29MHz transmit - Lime problems

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g8gtz
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Joined: Wed Apr 07, 2010 6:26 pm

29MHz transmit - Lime problems

Post by g8gtz » Sun May 16, 2021 5:52 am

I've seen 2 posts suggesting that the Lime Mini can be used for transmit on 29MHz.

Unfortunately, this is not true as the Lime LO is switched to 31MHz for lower band operation and appears at about -40dB down on the main signal - there is also a mixing product starting to appear 2 Mhz up at 33MHz.

The only way appears to be a transverter which could be a simple design based similar to Mike's 71MHz rx converter.

Noel
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g0mjw
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Re: 29MHz transmit - Lime problems

Post by g0mjw » Sun May 16, 2021 8:07 am

...which is why I designed a transverter earlier in the year. I am still waiting for the PCBs to arrive. I think the Lime can do better than that though.

Mike

G4FRE
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Re: 29MHz transmit - Lime problems

Post by G4FRE » Sun May 16, 2021 8:12 am

Exactly the results i found while experimenting over christmas and now why i use transverters for 21MHz and 29MHz from 146.5MHz

Interestingly you get a 30MHZ signal if you generate 28 or 29MHz signal but if you generate 31 or 32MHz the 30MHZ signal goes away!

Dave
g4fre

G3VZV
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Re: 29MHz transmit - Lime problems

Post by G3VZV » Sun May 16, 2021 9:20 am

Thanks for the info Mike,
Please put me down for some beta testing!
thanks
73
Graham G3VZV

g0mjw
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Re: 29MHz transmit - Lime problems

Post by g0mjw » Sun May 16, 2021 9:52 am

OK - expecting PCBs soon. I want to know if I can get away without tuneable coils which are difficult to get these days. Also it's quite small SMD based to fit in a little box. I have decided to try using an LO module - I was originally using G4DDKs Butler oscillator circuit but discovered it is now very hard (expensive) to get 116 MHz crystals.

Mike
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M5TXJ
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Re: 29MHz transmit - Lime problems

Post by M5TXJ » Sun May 16, 2021 10:15 am

Hi Mike,

Looks like it will be good to reproduce by others. I've enjoyed keeping up with your progress as seen on the Thursday night net. Might be good up here in the wilds of Cumbria! More power to your elbow as they say.

73 Dave.
...are you sure I can't use a pair of 813's?... :shock:

g0mjw
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Re: 29MHz transmit - Lime problems

Post by g0mjw » Fri May 21, 2021 9:20 pm

The last few bits finally arrived so I completed it this evening. OK so I didn't solder in the BNCs fully - just in case. Seems to work. For reference, I use a 120MHz oscillator so 29 MHz coverts to 149 MHz.
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On receive there is 10 dB gain but anything more than -20 dBm input starts to create spurii. That's probably not surprising as the PGA 103+ isn't specified on 10m, largely due to the input match, but has well over 26dB gain and would be over-driving the mixer. Maybe a different pin compatible device with lower gain might be better. I had planned to use a Gali 59+ but I don't have any in stock. Even so, on 10m it might be sufficient just to have the mixer and dispense with the input amplifier. Meanwhile if the input limits are an issue, stick an attenuator in front of it. The bandwidth was as expected, the 10m bandpass filter is not very sharp about 23MHz to 30 MHz -20 dB. It's not great as I used standard value parts. There is quite a lot of LO leakage but not so much to be a problem.
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On transmit the gain was also 10 dB, that's partially governed by the input attenuator, in this case 10 dB. It will easily give 10 dBm out but harmonics start to be an issue and in any case at that level the spectral regrowth would be too severe. The spurii are at least 60 dB down. The bandwidth is quite high as there is only a low pass filter to limit it.

Mike

g0mjw
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Re: 29MHz transmit - Lime problems

Post by g0mjw » Sun Jun 27, 2021 3:07 pm

Time for an update. With the first version of the converter I can generate DVB-T at 29.5 MHz that looks almost as clean as it does at 149.5 MHz. This is with the Lime / Portsdown.
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Ignore the date and time as the analyser RTC is not set correctly.

The shoulders are at least 50 dB down. With my driver amplifier inline I get a similar result - this is to deliver the drive for an MRF300AN PA based on the NXP standard design.

A few tests with the Pallet ensured, initially I have over 100W but it wasn't very clean, that was with 100mA bias current so as expected. More sensible tests arrived at 1A appearing about the optimum bias. Two plots, 80W output and 60W output.
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So it's not super clean but it's not bad. At much lower power it gets better. Next post...
Last edited by g0mjw on Sun Jun 27, 2021 3:30 pm, edited 2 times in total.

g0mjw
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Re: 29MHz transmit - Lime problems

Post by g0mjw » Sun Jun 27, 2021 3:15 pm

At 30W it is not at all bad. This is only 10% of the device maximum, it is taking 2A ant 50V so 30% efficient.
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More work to do but some hope. I should be able to get a lot more power out in DVB-S2 so just to prove that, here we are at 120W. Not pretty but...
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Mike

g0mjw
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Re: 29MHz transmit - Lime problems

Post by g0mjw » Sun Jun 27, 2021 3:24 pm

For DVB-S2 with plenty of bias I can get 150W out with about 1.2A bias.
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It will be interesting to see if a pair on 6m is any better or worse.

Mike

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