Hi Bert, No need to block the DC. These instruments already have DC blocks (and a switchable low current DC sink to "hold" the line from the exchange) because the telephone line has a DC voltage of typically 48V on it already. For comparison I just use 600 ohm termination and dBm, dBmV can be calculated or use a look-up table. To compare a batch you can zero the meter on the first or known good unit and just read the delta. Robert G8RPI.
________________________________ From: Burt I. Weiner <[email protected]> To: [email protected] Sent: Thursday, 31 January 2013, 19:19 Subject: [time-nuts] Low noise power supplies? Bob, I have a pair of HP-3551A's*. I'm very familiar with making transmission lines measurements, and it seems that measuring power supply noise would be the same, except that you want to block the DC from the input of the instrument. What has your procedure been and what numbers have you come up with? Since these instruments read in dBm0, do you reference from the supply's voltage and then convert to mV (difference)? * One that I picked up off eBay for $75.00 looks new and came with the complete manual, and the battery will run it most of the day. Thanks, Burt, K6OQK > Subject: Re: [time-nuts] Low noise power supplies? > > A non-standard but repeatable way to measure power supply noise is to use a > Transmission Impairment Measuring Set (TIMS) such as the HP3945(6)A or 3551 > (2)A. These were intended for use in pairs to assess analog telephone lines > for data use. As well as an AF generator, frequency counter, amplifier, > monitor speaker and level meter they will measure broadband noise. Being > designed for POTS they will also withstand at least 50V DC at the input while > measuring the noise. You can also apply internal filters if required. The > last digit designates a North American? (BELL) or European (CCITT) standard > unit, but broadband noise is the same. They can be picked up really cheaply > now (list was$3000-$5000) and make a nice compact audio test set. > > Robert G8RPI. > > > Burt I. Weiner Associates Broadcast Technical Services Glendale, California U.S.A. [email protected] www.biwa.cc K6OQK _______________________________________________ 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.
