Apache Labs Community Forum

I noticed some unusual peaks in my 28/70 MHz transverter TX output. As well as the main 70MHz peak I also see peaks at 14, 28, 42, 56 and 84 MHz from the transverter- some at -25dB down from the main peak (picture attached). When I checked the purity of my 200D XVTR TX output it had no detectable 2nd harmonic (56MHz) but the 3rd harmonic (84MHz) was clearly there at -40dB (see 2nd attached spectrum). This caused me some confusion because the LO in the transverter runs at 42MHz (to give 70MHz) so 84MHz would also be a 2nd harmonic, so two potential sources of this peak.

So, for those who have used the ANAN XVTR out port, do you see a clean spectrum at 28MHz? I am now starting to believe that at least part of the bad TX spectrum from the transverter is caused by the 84MHz peak from the 200D XVTR output. You can do the numbers with the 28, 42, 84 MHz signals to generate all the peaks in the bad transverter output. The final transmitted signal after my 70MHz PA is not so bad because of filtering but the unwanted peaks between 14-84 MHz are still present at around -40dB.

One aspect I have not looked at is the VHF PA gain settings in Thetis. I have left all these VHF 0-13 gains at the default values of 56.2. Do these have any effect on the XVTR output?

I suspect there is something that I am not understanding here, so would welcome input from those who have successfully used transverters.

73 Dave G3ZQH


For reference I am using 200D with the Ukrainian 28/70 transverter board with Thetis 2.8.11 and Protocol 2 firmware 1.9.

Hi Dave,

- as far as I remember the 70/28 transverter has a LPF at the Input, so this should add significant attenuation to the 3rd harmonic.
- I don't have the 200D, but on all of my ANANs (10/100/7000) PA GAIN DOES HAVE AN EFFECT OF THE XVERT output!!!
- IM3/IM5 are the figures to take care of at first.....

GL

Volker

Thanks Volker
Yes, you are right the VHF gain does directly effect the XVTR signal- I should have realised this! For HF we adjust gain to correct the power output (to 100 drive=100W normally) but what do we adjust gain for at VHF? I can't find any information. I can see the effect on XVTR power out as I change the setting. There is also a big change in the 28MHz XVTR TX spectrum with it being much cleaner at lower gain numbers. Maybe we should use the same setting as for HF 28MHz?

For IMD, I haven't made any measurements yet, but I will do this once I have worked out if I can use the two-tone test with the XVTR output.

I use an additional LPF in line with the transverter and this effectively removes 140MHz 2nd harmonic of 70MHz signal, but still I have 84MHz present at a higher level than I would like.

I'm still learning about how to optimise the transverter...

73 Dave G3ZQH

In case of overdriving your mixer with too much 28MHz injection, 3rd harmonic will increase as well. It is up to you to adjust the input level of the mixer. You can use attenuators and/or reduce PA gain (increasing value). As a rule of thumb, do not exceed 0,1mW for a 7dBm mixer. Go for IM3 first (in my opinion it should be better than -25dBc) and then care about harmonics.

73

Volker

Hi Volker
Now I have done some IMD measurements. On the Ukrainian XVTR the best I see is IM3 -30dBc, IM5 -38, IM7 -47 at gain=50. This is at 1-1.5W out, which is what I need to drive my 70MHz PA. Actually there was little difference in IMD with the 200D gain settings over 40 to 70 range at 1W from the XVTR.

At these settings the spurious peaks at 42, 56 and 84MHz from the transverter were all more than -40dB down from the main 70MHz output. No 140MHz signal detected at all.

So overall, perhaps not too bad. With the XVTR driving my PA to 100W I see IM3 -30, IM5 -27, IM7 -35. At 200W this is IM3 -20, IM5 -36, IM7 -35. These are not great but the PA and transverter are still open on my bench, so I hope once they are housed properly the IMD may improve a bit.

Now, I just need to check the PA spectrum. Also work out how to integrate Pure Signal into this! That should certainly help with IMD.

73 and thanks for your help with this.

Dave G3ZQH

Congrats! In order to improve suppression of unwanted spurii, try adding a band pass between the transverter and the PA, e.g.

https://electronicprojectsforfun.wordpr ... e-4m-band/

The cheap OWF filters cannot handle 30dBm.

73 es GL

Volker