[time-nuts] Noise down-converter project

[ed breya]

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