Extending Langstone to include 10GHz
Posted: Sun Sep 13, 2020 12:17 pm
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
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