Intermistic Repair of Low Cost Spectrum Analyzer.

 I purchased a low cost spectrum analyzer from China.

This is the LTDZ spectrum analyzer that covers a frequency range of 35 to 4400MHz, with some limitations. 

The first limitation is that the frequency coverage in a single sweep is limited to 350MHz. This does make the analyzer more cumbersome to use, but id does make it possible to "see" signals up to 4.4GHz, and that I could not do before.

The second limitation lies in the type of detection. The LTDZ uses a direct conversion approach to the frequency making the signal visible twice (no image rejection) in narrow sweeps.

The analyzer does have different "IF" bandwidth settings ranging from 5kHz to 500kHz, so it makes for a fair, but far from excellent, resolution of signals in the low microwave range. I suspect the dynamic range will be the limiting factor for this device, but a good indication of spurious signals is better than none.

The model I purchased has no tracking generator, but it it in a shielded case and it does have a built-in screen and battery, making it a good portable device. Just make sure there are no strong transmitter signals nearby, if you use it with an antenna - another repair job to do. I speak from experience, I destroyed the input mixer of an LTDZ, just by having a hand-held transceiver transmitting nearby. 

On the casing it says that you should have no more than "10dB" at the input. I have to assume this means 10dBm. In any case, the linearity of the mixer is probably not sufficient to make good measurements at this level, I suspect that the maximum input level for decent linearity is probably around -20dBm, but this will have to be tested.

When the device arrived the ON/OFF switch had been destroyed. I decided that, rather than having the hassle of sending the device back to China and wait for a repair, I would change the switch myself. OK, I do not have such a small switch at hand, so a preliminary, "emergency" repair has been done.

The existing switch has been removed, and a larger switch has been connected with two wires to the solder pads of the original switch. The external switch is attached to the outside of the casing, duct tape is such a good invention (if it was good enough for the astronaut on Apollo 13, it must be good enough for me ;) ).

I now have a working LTDZ.

One thing, however may need to be attended to. When it is running, the battery indicator seems to jump from a good battery level, sometimes to a zero battery level. It does not seem to interfere with the functionality, but it is annoying to see the battery indicator show an empty battery periodically.

The missing tracking generator is not a huge loss for two reasons:

I do have a NanoVNA v.2, working from 50kHz to 3GHz, so I can make filter and antenna measurements up to 3GHz. Also, when I get the software installed, I do have a PCB based LTDZ board with a tracking generator and a directional coupler covering 3.5 - 8.8 GHz (Would likely work nicely on 3.4GHz, too), so filter measurements are not a problem on the 3.4GHz band. With an external signal generator and a wideband power meter measurements of filters and antennas should work nicely on 5.7GHz.

I do have a signal generator covering up to 13GHz, and with another directional coupler (that I have) and a wideband power meter, measuring on the 10GHz band is possible.

So, in the not too far future a power meter capable of measuring up to the 10GHz band is in the planning. While I do have a bolometer HP-432A, this is not portable, so measuring antennas will be a bit tricky, therefore I intend to get a digital-readout (logarithmic) power meter up and running, and get it calibrated as well as possible.

Oh, yes, I have collected a few pieces of test equipment in my time, but a bit more is needed, now that I have entered a phase of my life with more time for experimentation. 

Small Update on the QO-100 Uplink.

 The uplink signal using the single patch antenna resulted in quite a weak signal on the satellite, There were even instances where a station simply continued calling CQ while I called them, I could easily hear my signal on the downlink, but it was not strong enough to get any attention.

While I am planning on improving the set-up with a bigger dish and with a helical feed antenna, I was browsing for QO-100 feed antennas, and came across this 4-patch feed which has circular polarization. I realized that I have a small WiFi antenna, a so-called 14dBi panel that I estimated having 4 patches phased together, a bit like the feed in the link, albeit this just with linear polarization.

I taped the panel to the dish, got it aligned, and sure enough, the signal is a few dB stronger than before.

Further, I had removed a 10dB attenuator between the 432MHz TX and the up-converter, and when I received a report on a spurious signal on the CW signal I tested with reduced power, and sure enough the distortion disappeared and the tone was clean again. The S/N was even better, now close to 10dB in SSB bandwidth, a quite comfortable level for receiving a CW signal.

Further, after some email exchanges with PA1GSJ, and some of my previous thoughts, I will be testing some more improvements on the uplink.

