Thanks, Davinleeds! That's also how my current receiver works, though it took me some time to figure that out!
Thanks for providing the links, holl_ands!
Originally Posted by holl_ands
He presents the double conversion tuner being used WITHOUT an RF Tuned Bandbass Filter on the input--which of course results in better Sensitivity (e.g. Noise Figure) by eliminating the loss in an RF inductor and Varactor Diode (voltage variable capacitor for tuning input RF filter).....and makes for a much more compact design that could fit into a USB Stick.
However, the RF Tuned Bandpass Filter (typical bandwidth of 10-20 MHz) on the input of a single conversion tuner plays a very important role in suppressing strong stations so that you can receive a desired weak signal. Hence, the primary use for a double conversion tuner would be on a cable system where all the signals are roughly the same strength, so the tuner doesn't have to try to receive a weak signal among many other strong signals.
Microtune cites improved performance for adjacent channel performance (D/U ratios), which would be nearly unaffected with or without an input RF Bandpass Filter. What has NOT yet been revealed is how much sensitivity is lost from overload from ALL the other channels across the entire band if an input RF Bandpass Filter is not used....hopefully, an OTA tuner would not forgo the input RF Bandpass filter...
Good point! Looking at the datasheet in the link you have provide, it is clear that the linearity of the LNA is a KEY specification for achieving good OTA sensitivity. I assume that they adjust the VLNA to some output power level (to meet some linearity requirement) and then filter after the first conversion to protect the linearity of the other stages. Clearly if there is a very strong signal also received with the desired small signal , then the gain of the VLNA will be limited and overall receiver sensitivity will suffer. How much it suffers is unknown, since I don't know the output power capability of the LNA.
For CATV mode, please note that the noise performance is 2 dB worse than in OTA mode. It's very possible that they switch in some sort of filtering structure prior to the LNA in this mode or perhaps increase its linearity by upping the bias current. (I note that such a filter is not shown in the datasheet, however.) If so, I suppose they could also choose to do this in the cases where OTA sensitivity would have been hurt more than 2 dB due to a large signal. That would limit the effect of the large blocking channels to no more than about a 2-dB hit in sensitivity. This is all conjecture on my part, but it seems reasonable.
For me, personally, I don't think it would be a big issue since the strongest station is about 25 miles away and it is not in the direction of the farther stations which I desire to receive. However, I realize this is not true for many...