Frequency extension for Spectrum/Network Analyzer
I have HP3582A (0.02Hz to 25.6KHz) and Anritsu Network analyze of 30MHz range.
The obvious way is to use a down converter to convert the desired frequency band down to the frequency range of your spectrum analyzer.
Such converter is designed with an input mixer and a local oscillator. The oscillator frequency is set as a difference between input RF frequency and the IF frequency which falls into the frequency range of your instrument.
For example, if your spectrum analyzer covers RF input from 10 kHz to 30 MHz, to observe frequency around 100 MHz, you can use a local oscillator at 80 MHz; this will "shift" 100 MHz to 20 MHz which you can observe on the spectrum analyzer, and still leave +/- 10 MHz around. So your input RF range will become ~81 MHz to ~110 MHz with the down converter.
@ jiripolvika
I know there are many companies making harmonix mixer but only for mmwave region (>50GHz). not for very low frequencies.
You can use harmonic mixers but a "straight" converter is better due to a lower conversion loss. If you are interested in lower frequency band than covered by your spectrum analyzer, you can build or buy an upconverter.
For low frequencies, some good designs are used by those observing atmospheric whistles and other natural low-frequency phenomena.
it is very useful indeed.
Can you recommend some local oscillator/mixer by HP that works at such low frequencies
I am sorry I cannot. My interest is rather in microwave and mm-wave region. But you can go to Agilent website and look through their catalog. If you wish to make something yourself, google SID and Atmospheric Whistles; there are amateur groups making and using very low frequency receivers to observe solar effects on Earth atmosphere.
Radio amateurs also have hard and software for a direct spectrum analysis at lower frequencies, from DC to ..30 MHz. This way may be better than complementing a RF spectrum analyzer.