I've got, and love, an RFSpace Inc. SDR-IQ software-defined receiver.
It covers 100 Hz to 30 MHz, and lets me eyeball any 190 KHz (or
narrower) chunk all at once, which is rather particularly nice. I'd
like to be able to use it on 30 to 60 MHz and higher, which means that
I need a downconverter. It seems to me that a suitable downconverter
would let me set a switch to choose which 30 MHz band got mixed down
to the roughly 0-30 MHz baseband.

I'd like to have this work up through the 70 cm. ham band for sure,
and as much higher as I can get. I know that construction techniques
change as one gets above HF, and change _radically_ at UHF and above.
How hard is this going to be, overall?

The general setup I envision is something rather like

LO
n*30MHz
(n to |
(n+1)) |
* 30 MHz 30 MHz wide V 0 to 30 MHz
input -- bandpass filter -- mixer -- output
to SDR-IQ

Some questions for the (perhaps not older: I'm 60) wiser heads with
more design-and-construction experience: is this at all a reasonable
thing to do? Does anyone have any ideas on suitable mixers? Am I
looking at a disparate-enough set of frequency bands that I will wind
up having to use multiple mixers, possibly one per 30 MHz band, or
per octave? It is trivially obvious to me that I'll need different
antennas.

Is it reasonable to look at using a comb generator which is driven by
a 30 MHz reference, so that I just take the appropriate harmonic and
amplify it to a power level that suits the mixer I have to use for
that particular 30 MHz band? Would it be better, perhaps, to use DDS
to generate the LO frequencies I want?

--
Mike Andrews, W5EGO

Tired old sysadmin