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Extending Langstone to include 10GHz

Posted: Sun Sep 13, 2020 12:17 pm
by g4eml
Prompted by the Hydra add-on module for adding 10 GHz to the Pluto, which adds a doubler on transmit and a LNB down converter on receive I thought I would have a further look at the possibility of using Harmonic mixing with the bare Pluto.

It has been previously discovered and noted in this forum that the LO of the Pluto has significant harmonic content and it would appear that this can be baseband modulated to produce signals above the normal 6GHz limit of the Pluto. The same method could probably be used to extend the receive coverage.

I added a feature to Langstone to configure the harmonic to be used for the transmitter and receiver LOs.

My initial test was using the 3rd harmonic for transmit (3456.066666 x 3 = 10368.2) I fed the Pluto Output through a coax to WG16 transition as a simple antenna and high pass filter. This immediately gave results and I could hear my SSB transmission quite strongly on 10GHz using my transverter. However the sideband was inverted. If i selected USB on the Langstone I had to select LSB on the transverter.

I tried again using the 5th harmonic (2073.64 x 5 = 10368.2) and this time the sideband was not inverted. The signal strength seemed about the same.
Using the second or fourth harmonics seemed to produce double sidebands as if the IQ mixer was not cancelling the image.

Maybe someone with more knowledge of IQ mixers could comment on this. Is this the expected behaviour of the mixers when using Harmonics? Or is it just an indication of poor balance at frequencies outside the normal range?

Trying receive also seemed to work in a similar way. Using the 5th harmonic of the LO I could receive my 10GHz transmissions as normal but using the 3rd harmonic the sideband was inverted and the tuning was reversed.

So it looks like both 10GHz transmit and receive are possible on the Pluto by using the 5th harmonic of the LOs.

The signals are not spread out as I would have expected if I was just listening to the 5th harmonic of the output. FM deviation is the same on 10GHz as it is at 2073. This tends to confirm that the fifth harmonic of the LO is being modulated by the baseband.

I tried adding an external MMIC amplifier and this increased the signal, both when I used it for receive and for transmit. On receive I could just about detect the noise from the amplifier on the Langstone waterfall so that would tend to indicate that with enough gain the receive sensitivity should be reasonable.

Just to prove the fact that I was not just listening to a fifth harmonic I removed the waveguide filter and fed the 2073MHz output of the Pluto directly into the MMIC. This would certainly generate harmonics at 10368. The resulting 10GHz signal was obviously spread out, FM deviation was three times wider and SSB was unreadable. So it is vital that there is a 10Ghz filter before any external amplifiers to remove the fundamental component.

I will do some more experimenting and try to take some measurements but it looks like it should be possible to design a simple 10GHz add-on board for the Pluto with a couple of pipe-cap filters and MMICs.

Colin G4EML

Re: Extending Langstone to include 10GHz

Posted: Sun Sep 13, 2020 2:40 pm
by radiogareth
And the best just got better :-) Makes me wish I didn't flog off about 40Kg of heatsink/weather proof box and the contained sundry bits and pieces, including a 250mW amp on 10GHz. 4 years ago so before the wonders of the Pluto....
Have you seen the HB100 10GHz FB group? 9 Using HB100 DRO Microwave doppler modules for some great FM WB TV. Wonder if there's anything relevant on there (not much use for NB as is). ... e=5F8549AA

Happy to make Pipe-cap filters and such and experiment.

Re: Extending Langstone to include 10GHz

Posted: Sun Sep 13, 2020 5:14 pm
by g4eml
From initial testing it looks like the output power on 10GHz is around -30dbm. I have a couple of 30db gain low noise amplifier modules, with one of those in line I am getting around 0dbm output (1mW).

The receive side seems to also need about 30db of gain to bring it up to a reasonable sensitivity, with that connected a quick test against my Kuhne transverter shows the sensitivity to be similar.

So the board might need to be a bit more complicated than originally thought. Probably 3 NLB-310 MMICs for transmit and the same for receive. Or maybe fashion something from a Franco board. The filtering can be quite simple. Either a couple of pipe caps or even just a short length of WG16 with an SMA transition at each end.

Alternatively with just a changeover relay and a coax to waveguide transition it would make a useful test source or ultra QRP rig.


Re: Extending Langstone to include 10GHz

Posted: Wed Sep 16, 2020 12:06 am
by g4eml
Langstone has been updated to add some new experimental features.

New setup menu entries for harmonic mixing.
It is now possible to set harmonic values of 1 to 5 on both transmit and receive. By setting a value of 5 it is possible to receive and transmit on 10GHz. The frequency display is set to the 10GHz frequency and the Pluto then operates at 2GHz and uses the 5th harmonic of the LOs. Output power is very low (about -30dbm) and needs to be externally filtered to remove the fundamental signal. Receive noise figure is likewise about 30db and needs an external preamp and filtering. As it stands, without filtering or amplification it is not of much use other than short range local testing. Filtering can be as simple as feeding into a waveguide antenna. That will effectively remove the 2GHz signal which will be well below cutoff.

Support has also been added for a smaller touch display. The 4” Pimoroni Hyperpixel4. Currently using this display means that all of the GPIO is occupied so there is none left for the Langstone. It is planned to add support for a low cost I2C module to restore the necessary IO.