Lime SDR Transmit Experiments

 As a receiver.. the Lime SDR-USB, like most SDRs in general has lots of problems, in part no doubt generally resulting from square waves used within the processing circuitry, which result in less than ideal reception. There are also problems from extraneous noise sources from the computer needed to work the SDR, its display, power supplies, and other local noise from electric lights etc etc. All this can often be ignored in practice. For example you can tune to a really weak radio beacon and hear it perfectly well.. being completely oblivious to the zillions of interference spikes and receiver spurii occupying the spectrum outside the frequency to which you are tuned. Alas.. although the extraneous noise spikes can be deemed irrelevant in receive mode, you cannot ignore SDR-generated spurii when transmitting. For example you may inadvertently transmit a signal on 121.5MHz (the aviation distress channel) or completely block an important airband channel used by your local airport.

Below is a picture of my Lime SDR with the top cover removed. The circuit board has two sets of receiver inputs, Low, High and Wideband together with two sets of transmitter outputs Low and High. Careful scrutiny of the spec of the board (even with factory modifications incorporated) will reveal that it precludes its use as an HF receiver unless an upconverter is used, but apparently allows its use as an HF transmitter, and conveniently, most amateur radio transmit modes are covered.

 

 The first test was to set the SDR frequency to exactly 100MHz and look at the output. In AM and FM the RF output popped up at exactly 100MHz and, using the drive control provided in the program (SDR Console Version 3.0.7 produced by Simon Brown G4ELI), I could set the RF level from -66dBm to +9.9dBm. However, what appeared on the spectrum analyser screen was not one, but several RF signals. Looking at the spectrum immediately around 100MHz I found that besides the correct 100MHz signal I found additional signals every 250KHz up and down. These were not as strong as the desired signal and varied in their relative strengths depending on the drive setting, for example, some 20dB down at each progression ie. -20dBm @ 100MHz, -40dBm @ 100.25MHz, -60dBm @ 100.50MHz and -80dBm @ 100.75MHz etc.

These spurii are clearly visible using a spectrum analyser, but would not be quite as obvious if one was to use a wattmeter. Many years ago I discovered this when tweaking the power output from a Microwave Modules 2m linear. A wattmeter adds up all the RF signals within its remit and indicates the total power. Effectively, each individual signal has RF energy which can be converted to heat and each spurious signal will add its own amount of heat to that produced by the desired output so a typical wattmeter displays for example not just the power in a 144MHz signal, but in addition the power of any signals at 288MHz and 432MHz. Twiddling a linear might result in 10% more power, but that extra 10% might be due to power in harmonics. I found that I could readily generate 100 watts from the MM linear but only 80 watts when tuning only the 144MHz signal. The extra 20 watts resulted from RF output at 288 and 432MHz. I recall the MM designers warning of this when discussing their linear back in the 1980s.

I connected my HP431C wattmeter to the Lime Tx aerial socket and read in excess of 10dBm although the true output should have been 9.9dBm.

In fact, if for example the spurii from the Lime are each 20dB down on the adjacent lower frequency, the result of the amount of power in each spurious signal isn't that great and may be considered as unimportant except under two circumstances... firstly.. consider what a sensitive receiver would display on its S-meter. At VHF S9 is reckoned to be -93dBm and given 6dB per S point which means that if sufficient attenuation is provided between the Lime TX output connector and a VHF receiver to produce an S9 reading for our 100MHz signal, the 100.25MHz spurious output will display a little under S6. This is not really good enough for a self-respecting radio ham. Maybe a spurious signal 45dB down is OK (something like S2/S3 compared with S9)? The second instance is when a spurious signal unintentionally interferes with another spectrum user especially if it pops up in a sensitive area (eg anywhere in the air-band).

 
 The pictures above and below may look similar at first sight, but the one above shows the SDR display immediately around 100MHz when "TX" is selected at full power, and the second, below, shows signals seen across the whole spectrum from zero to 1.5GHz. The indication is that when transmitting a 100MHz signal at 7.82dBm many other signals may also be transmitted. The chart below lists the harmonics and its pretty clear that a square wave is involved because odd harmonics are stronger than even. Note that the definition in the picture below is not adquate to show the spurious signals close to the fundamental, shown above. The danger comes when an untuned power amplifier follows the SDR transmitter.

 

 

 Table showing the strengths of the 100MHz transmit signal and its harmonics

 

 Freq MHz

 100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

 Level dBm

 +7.8

 -28

 -17

 -38

-16

-42

-18

-45

-17

-47

-22

-49

-29

 This isn't the whole story however, because if we recap and look closer at the transmitted signal, we see some more spurious signals. In fact the fundamental frequency plus its harmonics is each accompanied by a group of signals apparently based on 250KHz.

Below: this signal sitting at 100.25MHz is present at the Lime output socket when either unmodulated USB or LSB is transmitted at 100MHz.

Because we're using SSB the main signal is suppressed making spurii more apparent.

 

  Below: if AM or NFM is selected one sees the picture below. In this example the drive setting is reduced and shows the 100MHz output adjusted with the drive setting to -12dBm accompanied by 100.25MHz @ -17dBm, 100.50MHz @ -50dBm, 100.75MHz @-63dBm.

From my experiments with the upconverter it almost appears that a 100.25MHz signal is used to mix a processed RF signal of say 250KHz in a balanced mixer and what we see above is leakage through the mixer appearing at the output socket. Non-linearity is also then producing further signals at 250KHz intervals.

 

 Experiments continue... next, what about transmitting at HF?
 I set the frequency to 10MHz and pressed "Tx" but not much happened so I tried 30MHz and saw a little RF output. Clearly the Lime is not going to be much use as an HF transmitter unless it's used with a downconverter. Maybe something similar to the upconverter although of course for amateur radio use the transmit bands are tightly specified. Perhaps I need to work out the optimum frequencies for amateur radio use and then choose crystals to convert these down? As far as the VHF bands are concerned, maybe the Lime will be OK to drive a linear amp for the particular band. I found a few very odd things , but I'll just recap on my earlier findings. The picture below shows a signal of 1GHz from the Lime. It looks really good. A nicely defined RF spike. I'm feeding the Lime output through a 20dB attenuator so amplitude-wise it will be about +2dBm. But is all what it seems? See the second picture below this one...
 

 

 Above is a sweep of +/- 500KHz about 1GHz and you can see not a single clean spike of RF, but three spikes. The two unwanted signals are precisely +/- 250KHz from the main output signal. In fact the characteristics of the Lime output don't really differ from the previous tests at 100MHz except that the relative amplitudes of the spurious signals differ slightly. Sometimes the upper signal is bigger than the lower and sometimes smaller.

 As an aside... I carried out additional testing using a newer version of the program. This was Version 3.0.8 and installed on a second computer. The results were weird and were due to a problem with the software. Not in the operational coding, but in the installation. Maybe a stray file left over from an earlier build? The symptom was associated with a provision for offsetting the transmit frequency but after uninstalling and re-installing the problem disappeared and all was well. As a rule of thumb maybe it's worth re-installing your program if strange results are apparent?

 continues....
 
 

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