On 10/6/21 10:53 PM, msat via groups.io wrote:
First off, thanks to everyone for all the helpful responses! It made me realize just how loaded my question actually was. It also help unjam the gears in my head. That said, there's a lot of stuff here to respond to, so hopefully I don't forget anything I wanted to discuss.
To clarify, I intend to operate at a fixed frequency, no modulation, preferably somewhere in the 900MHz ISM band, driven by an amp operating at less than 5W. Each run of the experiment should only last a few minutes at most.
Probably due to me not knowing any better, I'm leaning towards making my own amp along with bypass filtering. The amp section would be based on modules such as:
@Jim Lux & Andy G4KNO
Your references to the FCC requirements and the given examples are very helpful. It provides a better perspective of what I'm dealing with here. It also shows me how much I still have to learn.
Regarding the monolithic filters in that band, I suspect they're a lot less common than they once were. I see reference to a lot of discontinued items. That said, some can still be found which is plenty for my needs as long as they provide sufficient performance. The datasheets for some of these leave a lot to be desired. I didn't hear of minicircuits until you mentioned it. It looks like they may have a viable solution.
I'd say Murata and Johanson are better bets for filters - a better selection - and then, there's the one posted by someone else. In any case, the filters are cheap.
BTW, it depends on if you have more time or more money, but MiniCircuits also has RF amplifiers. 5W is going to be around $600-1000, depending on how much compression you can tolerate. There are probably also countless surplus sources, but that brings other tradeoffs (time vs money).
@Roger Need
I actually had both the rf-tools page and that digikey part (along with some others) already opened in a browser tab. I made the mistake of making a really sharp 1st order bandpass without realizing how ridiculously tiny either the capacitor or inductor was. After widening the band enough to allow for more realistic components, the roll off was much more shallow, in turn requiring higher order filters. By that point, I became a bit more concerned regarding the complexity of the filter.
Which is why, if someone makes something like a ceramic filter, that's usually easier.
@OneOfEleven & John Gord
Thanks for the hardware suggestions (I'm actually considering a TinySA to test filters and amps), but since a critical aspect of my experiment is getting phase angle information, I either have to use the nanoVNA to generate the RF signal, or I don't use the nano at all and instead buy an old so-called "vector voltmeter" along with all the other hardware I'd need. I can't help but to think it should be possible to sufficiently attenuate frequencies outside the nano's 5th harmonic much easier and cheaper than to purchase a bunch of additional equipment.
Or, if all you need is phase, get yourself a source (a VCO and a pot or DAC to set the frequency), some filters, a PA, and a eval board for an I/Q demodulator to serve as the receiver, then run your I/Q through low pass filter (RC is fine) and digitize with a RPi or an Arduino (I favor the Teensy series, myself).? Use a sample of the transmitted signal as the LO for the demodulator.
So one thing that's not clear to me, particularly when it comes to dielectric-type filters, is whether they could be cascaded to increase Q without using amp inter-stages.
Maybe, maybe not - layout is important. They're cheap, try it and see.
