John- The BPFs in the 5 to 20MHz chain are just 7-pole LC filters with the goal of trying to keep any other harmonics other than the desired at least -50dBc. Xtal filters would be the better choice, no doubt.
The -50dBc level is clearly not the best that one could get, but was enough for an earlier 240GHz project. I just used the same OCXOs and early stages of multipliers to get the latest system running on 630GHz. In the 241GHz system, I ended up building a direct frequency synthesizer to get 110MHz from a 10MHz drive signal. At the time, the Freq West PLL blocks I used wanted a VHF signal to drive the sampling detector to phase lock the L-band cavity VCO. The original Freq West units used 5th OT xtals for the commercial applications. By later experimentation, I found that the same sampling detector would also work with a much lower frequency reference and still lock the loop. The risk however is that the PLL might lock on the wrong harmonic of the reference (i.e.: value of N) or can have higher reference spur levels since the PLL was designed assuming a VHF reference and not an HF reference frequency. But this is not a commercial design project, and I can live with a difficult alignment procedure or initial power-up PLL lock troubles. But all this aside, my efforts are currently aimed at best close-in noise within the first 1KHz of BW around the carrier. The PLL bricks all seem to have several kHz of loop BW, so my close-in noise going from 20MHz to 1320MHz should be only slightly worse than 20Log(n), with n=66 in my case. But I'm not ruling out the chance of 1/f noise (or similar) showing up from the sampling detector or some other yet-to-be determined source. However my focus is currently on the 5MHz to 20MHz portion of the LO chain and to be sure the gain stages are not running near compression. I do still agree with your earlier comment about getting the most from that portion of the chain. -Brian, WA1ZMS -----Original Message----- From: [email protected] [mailto:[email protected]]on Behalf Of John Miles Sent: Tuesday, December 16, 2008 7:53 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Close-in phase noise question...more info... > More info on the LO chain: > > 1) 5MHz Wenzel OCXO <--Custom Osc for me. > 2) MSA-1105 buffer MMIC and lumped LPF > 3) 5MHz to 10MHz 1N5711 diode based doubler > 4) MSA-1105 buffer and lumped LPF > 5) 10MHz to 20MHz 1N5711 diode based doubler > 6) 20MHz BPF What kind of BPF? A really narrow crystal filter would be nice here. (You have basically reproduced the 8568A/B's 20 MHz reference section.) > 7) 20MHz drives sampling detector inside surplus Frequency > West PLL block to lock 1320MHz cavity oscillator. Sounds OK as long as the sampler loop's noise floor doesn't limit you. I haven't measured the in-band residual floor of any bricks but I'd be surprised if an SRD multiplier wouldn't be quieter. > 8) 1320MHz drives Frequency West SRD multiplier to 6.6GHz. If I wanted to get to several GHz with what's in my junk box right now, I would do what you did to get to 20 MHz, BPF it with a multipole crystal filter, and then use a few more multiplier stages to get somewhere between 100 MHz and 1 GHz, a la the 8662A reference section, depending on the choice of the next stage. That VHF drive signal would go into either an HP 33002A or 33004A SRD multiplier, or one of the Picosecond NLTL multipliers (e.g., http://www.picosecond.com/product/product.asp?prod_id=109 ) I picked up in their fire sale when they shut down their fab. > 9) 6.6GHz to 39.6GHz Milliwave diode multiplier/amp/filter > 10) 39.6GHz to 79.2GHz in varactor doubler > 11) 79.2GHz to 158.4GHz in varactor doubler > 12) 158.4GHz into x4 sub-harmonic mixer AFAIK the rest of the chain is fine. I'd focus on getting rid of the brick PLL, or at least taking pains to make sure that it's not the problem, before worrying about the MMICs in your early stages. Remember that there's no point in optimizing the PN of any one stage much below the input-referred residual noise of the following stage. MMICs, in saturation or not, are pretty quiet. Quieter than sampler loops anyway. -- john, KE5FX _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
