[time-nuts] Spice simulation of PSRR and phase noise

Bob kb8tq kb8tq at n1k.org
Tue Oct 24 08:24:32 EDT 2017


Hi

One would guess that they put them in parallel to get more drive. If that’s correct,
details of the loading are going to get into the simulation pretty quickly. 

In a lot of cases, these amplifiers were designed against a specific need. If you have
a signal source that is in the -180 dbc / Hz range, they are unlikely do perform well. 
In many cases a floor in the -140 dbc / Hz range was considered “good enough”. 
If you are simply driving common test gear, it probably *is* good enough. If the 
application was video rather than a standard the specs could have been very different. 

In the case of an amp with a LMH6702, you are not going to get super  close in 
phase noise. The device is *very* noisy under 1 MHz. It also starts to increase distortion
by 10 MHz so you will see up conversion. It probably did quite well against the intended
design spec. 

=====

If you need a system that will distribute one frequency today and a totally different
frequency tomorrow, broadband makes sense. For the more common task of 
something like “only 10 MHz”, it does not make much sense at all. Gain other 
 frequencies is just going to spread around noise from this or that source
of crud. Driving filters with op amps can be problematic. It often is easier to go another
way. 

Bob

> On Oct 24, 2017, at 6:09 AM, Anders Wallin <anders.e.e.wallin at gmail.com> wrote:
> 
> FWIW I recently took a peek inside a commercial distribution-amplifier and
> it seems to use two LMH6702 op-amps in parallel.
> There are two of these dual-LMH6702 stages with a 1:2 splitter after the
> first, and then a 1:4 splitter after the second stage. 8 outputs in total,
> with an additional op-amp driving each output.
> A simulation that shows the difference in PN between a single LMH6702 and
> the dual-op-amp idea would be nice.
> For far-out (>100Hz from carrier?) PN only SNR might matter, so a SPICE
> noise-simulation giving noise PSD at relevant (5-10MHz) frequencies might
> give something?
> 
> Anders
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