Spurious emissions

[kb7pst]

I am curious -- are there any tests/research/information around whether the radios used for OpenGD77 have issues with spurious emissions. I'm particularly interested in the DM-1701, but would be interested if there's data in general around these radios -- and if they are improved through using the OpenGD77 software.

[G4EML]

I don't know if anyone has recorded the results of any tests.

The performance will be the same with openGD77 as it is with the official firmware. The filtering is fixed in hardware so the firmware can have no effect.

We do know that the experimental 220MHz support on the GD77 has a very poor output spectrum because the radio has no filtering for that band. So it should only be used for receiving.

Colin G4EML

[VK3KYY]

I don't know if anyone has recorded the results of any tests.

From what I can remember Jason VK7ZJA (who is now silent key), did some tests on various radios including the GD-77, and he didn't mention any difference in the spurious emmisions when using the OpenGD77 firmware.

I also know that VK4JWT did various tests using the OpenGD77 firmware, which were mainly Rx sensitivity, and he didn't mention anything about additional spurious transmissions either.

AFIK. The PA in the DM-1701 / Retevis RT-84 is better than the UV380/RT-3S, because I think it has separate PA chains for the UHF and VHF, like the GD-77 and DM-1801.

However, AFIK, the MD-UV380 uses a single wideband PA for both VHF and UHF , with a single filter just before the antenna connector.
This PA design is similar to Baofeng RD-5R, which is known to be worse for spurious emissions, however I don't think anyone has done any comparative testing between the UV-380 and DM-1701 and RD-5R etc, to see if having separate PA chains for VHF and UHF results in less spurious emissions.

[kb7pst]

I have good news and bad news.

Methodology: DM-1701 connected to a Signalhound SA-44 with a 40dB attenuator and transmitting at 250mw. Checked it against a Yaesu VX-5 transmitting 300mw.

2m on the DM-1701:

bm-1701-harmonics-2m-250mw.png (149.4 KiB) Viewed 2343 times

2m on the VX-5r:

vx5r-2m.png (140.55 KiB) Viewed 2343 times

70cm on the DM-1701:

bm-1701-harmonics-70cm-250mw.png (138.3 KiB) Viewed 2343 times

(I didn't check 70cm on the VX-5r)

Tl;Dr - My read is that harmonics on 2m aren't great. 70cm seems pretty good (and all of my local DMR repeaters are on 70cm)

[VK3KYY]

I wonder how this compares with the FCC report on this radio....

I presume it has FCC approval.

Its unlilkely that the OpenGD77 firmware effects the spurious emissions level as there are no software controllable filters in this radio.

Did you try comparing the emissions when using the official firmware.

[kb7pst]

I did not try testing with the official firmware. I might do that when I have a bit of free time in the future.

[VK3KYY]

I did not try testing with the official firmware. I might do that when I have a bit of free time in the future.

thanks

[G4EML]

Out of interest I had a quick look at the FCC documents for the radio. The supplied spectrum plots don't make any sense to me. Some show no harmonics at all and some show the fundamental at a lower level than the harmonics! It also appears that they were done in a chamber with the antenna connected and not by direct connection.

I don't however think that this is unique to this radio. I suspect many of the cheaper radios are not good. On their own antennas, not too much of a problem, but hams tend to connect them to large antennas.

Colin

[SA0BUX]

Out of interest I had a quick look at the FCC documents for the radio. The supplied spectrum plots don't make any sense to me. Some show no harmonics at all and some show the fundamental at a lower level than the harmonics! It also appears that they were done in a chamber with the antenna connected and not by direct connection.

I don't however think that this is unique to this radio. I suspect many of the cheaper radios are not good. On their own antennas, not too much of a problem, but hams tend to connect them to large antennas.

Colin

I read this test report for MD-9600, I guess it's a late model
https://fcc.report/FCC-ID/POD-DMR3/5195309.pdf

Very strange curves where the fundamental frequency levels seems lower than the harmonics.

My V2 is quit terrible on 144 MHz as the second harmonic that is 432 MHz is rather strong due to the filter design.

Now when I have the TinySA Ultra I started to play with it, I have several 30 & 40 dB power attenuaters and a 0-110 dB step attenuater that I have combined to be able to get a low ( - 30 dbm) signal into the TinySA.

So now I will test and compare my radios and post a link to my curves.

[G4EML]

Since my last post I have had the MD9600 on the spectrum analyser. As you have found the 3rd harmonic of 2M is a little stronger than it ought to be.
The radio does have separate low pass filters for VHF and UHF, effectively they are connected in parallel with the unused filter being isolated by a diode switch.

I think this is where the problem occurs. when on VHF the VHF lowpass filter works fine on its own, but part of the signal also gets applied to the 70cm low pass filter due to the leakage through the diode switch. The 70 cm filter will have low loss at the third harmonic frequency. So the attenuation of the third harmonic of 2m cannot be any more than the isolation of the diode switch. 30dB or so would be typical for this.

I proved this theory by shorting out the UHF filter at its mid point, thus reducing any through signal. The third harmonic of 2m improved significantly.

I suspect it might be possible to improve the situation by adding an extra diode switch in the 70cm lowpass filter to short it out when on 2m. However I have not tried this.

Colin G4EML