Hi all, I'm continuing with my distribution amplifier project described here: http://www.anderswallin.net/2015/12/frequency-distribution-amplifier-first-tests/ http://www.anderswallin.net/2015/12/frequency-distribution-amplifier-v2-simulations/
The conclusion from those v1 board tests were that the AD8055-based amplifier phase noise floor is at around -156 dBc/Hz while an old Symmetricom 6502B I have in the lab is at -163 dBc/Hz. It would be nice if the new design was at least as good as the 6502B ;) On the v2 board I improved the power-supply with more filtering (BNX025) and changed the layout to take SO-8 op-amps with an analog-devices style feedback pin-1 and exposed bottom/GND pad. With an AD4899-1 the results are in principle OK with small amplitude input, but I think the limited slew-rate of (310V/us) causes compression/distortion quite quickly and the phase noise is actually worse (-150 dBc/Hz) at +7 dBm output compared to +4 dBm signal level ( ca -155 dBc/Hz). Contrary to simulations I didn't get 12 dB better phase noise out of the AD4899-1 - beware of simulations I guess is the lesson! ;) I also tried an AD8000, but this op-amp is not stable on my layout (oscillates at 470 MHz). Yesterday I opened up the 6502B to reveal a LMH6702-based input-stage and an LMH6609-based output stage. The files are on dropbox: https://www.dropbox.com/sh/3vxyu87ax5cqjyf/AACQWDG3lN5lpnvTAqRXKW9ia?dl=0 Maybe someone in the group already has a schematic, home made or official, for the 6502B? To me the signal path looks like: R1 is 50 ohm (switchable) termination to GND. When in hi-Z mode the input impedance is R2=2k. The input stage LMH6702 looks like an inverting amplifier with gain -RF1/R = -133/100. The distribution to output stages is via a small resistor R7=10R, and then there is (small?) loading of the input-stage by the level-detector/alarm. The output stage looks like a non-inverting circuit with a 1k/1k R21/R22 divider on the input and gain of 1+R23/R19 = 2. Any comments/ideas of why they use a different op-amp for the output stage? The powersupply is nothing fancy at all: LM317 and LM337, two devices in parallel I think. What are the parts "Dale 100" and "Dale R27"? Someone suggested resistor networks to me. "Dale 100" is used as the R in the first of two RC-filter stages on all power-supply pins. What is the benefit of 2 (or 3?) resistors in parallel? "Dale R27" is used on the output, just after a 51R1 output-impedance resistor and before a (very small?) capacitor C15 to ground. Almost half of the components deal with the alarm-signal, which to me looks like a level detector built around the dual-opamp LM358, where input and 10x outputs possibly share a common single alarm-trace? For the next version I will either try to make a stable schematic/layout for the AD8000 or try the LMH6702/LMH6609. I will skip the alarm-circuit of the 6502B, and perhaps also change to bipolar simpler supplies as compared to the unipolar TADD-1/v1/v2 design. If anyone wants the kicad-schematic I threw together just email me. Anders _______________________________________________ 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.
