The excellent G4DDK Anglian transverter range has the facility to "injection lock" it's 116MHz Butler Crystal oscillator to a stable 116MHz source such as a synthesiser. This allows the whole transverter to be locked to a stable reference such as a 10MHz GPSDO. Injection locking does this but still retains the clean, low phase noise of it's Butler Crystal oscillator. Using just the synthesiser as an external LO would have meant that the whole transverter's noise performance would have been governed by the synthesiser's phase noise performance. Using this method, the crystal is just "nudged" on to the exact frequency by the synthesiser.
This got me wondering if I could do the same with the single FET oscillator used in all the previous-generation Kuhne G2 series of microwave transverters. I have tested this out on a 5.7GHz G2 with a 117MHz crystal LO, but the oscillator circuit used in the G2 is common to all the Kuhne transverters below 24GHz so should work in all of them given the correct crystal frequency.
Most microwavers know that the G2 series have the facility to remove the crystal and inject an external LO. It's a standard mod used by Kuhne themselves in the "external LO" version of their oscillator chains. The injection point is via a 100pF to one end of the crystal position, so I tried to inject a signal at that point while the crystal was in position and oscillating and see what happened.
I used a +5dBm from a G4JNT LMX2541 fractional N synthesiser. This board is simple has good progamming support and has an on chip VCO. It is not though, in the top class for phase noise!
I monitored the transverter LO at the output of the first tripler at 351MHz with a spectrum analyser and a Rubidium - locked frequency counter. As you would expect, a nice clean signal for the crystal but slightly LF of the required frequency. On connection of the external 117MHz synthesiser,the oscillator immediately locked up showing 351.0000MHz but sadly the output noise spectrum immediately degraded to match the synthesiser, not the nice clean crystal. I reduced the synthesiser level and watched the spectrum and frequency. As I reduced the synthesiser drive level, the crystal stayed in in lock and the output noise reduced.
"result!"
With about 20dB attenuation - a drive level of -15dBm, the frequency was still 351.0000 and locked but the noise now looked like the unlocked crystal.
So there we have it.
Without removing the crystal, just like the Anglian, you can injection lock your Kuhne G2 transverter to a synthesiser such as the LMX2451 or ADF4350/1 and not spoil the phase noise performance!
Have a Happy and quiet (low phase noise) Christmas!
73 John
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4 comments:
Interesting. I am upgrading my system to put all 4 DB6NT transverters 13-3cms at the masthead. At the moment, they are running on crystals as I was concerned about phase noise due to the presence of several strong local. I was wondering if I could feed an LO up some coax - maybe I can.
Mike
If you were really clever you could hack Andy's lmx2541 control PIC code to allow you to select 1 of 4 LO frequencies and feed a single coax up to a masthead splitter to each transverter. Then you could do a remote band select of the lock signal.
interestingly the optimum drive level to injection lock the g4ddk iceni/anglian transverter for cleanest signal is also around the same level. Using 0dbm, as some have tried gives a really dirty signal
Just tried this on my 13cm db6nt, but it just stopped the oscillator.
Was using -15dBm through 100pf into the "cold" same point shown on db6nt's kit manual
Obviously I am doing something wrong...
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