On 5/28/06, Peter wrote: > Vasile Surducan gmail.com> writes: > > > Would work yes, but will be almost unuseless. The simple mixer must > > be replaced with an IQ one, else would be necessary two or three IF > > stages with all the problems. The biggest one will be the frequency > > fluctuation in the audio side and the limited bandwith or distortion. > > Also will be necessary a real VCO and not a white noise generator. > > There are many analyzers like this one on the web, the best one is > > using a tuner > > and claim to work ok till 2Ghz. I'm seeing every day how works a good > > spectrum analyzer (for it's 30 years old age) TEK492 and if that one > > can't be used for fast signal analyses, all analyzers having the same > > principle (unfortunately) can't. > > So depends what do you need. > > I wrote: > > > > Finally an affordable solution for > > > filter and amplifier design for homebrew UHF and microwave ? > > > > > > http://www.qsl.net/7n3wvm/Fil_Meas.html > > > > > > Peter > > Could you elaborate on this ? Why is an IQ mixer needed ? Well, a DC to microwave spectrum analyzer can't be made with one IF stage only if you're using an zero IF IQ mixer, a damn good reference oscillator -not a DDS which has a really poor SFDR on the whole oscillator band (50-60dB if you're a very good PCB and filter designer) with the eliptical filter adjusted on the maximum frequency range and a sinx/x amplitude dependence (and you need about 90...120 dB SNR if you want to see something on your screen with small signals on the input) -nor a PLL oscillator which has a poor signal to noise ratio (dBc) because of it's working principle. So you need something else or a combinations above... A mixer from DC to 4GHz is good, but not if you're trying to get audio, mixing 4.000.000,000 MHz local oscilator with an input signal of 4.000.000,010 MHz, and you believe you'll really get 10KHz without crazy fluctuations... If you're interested I could send you a few links about home made spectrum analyzers. But I don't recommend building any if you may buy a new one, with about $1500, covering 1Mhz to 7GHz range and showing you at least the frequency and amplitude. greetings, Vasile The way I see it the > most needed thing is low (very low) phase noise of the carrier so the LO does > not swamp the mixer and the audio after it (although I am unsure what happens > with the phase noise in a direct conversion receiver when an actual signal is > not present). The scheme is the same as used in DSB/CW direct conversion > receivers, whose performance is rather good usually. Also using a noise source > as input makes it possible to go to very high frequencies without the usual > problems. No ? Also it is a very simple and inexpensive scheme. With a $10 mixer > it could cover dc to 4GHz. If a stabilised Gunnplexer is used then microwave > filter (and Gunn and varactor bias, by voltage, to match an existing filter or > structure) tuning becomes possible. What am I missing ? Ok, dynamic range is > probably 55dB or less (sound card dynamic range). With a good sound card and a > preamplifier this could be 90-100dB. Many people already have such a good sound > card. If the LO is replaced with a stepwise DSS or PLL then a spectrum can be > taken with say 40kHz 'windows' from DC (almost) to 1GHz. The DSS could be > controlled by a USB to parallel control for example. > > Peter > > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist