Continuing on, the 70 MHz for the LO is tapped off at a leveled low
impedance point, that feeds the normal 70 MHz 0 dBm output on the front
panel. The tap off point is probably around +3 dBm, and I added a higher
R attenuator to get about -10 dBm for the power amp. This CATV amp is
made for 24-30 V operation, but works OK on 15 V, with much less output
power available, and high distortion (obvious on a scope), but still
plenty of gain (35 dB). The output runs about 25 dBm, while the
saturated output power limit is about 28 dBm, which are just about right
for good drive level, but not too much fault power, to avoid mixer
damage if anything goes wrong. The output is already way into
compression, but that's OK. A 6 dB pad connects it to the mixer,
providing nominal drive around 19 dBm, or 22 dBm fault, which is the
mixer's maximum power rating.
That all is what was planned, but what actually shows is that the mixer
looks like a lower Z, well below 50 ohms. I set up the drive with a
built in monitor port that provides a -26 dB view, that showed about
right with a 50 ohm load in place of the mixer, but much lower with the
mixer - it looks like about 15 dBm. It seems to run fine, but is a
little odd. I don't want to push it too hard without more study, so it
is what is is for now.
The maximum noise power comes in at around -70 dBm/Hz from 75 ohms, and
it turns out that a min-loss 75-50 ohm broadband pad is just about right
to knock off 6 dB, putting the R input total power level around +1 dBm,
and peak up to +16 dBm due to crest factor. This is totally safe for the
mixer, and provides good power output. The crest factor will be degraded
somewhat due to running into the LO limit, but only at the highest power
settings. It should be preserved well at lower power.
The chosen mixer is the WJ M9D, which I've discussed previously. Since
this setup is a DSB down-conversion, the conversion loss is less (about
twice as good) than for SSB. I estimate it at around 4 dB, which seems
to agree with my measurements so far. Interestingly, the 50-90 MHz noise
power is not like a typical up-converted baseband signal. Each
"sideband" around the 70 MHz is not redundant to other - they are
independent and uncorrelated (I would think) noise, and simply add
together.
So anyway, ignoring the losses, half of the incident noise power is
converted to the 0 to about 25 MHz range, and the other half goes mostly
to the upper image centered at 140 MHz, and the higher order products.
The IF spectrum viewed on the SA is interesting. The DC-25 MHz portion
is the biggest, and dead-flat in the scale of things. The upper image
looks about 3 dB less, to account for all the rest of the power
contained in the higher products - they are quite large, and go out
quite a way.
That's all for now. Next up will be more mixer and filter stuff.
Ed
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