Full duplex in sat range is not exact,name of, but it's mean TX and RX modes run via same path - AERIAL of OWNER’s TRX. May be is not exact name , but is it. RX signal from LNB is really strong,so 20dB pad is not a problem. Few radioamateurs add cable to rerduce signal level up to 40 dB! If you compare,many constructions include attenuator on RX path to reduce a lot of power from LNB. In that case sensetivity still at desired level. Supression of conversions or unwanted products. Because in every point or in every mixer is single frequency signals,try to calculate where you can see unwanted products.
My oldmen teacher (RIP few years ago), was teach me 35 years .ago and i really remember many lessons and themes, but one geniuos phrase i'm remember perfect: "every RF circuit will working ideal on the paper even mistakes is in".
I will explane you all part and part from this design, and because this project is totaly my from first block diags up to synth calc include phase noise,loop filter, program and other blocks. Also, you can immaginary about fantastic parts, but if you have not compact, low power consuption and RF level VCO, your project will on paper only.
Off course i can make any manupulations with frequency sources, multiply, divide, add or substruct, but first all i need choosing parts with cost-efficiency line. In thats case must to see what's market have too.
If i talk about design for HF transverter, in any case it must to give to owner working (by request) in one of HF bands from 3,5 to 28 MHz.
I will attache screenshots of Appcad 's run spurs of mixer. I open scale of 5-th product of every mixer combinations.
Now about choosed parts - my opinion is VCO from Maxim series MAX262x & VCO from Alps E-D07A.
Now show me please VCO for 720 MHz mid range with same parameters. Believe me, I testing market before making first step.
You are change my circuits, but have not parts. I receive 711,5 MHz by product of 1660,5-949 MHz. In the next mixing stage I’m add 1660,5 to 711,5 MHz, there additional TRX freq of 28 MHz will give 2400 MHz. To change final product to desired HF band, I reprogram & lock PLL 949 MHz to 935 MHz e.t.c. For working on 20 m band, mixing must to give 2386 MHz. Also other ranges of HF.
It’s good or not I don’t know. Now I wait for mixers, so important parts is tested – PLL lock and software test.
I hope that you have all answers for your questions.