[time-nuts] Distribution Amplifier: a look inside the 6502B

Anders Wallin anders.e.e.wallin at gmail.com
Sat Jan 16 08:59:59 EST 2016


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



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