traced the problem to a dirty LO chain. 

We were able to clean it up substantially by inserting a 3 pole BPF between the 10 mW transverter output and the next amplifier stage.  The correct way to fix this in my opinion would be to incorporate another of these filters in the output of the LO chain.  We didnt get that far with it today though.

When we hooked up the 2W PA, there was no output from the amp, and we didnt have time to troubleshoot it.  It was working when we began testing, so I am guessing it is something simple, such as a short in the RF path somewhere.  Since the TX is now clean, I can now troubleshoot the amp and try to maximise the output power.

Another possible problem with the dirty LO is that of receive performance.  With a dirty LO, the receiver will be receiving on 2 different frequencies, cutting the gain down by 3dB (or more) on the desired 1296 MHz, and also increasing the noise by 3 dB (or more),  I never really noticed a receive problem with the exception of the RF stage oscillating, which was a confirmed problem.  So, with that discovery, work can begin cleaning up the LO, and eventually I should get a 1296 rig on the air , hopefully before summer.

Wednesday, March 3, 2010

1296 Transverter Progress

For quite awhile, I have been battling a low power problem with an older circa 1995 Down East Microwave 1296-144 transverter.  At first I thought the PA was blown, I removed it and installed an outboard PA with no success.  Eventually I replaced the entire transmit chain with still no improvement.

Most recently, I replaced the mixer IC.  The original mixer chip is no longer made, however a newer model which spec's somewhat better than the original was installed.

I never reinstalled the PA module.  I am going to do some testing very soon with the +10dBm output from the driver.  If everything checks out, I'll continue to use the outboard 2W module, then feed that into the 30W PA.  Keeping the rig this way, in my opinion would be more versatile as far as making improvements and upgrades on the circuit.  Because everything is interconnected using UT-141 copper hardline, things such as bandpass filters and amplifier replacements can be more easily tried.

I am hoping to put the rig on a spectrum analyser and power meter to determine if the transverter is working properly.  If it is, I plan on operating some 1296 up on the Lake, as well as possibly some mountaintop operations this summer.  Stay tuned to my blog, I'll report here on the performance once I am able to make the proper tests.

Softrock 6.3 RXTX with USB Control

I ordered and built the USB I2C controller board for my Softrock 6.3 RXTX.  Adding this option opens up a whole new world for your Softrock.  Not only does it allow you to tune the entire bands, but also allows you to have general coverage receive capability with the rig.

It also allows external programs to take control of the radio.  This is REALLY cool, as you can run Powersdr in the background and use software such as fldigi and WSPR to actually control the frequency.  Fldigi logging works great too, as it can read the frequency of the radio and it puts that into the log automatically.

Another added feature is you no longer need the serial cable to key the transmitter.  This is all done now via the USB port.  Of course, you still need to set up virtual com ports so the additional software can communicate PTT to Powersdr. 

I have been using PowerSDR v. with this new setup.  This version has been designed for use with CURRENT firmware in your softrock, and it works quite well.  I find the audio is less choppy with this version as well, and it's a lot prettier on the screen with its customizable skins.

Upgrading your Softrock with this little $11 board is more than worth adding the option. 

On a sidenote, the other night I thought I would try some digital communications running barefoot 1 watt from the radio, and worked a fellow in Arkansas using Olivia 500/16 on 40 meters.  I was running 1 watt, he was running 40 watts and was using a Flex-5000.  We maintained contact for 1 1/2 hours, talking about SDR in general.  Was a real fun QSO.  I'm looking forward to working my first Softrock to Softrock rig, as I think that would be quite interesting.
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