With the 11729B/C it's easy to reroute the mixer output directly to the front panel. The internal mixer is decent, but it can also be easily be replaced with a high power unit if that's desired. Starting with a box which needs no S/W to drive it can be useful.
Pete Rawson On May 7, 2010, at 1:13 PM, John Miles wrote: > It's not really the mixer's fault -- there are a couple of other things to > keep in mind with the 11729. The most important is that you can't measure > 10 MHz sources with it unless you install an LPF in front of the LNA, > something with a cutoff on the order of 1-2 MHz. At 10 MHz and below, the > USB output from the phase detector at <= 20 MHz will drive the LNA into > compression. The manual warns you against using IFs in that range with > microwave downconversion, but the same problem applies at baseband, and over > a wider frequency range than they say it does. Without the additional > filter in place, you shouldn't try to measure anything under about 30-40 > MHz. > > You will lose the ability to view PN at offsets greater than the LPF cutoff, > of course, but the 1-10 MHz decade isn't usually very interesting anyway. > > Second, the ALC-limited amplifier in the 11729 seems to have a PN floor in > the -155 to -160 dBc/Hz neighborhood, while as you noticed the 11848A brings > its mixer ports directly to the front panel. (You still have to use > outboard isolation amps to measure OCXOs with the 11848A, but they can be > much quieter than what's in the 11729). > > The specs in the 11729 manual are worse because they are talking about the > best results achievable with microwave downconversion, I believe, and/or the > best results achievable with downmixing from the 8662A as Christophe says. > The baseband performance is much better than specified, if you use a clean > reference, but it's still not quite what you need to measure 10811s and the > like. For that you're better off gaining direct access to the mixer. > > It helps to think of the 11729 as a collection of useful modules that can be > rearranged as needed for specific low-noise measurements. Adding the > pre-LNA filter is job #1, IMO. > > -- john, KE5FX > > >> -----Original Message----- >> From: [email protected] [mailto:[email protected]]on >> Behalf Of Christophe Huygens >> Sent: Friday, May 07, 2010 7:17 AM >> To: Discussion of precise time and frequency measurement >> Subject: Re: [time-nuts] HP 11729C versus 11848A >> >> >> I don t have my notes here, but are these numbers >> not for the 11729C system including the 8662A >> rather than for the unit standalone? So as long >> as you are measuring > 20-30 MHz the difference >> 848/729 should be much smaller than indicated, >> or be attributed to the ref? >> >> Xtof. >> >> >> >> Adrian wrote: >>> I tried to measure phase noise of a 10811A, but found out that the >>> specified PN is below the noise floor of my 11729C. >>> >>> Can anyone tell why the (phase detector method) PN noise floor is so >>> much different between the two units? >>> >>> 11729C at 100 Hz -126 sBc/Hz (-133 dBc/Hz typ.) >>> 11729C at 1 kHz -135 dBc/Hz (-140 dBc/Hz typ.) >>> >>> 11848A at 100 Hz -150 dBc/Hz (-160 dBc/Hz typ.) >>> 11848A at 1 kHz -160 dBc/Hz (-170 dBc/Hz typ.) >>> >>> Some 25 dB is quite a difference, isn't it? >>> >>> Basically, both units apper to be not that much different, except that >>> the 11729C has an IF amp and power splitter between the input and the >>> PD L port, while on the 11848A the L input is fed directly into the >>> mixer. There are some differences in the LNA circuits, but that >>> shouldn't be responsible for the huge noise floor difference. >>> >>> 11929C requires 0 dBm (-5...+10 dBm) 'L' (MW Test Signal) input level, >>> that is amplified by the IF amp to >+10 dBm at the mixer input. Btw. >>> the IF amp saturates at input levels grater than -50 dBm. For the 'R' >>> input (5-1280 MHz), the manual specifies -1...+1 dBm. >>> >>> For the 11848A, the L input is +15...+23 dBm, and 0...+23 dBm at the R >>> input. Below +15 dBm L and R, the system degrades considerably. >>> Reducing 'L' to +7 dBm adds 10 dB to the noise floor. Reducing 'R' >>> below +15 dBm adds directly to the noise floor. So, reducing it to 0 >>> dBm would add 15 dB to the noise floor. >>> >>> So, it looks like the 11729C phase detector is more like a +10 dBm >>> mixer, while the 11848A has a +17...+23 dBm mixer. >>> >>> Replacing the 11729C PD with a ultra high level mixer should get the >>> noise floor close to 11848A specs. It would just require to feed L and >>> R directly into the mixer rather than using the instrument inputs. >>> >>> Any thoughts / experiences referring to this? >>> >>> Adrian >>> >>> _______________________________________________ >>> 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. >> >> >> -- >> >> >> Christophe Huygens Dept. Computer Science E-mail: >> [email protected] Celestijnenlaan 200 A. bus 2402 B-3001 >> Leuven, Belgium Tel: +32 16 32 70 88, Fax: +32 16 32 79 96 >> >> >> Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm >> _______________________________________________ >> 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. _______________________________________________ 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.
