HI Spectral Line Monitoring
So how can I effectively use a narrow bandwidth receiver with a basic frequency stability specification of parts in 10E-6, to monitor frequencies in the range of 1.4E+9. One way would be to lock the local oscillator (LO) to a relatively stable frequency source. The G305e SDR does not have the capability to do this without serious modification, something I would prefer not to do. I do have a signal generator that I can use to inject a nominal or offset signal into the front end of the receiver but it's basic frequency stability is about the same as the receiver, possibly doubling the (worst case) error. The signal generator does have the capability to phase lock to an external 10MHz frequency standard.
Enter the Thunderbolt
Recently, I added a Trimble Thunderbolt GPS Disciplined Oscillator to the inventory for a couple of reasons/projects: (see the GPS Timing page)
Use it somehow to stabilise the G305e SDR on the telescope for this project.
Use it to generate a stable and accurate LO for two USRP SDRs configured as I & Q inputs on the telescope for this project.
An accurate UTC time source for observing and recording occultation events (optical astronomy)
An accurate UTC time/frequency source for synchronising to a long baseline interferometer, now that the RA group is developing the HFI project.
If I use the Thunderbolt to stabilise the frequency of the signal generator output, all I should need to do is track the signal at an offset of say -100Hz (1420.40566MHz) and monitor any response other than the reference trace.
This would display any HI responses from sources at the nominal 1420.40576MHz
local sources within the Milky Way galaxy do not have large doppler shifts (red shift below nominal) or blue shift (above nominal).
Other HI signals with significant offsets would be from outside the Milky Way Galaxy and probably not within the 15kHz maximum IF of the SDR.
2 images of the injected 100Hz offset signal, level is about -105dBm @ approx. 1 metre from the feedhorn.
The G305e SDR has an Automatic Frequency Control (AFC) function which will allow this to work, but there are several minor disadvantages with this arrangement.
the receiver requires a significant input signal level to operate the AFC correctly
the reference trace is always centred on the display, halving the usable available bandwidth
sources red shifted by 90 to 110Hz would be masked by the reference signal response.