[Discuss-gnuradio] Frontend to build a Spectrum Analyzer

[Stefan Mangard]

Hi everybody,

I am a student working a university. My background is mainly in computer 
science and VLSI design - unfortunately, I am not an expert in building 
RF frontends yet.

However, I would like to achive the following goal and I think my task 
has a lot in common with GNU radio.

We have a quite nice PCI oscilloscope at our university that I can use. 
It has a resolution of 8 bit and a sampling rate of up to 1GHz.

I would like to build an RF fronted for this oscilloscope that allows me 
to use the oscilloscope as spectrum analyzer and as a tuner to receive 
short signals (I don't have a real-time decoding constraint - for my 
application, I just need to be able to receive a signal in a given 
frequency band for a few milliseconds - i.e. until the memory of the 
oscilloscope is full). Because of the limited resolution of the 
osciloscope I need to do filtering in the analog world.

So, supposedly I need an architecture like this:

antenna - amplifier - mixer - Bandpass - amplifier - oscilloscope
                       |
                       |
                   Oscillator

It should be possible to control the oscillator via the PC. 
Additionally, the Bandpass should be switchable between different 
bandwiths (at least there should be a mode for a few kHz and one for a 
few MHz).

I think the scenario is pretty much the same as in a software radio, 
where you want to receive all kinds of transmissions in all the 
different frequency bands.

In my application I would like to be able to detect signals up to 1GHz.

I am not experienced in doing RF designs and so I am looking for some 
hints concerning to following questions:

- Can I buy suitable RF Frontends somewhere and just plug it between the 
Antenna and the Oscilloscope?

- Are there components in the HAM radio world I could use?

- How do you generate the signal for the mixing in GNU Radio? As far as 
I know VCOs are only available for a limited frequency range?

- How do you do the bandpass filtering in GNU radio? I suppose you also 
need an analog filter, because of the limited resolution of the A/D 
converter.

Thanks for any hints,

Stefan