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.
Have you seen the HB100 10GHz FB group? 9https://www.facebook.com/groups/603514236508122) 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).
https://scontent-lhr8-1.xx.fbcdn.net/v/ ... e=5F8549AA
Happy to make Pipe-cap filters and such and experiment.
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.
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.
Many thanks for all your hard work with Langstone. I have mine working on the bench with no problems but would like to pick your brains about using Langstone on 10GHz.
I am trying to receive a marker on 10368.15GHz.The marker is an AD chip locked to a 10MHz OCXO so its frequency is known accurately,
I have set the Langstone Rx harmonic to X5 and am feeding the marker signal into the Pluto Rx port via a small 10GHz horn, coax to WG transition, broadband amp (2-18GHz, 26dB gain, 5dB NF) - see picture.
How do I estimate what the received frequency will be for a given Pluto TCXO error?
As I tune the Langstone, I seem to see many signals at 10GHz which are spaced by about 50kHz with no obvious main signal around the expected frequency.
Any comments would be much appreciated.
73 Barry G8AGN
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When I receive 10GHz I don't see any significant spurious signals so I am not sure why you are seeing that. I wonder if the amplifier is overloading the Pluto if the signal is very strong. It might be worth just connecting the transition directly to the Pluto, for local signals it is should be sensitive enough.
The LO in the Pluto will be running at 1/5th of the receive frequency. So for 10368 that will be around 2073. If you know the Plutos TCXO error at 2 GHz then the error at 10 GHz will be 5 times greater.
Just to confirm, when set for x5 mixing you should see the annunciator 'MULT' at the top of the display. You also set the displayed frequency to 10368.150
The way you have it set up should work but ideally there needs to be a filter between the amplifier and the Pluto on receive and transmit to remove any 2Ghz content. Whist it will receive 10Ghz it is also still receiving at 2 Ghz, So any noise or signals at the lower frequency will degrade the 10Ghz performance. I have pipecap filters connected directly to the Pluto on both the Tx and Rx ports. Alternatively a short length of WG16 with coax transitions on each end would act as a good high pass filter.
I have just checked and don't see MULT showing on the display. In the SET menu, I chose X5 in both the Tx and Rx in the Harmonic Mixing settings.
I note that I also have
Rx Offset = -9936.000000 Rx Freq = 432.150000
Is this correct? I haven't touched these values so assume they were set automatically?
The default Langstone band for 10GHz is set up for a transverter. It sounds like you are using that one. To use the harmonic mixing you need to change the settings.
You need to set the Tx and Rx offsets to 0, the harmonic mixing to X5 and the displayed frequency to 10368.xxx
You can either modify the existing 10GHz band or change one of the others. The 12 bands are really just 12 VFOs so that can be set to anything you want.
Maybe I need to add a new default band for 10GHz.
that seems to have done the trick!
My marker is on 10368.150 whereas the Pluto is receiving at 10368.2488 so how do I calculate the required TCXO shift?
Is it ( 248.8 -150)/(5 x 40) = 494 Hz down i.e. 39999506 ?
Langstone is fun! I have tried it on transmit on 1296 - 5760 but not yet on 10.
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