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

Bob Camp kb8tq at n1k.org
Sun Jan 17 01:43:09 UTC 2016


Ok, so:

Does broadband phase noise matter in a distribution amp? If so why?

Does ADEV matter on a distribution amp? If so why?

Does long term accuracy matter on a distribution system? (yes of course it does).

Is there any benefit to a distribution amp having a lower floor than the source driving it? If so why?

Is a source likely to have excellent broadband phase noise, good ADEV, and excellent long term accuracy all at once?
If so how much does that source cost? Do you have one?

What is your distribution system driving? Does it care about low broadband, good ADEV *and* good long term accuracy? 
If so, what is it?


The answer to that multitude of questions is what drives the design of a distribution system. Normally designs are done to target
a set of targets for a set of customers. A typical instrument “target” is not in any way impacted by broadband phase noise. There are a 
lot of customers who run instruments off of distribution systems. They *do* care about long term accuracy and ADEV. 

The gotcha is that you have tradeoff’s between each of the parameters. The best approach is to target what you need and ignore the rest. 
To hit -170 dbc/ Hz phase noise broadband and a 7 dbm output, you need about an 11 db noise figure. (assuming it all goes one way). For
good ADEV performance, a noise figure of 41 db is plenty good enough. A couple of pico seconds of delay variance have zero impact on 
broadband phase noise. They are a big deal for ADEV, they have marginal impact on long term accuracy. 

Isolation between outputs likely has very little impact on broadband noise. It’s also unlikely to impact ADEV. Plugging this and that with a “pop”
likely has a bit impact on long term accuracy. Lots of issues. 


So, for broadband phase noise - you need a lot noise figure amp. It also needs to be able to handle the intended output level *and* achieve
that noise level (not as easy as you might think). Add in the need to do all this into a specified load (say 50 ohms), that narrows things down 
a bit more. Isolation is likely to be a function of the package and how many packages you use. At 40 db per package, you may need three
packages to get a 120 db isolation number. 

A logic gate can get you to -174 dbc/Hz broadband. Does it do what else you need to do? Cascading op-amps may (or may not) be easier than 
cascading logic gates. Driving an op-amp into an op-amp gives you a pretty high load on each of the cascades. The final op-amp needs to be 
able to drive 50 ohms. With the gates, the last gate (or combo of gates) needs to be able to drive 50 ohms.

Lots of fun!


> On Jan 16, 2016, at 8:59 AM, Anders Wallin <anders.e.e.wallin at gmail.com> wrote:
> 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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