[time-nuts] SMD TADD-1 distribution amplifier - seeking comments and suggestions?

Jörg Logemann joerg.logemann at lai.de
Thu Dec 17 18:08:56 EST 2015


Anders,
the TL071 is not the right choice as it has a unity gain bandwidth of
only 3MHz! You should use an OP with a unity gain bandwidth of ten times
the wanted frequency at least (I guess 10MHz, so a 100MHz OP). Modern
OPs are AD8045 (preferable), AD8099 (too fast and too good for this
purpose;-), OPA847 and many others. The AD8045 can drive 70mA, so you
could drive up to 15 outputs with 0dBm or up to 5 outputs with +10dBm
simply by serial resistors (50 Ohm) from the OP output to each
coaxconnector (+dc-block).

Joerg, DL2NI

Am 17.12.2015 um 20:44 schrieb Anders Wallin:
> First prototype assembled today, tested with 12 VDC SMPS wall-wart supply
> and with 12 V lead-acid battery.
> Compared against a SRS FS710 and a Symmetricom 6502 and John Ackermann's
> 2007 plot:
> http://www.anderswallin.net/2015/12/frequency-distribution-amplifier-first-tests/
>
> What does the 6502 do differently? It is maybe 5 to 8 dBc/Hz better both in
> PN and AM from around 1 kHz and out.
>
> My 'ultra-low-noise DC-supply' in the form of a lead-acid battery improves
> things somewhat, but some spurs still remain:
> http://www.anderswallin.net/wp-content/uploads/2015/12/2015-12-17_fda_spurs_and_comments.png
> interpretations and explanations are welcome!
> The board was not enclosed in a metal can for these tests.
>
> The regulator is an LP38798
> http://www.ti.com/lit/ds/symlink/lp38798.pdf
>
>
> Anders
>
>
> On Tue, Dec 1, 2015 at 12:54 AM, Bruce Griffiths <bruce.griffiths at xtra.co.nz
>> wrote:
>> Anders
>> U101 only needs to have a gain of 1 at dc so replace R104 and R105 with a
>> capacitor connected to ground.
>> Adjust the other components of the gain determining network
>> accordingly.Also the junction of the power supply divider R102 and R103
>> should be heavily capacitively bypassed to ground and a resistor placed
>> between this junction and U101's noninverting input.
>>
>> Excess Sub Hz PN is usually due power supply noise or thermal fluctuations
>> due to air currents/ convection.
>> Most of the so called super regulator circuits  should be more than quiet
>> enough with an LM329 reference.A damped LCR filter at the input will
>> improve the PSRR at switchmode output frequencies.
>>
>> Bruce
>>
>>
>>
>>     On Tuesday, 1 December 2015 9:02 AM, Anders Wallin <
>> anders.e.e.wallin at gmail.com> wrote:
>>
>>
>>  HI all,
>> I need to build a few distribution amplifiers (>90% for 10MHz, sometimes
>> maybe 5MHz) and instead of reinventing the wheel I decided to try to
>> modernize the TADD-1 into an all (almost) SMD design. Here are some draft
>> sketches:
>>
>> http://www.anderswallin.net/2015/11/frequency-distribution-amplifier-plans-a-k-a-smd-tadd-1/
>>
>> Does this sound/look reasonable or crazy?
>> Any suggestions for op-amps to try and/or compare to the AD8055?
>> What causes the extra phase-noise below 1 Hz offset in John A's result:
>> https://www.febo.com/pages/amplifier_phase_noise/amplifier_phase_noise.png
>>
>> Suggestions for a low noise DC-regulator circuit? The 12-24VDC supplied to
>> this board will most likely come from a switched-mode PSU, so filtering of
>> common-mode noise is mandatory.
>> I found the TI LP38798 shown in the schematic by googling - if someone has
>> a proven a measured design that would be a safer choice. In any case more
>> filtering (e.g. ferriites) is probably a good idea.
>>
>> This design will be available on my blog or on github when it is done - if
>> anyone is interested.
>>
>> Thanks,
>> Anders
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-- 
Dipl.Ing.(FH) Jörg Logemann 
Veilchenstraße 3 
D-89150 Laichingen 
Germany 
Tel. +49 7333 922506 
Mobil  +49 171 2814359
Fax. +49 7333 922507 
mail: joerg.logemann at t-online.de




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