On Sat, Oct 21, 2023 at 01:21 PM, John Z wrote:

Any notion that the Tayloe input is a simple low impedance load is now dispelled.

I don't think anyone advanced such a notion in this group as far as I know. I also don't recall anyone who argued against that the input impedance is dependent on fLO.

About 120ohm on 20m and 38ohm on 10m is a lot lower than what was predicted from your simulation that omitted parasitics.

Also, keep in mind that QMX uses an IF of 12kHz so the actual operating impedance is much lower than those peak impedance right at fLO. Exactly how much, I'll wait for a non-blurry closeup traces.

With my QMX the voltage was limited to 4.2V, which was unfortunately too much for the MS5351S (IC204). This resulted in an unclean output signal and even a doubling of the set frequency: /g/QRPLabs/topic/101560193

73 - J?rg, DG0JDE

Yes for both. Many people pointed out those things. I think Hans is working on those.

I think one of your zooms is mixed up (supposed to be 20m appears to be actually 10m) but I get the idea. More of an issue I have is the blurry images that makes it hard to read...

The input impedance appears to be about 120ohm up to 17m but goes down much lower at 15m and higher.

In order to take advantage of the input impedance characteristics to maximum advantage for the preselecter purpose is to keep the signal impedance high. Since 15m and higher need a lower input impedance than 17, this is probably most easily achieved by a high-pass L-match (series C to the source, shunt L at the load). If done well, some 6dB sensitivity gain can be expected on lower bands, less gain on higher bands.

I said L-match above and in another thread since the explicitly added components look like an L network. However, keep in mind the input end has a significant shunt cap (BSS123, MUX, PCB pattern, etc.) so the actual function is more like a pi-match if you start with the end of LPF output. What I did in my 60-10m QMX is a pi-match with the inductor in the middle. That serves the same impedance matching function, except it is a low-pass configuration (because I wanted to use the existing network topology with minimum modifications).

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JZ, keep up with good work on the refinement. What is the actual complex impedance profile +/- 100kHz of fLO, where fLO is between 3MHz and 30MHz? Then implement realistic strays?

If the impedance is very high and well behaved, after you implement all the strays realistically, one approach to clean up the whole BPF issue while increasing the overall receiver sensitivity may be to increase the input transformer winding ratio and/or replacing the BPF with a high-pass L-match (series C to the source, shunt L to the load) which will behave like a band-pass anyway. I'm not sure what the actual input impedance will be though, I get an impression that this line may be around 75 or 150 ohm, a bit reactive. For the high bands, where I replaced the BPF with a pi-mach, whatever the load impedance is, it doesn't really matter much because when I tweak the pi-match inductor it's close enough for a receiver work.

At least, this explains why some people have very different LPF performance (distance, direction of winding), and why QMX and QDX have very different BPF behavior (because BPF and LPF directly interact through the signal line impedance profile). I never built QDX myself but photos on the QRP Labs website show that the LPF inductors are sparse and turned 90 degrees to each other to avoid this magnetic coupling.

Something like this? I’m hoping to get this onto Etsy soon ?it’s resin printed, hand finished and painted ?knobs too ?

Here it is in use.?

--
Colin - K6JTH?

73 Lex PH2LB?