- a circular polarized feed antenna, such as a helical.

- replacing the low cost satellite cable with RG-6 may provide a sufficiently low loss to eliminate the "driver" Edup amplifier and the extra filter in the outdoor unit.

- a larger dish for the uplink is contemplated, such as a 100cm one.

- I do have a 80cm dish, and I might replace the 60cm receive dish with this one.

- a better reference oscillator (set) 10/25MHz is on the way, so I know better which frequencies I work on

- a second receive converter , so I can use my dual band 2m/70cm TRX in satellite mode (70cm for the uplink and 2m for the downlink), and having the TX and RX tracking.

This is quite a list, so I will take my time and slowly improve/optimize the satellite system.

Up-link Up Converter and System for QO-100, and Some Success.

Finally, I got some test equipment for 2.4GHz up and running. It is not yet complete, but I could start testing the up converter and power amplifiers in the lab.

A few tests and measurements were done, with the following results.

For temperature stability reasons I decided to place the up-converter per se indoors. The unit can deliver between 1.5 and 2W. The outdoor unit contains a Chinese WiFi booster, a so-called 8W unit, the Edup AB-003. Tests show the saturated output power just under 4W, and with 3W it would probably have sufficient linearity for SSB on the satellite.

The test was done in the lab, with a cable length approximately the length that will be used from the indoor to the outdoor unit, 15 - 20m. I decided to go low cost and use low cost, i.e higher loss, cable, since a really low loss cable would be overly expensive and un-flexible.Loss is about 15dB.

With this setup it was impossible to get more than 2W out of the Edup (actually a bit less). Ach! too little drive for the Edup. What to do?

Next test, simulated indoor unit with an extra Edup amplifier, only a little less than 3W out. Hmmm! Now what?

One more test: (Simulated) outdoor unit with 2 Edup amplifiers in series. That helps. Just under 4W is now easily possible, the system gain is good. 

Next "problem": With lower gain/output of the indoor up-converter the local oscillator and image rejection is reduced. I want as clean a signal as I can reasonably get. OK, a 2.4GHz PCB "hairpin" filter mounted between the two Edup amplifiers in the outdoor unit should do it. The filter should improve the LO rejection about 30dB, easily compensating for the reduced rejection. On top of that the output of the up-converter has to be increased due to 5-6dB loss in the filter. Still sufficient system gain, as expected. The image rejection for a 432MHz IF is improved about 45dB. All looks good. 

Testing this setup results in just under 4W saturated out of the Edup amplifiers, and we are ready for an initial live test. The amplifiers and the filter are mounted in an (electrician's) distribution box A 2.4GHz patch antenna is connected to an extended cable. At this point, the first test will provide 2W saturated at the patch antenna that will be mounted on a 60cm dish. This should deliver a good CW signal over the transponder. SSB, however will sound ugly because of the threshold for the RF sense in the amplifiers.

A set of attenuators in series have been connected, and should provide sufficient attenuation of the 35W from the transceiver, not over-driving or destroying the up-converter mixer.

The system has now been set up, and is tested. It is far from ideal, many improvements and optimizations are possible.

The test with CW is a qualified success. The signals are not very strong, and the first evening there were no replies to my CQ calls, even though I could receive my own signal (too late for much activity). The afternoon/evening after (today, Saturday) 12 QSOs have been made, and most of the stations could receive my faint signals.

I was warned that the patch antenna is not ideal, so at some stage I will make a helical feed - and yes, I will have to make sure to get it wound the correct way.

A shorter cable from the outdoor PA, combined with the helical, should provide at least 5dB better signal-to-noise ratio, and a bigger dish (the present one is 60cm) will provide even more gain. It looks like I can find a 110cm dish from a local amateur.

A modification of the Edup amplifiers for constant TX mode will have to be done, so SSB transmission will be possible. About 3W to the antenna should be possible. A higher power amplifier may be convenient to have.

All in all much to do to improve the system, a part of it (indoor activities) can be done anytime, but a bit of the outdoor stuff should be done soon, before winter.

 Other improvements will be increased frequency stability and precision. At the moment I have completely separate transmission and receiving systems, and the frequency offsets are different, so tracking transmit and receive frequencies is cumbersome. I will likely end up with a GPS locked system, but slowly, slowly, not too hasty.