[time-nuts] OP-Amps for 10MHz distribution...?

Bob Camp lists at rtty.us
Tue Feb 28 22:20:56 UTC 2012


I think the tradeoff of "harmonics still better than -60dbc" is fine if the
floor does indeed drop a few more db. For time nut applications, it's going
to be the phase noise close to carrier that matters. Harmonics with
reasonable terminations would not be the big issue. 

It will be interesting to see if the supplies really do contribute to the
close in noise in this case. If so, there are a lot of ways to take care of
that. My guess is that a couple simple linear regulators will take care of
it if there's an issue there. 

If you have a setup to do so, take a look at the output return loss next
time it's up and running. One thing this approach *should* do is to give you
very good return loss.


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bruce Griffiths
Sent: Tuesday, February 28, 2012 1:21 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] OP-Amps for 10MHz distribution...?

The datasheet indicates that an input of +13dBm should be OK albeit with 
somewhat more 3rd harmonic distortion (~-65dBc) in the output.
I'll unearth the prototype single transistor (2N5943) transformer 
feedback discrete buffer and use it to boost the signal before the splitter.
I'll also measure its phase noise, or at least attempt to do so as its 
likely to be very quiet.

The OPA653 was mounted on the TI evaluation board using its internal 
feedback resistors to set the gain to 2x before the 2x attenuation 
produced by the source termination and load resistors.
The board was powered by a couple of HP E3611A bench supplies.
Its likely that the low frequency power supply noise modulation of 
internal opamp capacitances (and hence the output phase) is greater than 
that due to opamp noise.
Measurements with quieter power supplies may be useful.

The OPA653 is a voltage feedback amplifier, a current feedback amp like 
the AD8007 is likely to have a somewhat higher phase noise floor (+6dB) 
due to the inverting input noise current (22.5pA/rtHz) flowing in the 
feedback network resistors (2 x 500 ohm for a gain of 2 ).


Bob Camp wrote:
> Hi
> Very cool. How much power can you run through the device? Put another way,
if you drive it with +13 dbm do all the numbers get 5 db better?
> I doubt very many of us will be worrying about weather it's below -153 at
10 Hz or not.
> Bob
> On Feb 28, 2012, at 5:42 AM, Bruce Griffiths wrote:
>> The attached plot indicates the phase noise performance obtainable with a
wideband FET (OPA653) input opamp.
>> With a 10MHz +9dBm input, the phase noise floor is around -163dBc/Hz at
1kHz offset and around -154dBc/Hz at 10Hz offset.
>> A quieter test source would be useful particularly for offsets below
>> Bruce

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