Re: QMX RX BPF troubleshooting, a new clue
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So, I had the 20m sensitivity issues myself, and rather than futzing around with it I had the realization that the RF is passing through a 5th order lpf on its way into the radio, so another series inductor on top of that is really not going to make or break the high frequency rejection on receive. I ripped my L401 out completely and I've had nothing but improvements in performance. If anyone has a SPICE sim or an irl measurement that proves me wrong I'd love to see it but between the TX LPF and the inherent bandpass property of the Tayloe mixer it seems like extra filtering in the receive section just isn't all that necessary.
I don't agree... the LPF does indeed suffice for rejection of the harmonic content. I believe the BPF in general provides an improvement in performance. The QSD has very high performance but I believe a BPF ahead of it is still a good idea.?
The claimed inherent filtering of the QSD is useful but doesn't save the day totally. Think for example, of a superhet SSB receiver whose front end consists of a diode ring mixer, then there's a crystal ladder IF filter and a product detector. Typical situation. The crystal ladder IF filter provides the SSB sensitivity. The dynamic range and intermodulation performance (IP3) are determined by the characteristics of the diode ring mixer. We don't say that because an IF filter follows it, there is inherent bandpass filtering so we don't need to protect the front end from strong out of band signals. The IF filter provides the desired selectivity but the signals still have to get to it, through the mixer, first.?
I don't see that the QSD is fundamentally different in this regard. The 1:4 MUX switch is configured as a double balanced QSD. The double balance topology gives you good common mode signal rejection (good rejection of AM broadcast signals). The dynamic range and IP performance is very high but it is still fundamentally limited by this mixer; and still protecting it somewhat from out-of-band strong signals will help a little. The performance is determined by how perfect the switches are. They're a lot better (more perfect) than diode switches which is? why the performance is greater than diode ring mixers.?
If the band pass filter is eliminated then you may not notice any performance degradation and perhaps the few dB of rescued insertion loss could improve sensitivity slightly; but at the expense of vulnerability to out of band signals, which you may not notice having their detrimental effect. The same applies to removing front end filtering on a superhet receiver. You do have some other considerations; on a superhet receiver you need to filter out the image frequency. Well on a hard-switching detector like QSD we need to filter out reception at odd harmonics of the LO (the LPF can do this, as you said). You could remove the front end filtering of an SSB receiver and think everything was OK too. But still the designers wouldn't recommend it.?
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