Now I'll talk about the methods used for processing the LO signals. There are many ways to do it, but I used particular arrangements depending on what I had on hand in the junk parts department. My rule of thumb for TG system invisibility is to acquire the signals and get them to appropriate levels, while providing about 100 dB or more of reverse isolation between all activity in the TG, and the SA's LO sources.
The first LO (1LO) is fairly strong, at around 6-8 dBm, so I went without amplification, and used a cascade of one-octave 2-4 GHz isolators. Four of these, followed by the original 2-3.8 GHz BPF from the 8444A, followed by one more isolator, provides 1LO for the output mixer.
2LO is fairly small, around -26 dBm, so needs amplification. Three narrow band isolators, tested for good performance at 1.75 GHz, are used in this chain. First, an isolator, then a 30 dBg amplifier, then another isolator, then a 1.75 GHz BPF, then another isolator, provide 2LO for the IF mixer.
3LO is also fairly small, around -26 dBm - I picked off only a small amount from the 3LO module in the 8568, to minimize loading and interference with the signal that goes to the PLL synthesizer in there. This is the part I discovered late in the game - the need for 3LO rather than a fixed 301.4 MHz source.
It turned out that I had some big old RF modules from an AIL brand synthesizer I junked out twenty years ago, and had only kept these parts intact all this time because they had nice BPFs built in. The BPFs were the wrong frequencies, but close enough to be readily re-tuned for what I needed. The first module is an amplifier/filter/amplifier, and was ready to use pretty much as-is, by re-tuning the BPF and peaking circuits in the amplifier stages for 280 MHz operation. One drawback is that these modules were built for -12V power, but the 8444A PS only has -10V, and I didn't want to make extra power supplies. They work OK on -10V, but have less gain available. They can easily be modified if necessary, for more gain.
The second module was a mixer/amplifier/filter/amplifier/mixer - the exact function I needed, except for the second mixer, which I deleted. I tweaked it up for 301.4 MHz, and added a PIN circuit and a gain stage at the output.
So, the first module amplifies and filters the 280 MHz 3LO, which is then mixed in the second module with 21.4 MHz from a VTXO. The resulting 301.4 MHz is amplified and filtered, then amplified some more. The PIN stage provides gain control for the ALC.
The 301.4 MHz goes to the IF mixer, along with 2LO, to make 2050 MHz, which goes through an isolator, then a 2.05 GHz BPF, and another isolator, then to the output mixer, where it gets subtracted from 1LO to make the center frequency.
You can probably tell by now that I like using isolators, and have a pretty good stock of them. If I didn't, I would not choose to go this route, because to acquire them new would be very expensive, as with most packaged microwave parts. These functions and high isolation can all be readily attained by using lots of attenuation and lots of amplification, properly distributed, in a number of arrangements. It's also likely that so much isolation is unnecessary, depending on filters, amplifier and mixer characteristics, and signal levels - I've probably overdone it by 30-40 dB in some spots, but it's expedient to just have lots. With experimentation, the various sensitivities of the LO ports can be determined for all operating conditions, and just enough isolation can be applied. Even with little isolation, it doesn't hurt anything - it just limits the available dynamic range.
Next time I'll talk about the output section, low frequency limitations of the original output amplifier and mixer, and the low-band amplifier I had to build to reach nearly DC.
Ed
