The satellite Es’Hail-2 is expected to be launched by the end of 2016. This will be the first geostationary satellite carrying an amateur radio transponder. As the launch date comes nearer, it becomes interesting to find a low cost solution to receive its 10GHz downlink.
Several amateurs have been experimenting with low cost LNBFs designed to receive satellite TV. These operate in the Ku band and usually cover the frequencies 10.7GHz-12.75GHz. However, many of these LNBFs have also good performance in the X band, and particularly in the amateur 10GHz band (10GHz-10.5GHz). In fact, the ASTRA-type LNBFs have a local oscillator which can be setted to either 9.75GHz or 10.6GHz. The 9.75GHz local oscillator mixes 10.386GHz (the narrowband terrestrial subband) to 618MHz, which is a frequency covered by most SDRs and conventional scanners. The satellite subband, which is 10.45GHz-10.5GHz gets mixed down to 700MHz-750MHz, a frequency which is also easy to deal with.
The receive performance of these LNBFs seems to be very good in terms of noise figure, but their frequency stability is not so adequate for narrowband applications. The local oscillator is derived either from a DRO or a PLL controlled with a crystal oscillator. The PLL is more stable than the DRO, but since the crystal is not ovenized, a stability of 0.25ppm is the most one can expect from it. Of course, this is 2.5kHz at 10GHz, which is way too much. Some amateurs have been modifying these PLL based LNBFs to use an external frequency reference instead of the crystal oscillator. See for instance the tests done by G4JNT and F1CHF.
Many of the PLLs are driven by a 27MHz crystal oscillator. The idea is to replace this with an external 27MHz signal generated by a PLL which is driven by a very stable 10MHz oscillator (this frequency being the most common for a frequency reference). Ideally, the 10MHz oscillator should be GPS disciplined. Dieter DF9NP has designed a 27MHz PLL to be used for this purpose. Luis EA5DOM has a nice document describing how he modified an LNBF to use the PLL. This is probably the best solution in terms of performance, but it can get a bit expensive. The PLL alone is about 30€, which is more than the LNBF.
It is possible to get OCXOs in the surplus market for low prices, but 27MHz is not a usual frequency for an OCXO. However, QRPLabs provides a kit for an OCXO/Si5351A synthesizer. The Si5351 is a PLL/MultiSynth-based frequency synthesizer and the intended application of this kit is to be used with other kits such as the Ultimate3S QRSS/WSPR transmitter. The nice thing about this kit is that the Si5351 has to be driven from a 25MHz or 27MHz crystal, and so this kit contains a 27MHz OCXO. The 27MHz signal is accessible on one of pins in the output header of the kit. With a price tag of 15$, this is probably the cheapest way to obtain a stable and clean 27MHz signal.
The oven is built by hand using PCB pieces, so it will be interesting to see how stable it is. The assembly manual says that, in practise, the frequency stability for a 10MHz signal is better than a 0.01Hz drift in 2 minutes. This is equivalent to 10Hz at 10GHz, which more than enough stability for most narrowband applications. Another interesting aspect of this kit is the possibility of using the Si5351 instead of the crystal oscillator to generate and feed the LNBF a frequency different from 27MHz. For instance, a 27.515MHz signal will make the LNBF mix 10.368GHz down to 432MHz instead of 618MHz, permitting the use of a 70cm receiver. However, the phase noise of the Si5351 might not be so good. The Si5351A can generate 3 different frequencies from 2.5kHz to 200MHz simultaneously (with some restrictions), so it can also be a very useful tool to have around in the lab or shack for other applications.
Another possibility that some amateurs have experimented with is to use a 3MHz surplus OCXO and a simple multiply by 9 circuit to obtain a 27MHz reference.
I have already ordered an OCXO/Si5351A kit and started modifying my LNBF. I will test how well this solution works in a future post.