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

[Bob Martin]

I never had much luck with current feedback amplifiers such as the 
LMH6702.  Their input current noise (at the time) was too high for 
my needs and their output peaks at higher frequencies if the 
feedback resistors aren't optimal for the part.

  I had the best results with voltage feedback op amps like the 
MAX4104/MAX3404 when I needed gain on the input stage and the 
LMH6609 when I needed a buffer.

  My applications were broadband. If I remember correctly, 
aggressive bandwidth limiting can cause phase shift problems due to 
temperature changes unless one is careful in the design of the filter.

  I've successfully put as many as four op amps in parallel in an 
input stage to reduce phase noise.


Bob M (another bob)

On 10/24/2017 6:24 AM, Bob kb8tq wrote:
> 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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