THS3491 test

Update: see also

Update: phase-noise with Rg=Rf=4k7. 

THS3491 looks like a good op-amp for frequency distribution amplifiers.

I tried it with a slightly modified FDA-design. With 200R feedback/gain-resistors it self-oscillates, but with 1k resistors (976R suggested in datasheet) it is stable. However I see 16dB of gain-peaking!

Using +/-13V3 supply-rails it does drive a 20Vpp sine-wave-ish looking waveform into a 1 MOhm load.

Phase-noise TBD, if and when the gain-peaking is resolved.

Note stylish extra pins on PCB to ground pin1...


Compact frequency distribution amplifier

Stefan DK3SB, sent me this picture of his version of my frequency distribution amplifier. He modified the PCB to make it narrower (160mm), to fit a  Hammond
enclosure (about 25 euros from Farnell).

Nice work! KiCad sources and gebers hopefully to follow, on


Distribution Amp - S/N 16

Update2: Here's the 14-hour data together with a 14-hour noise-floor run. The ADEV and TDEV results are more or less instrument-limited. Only for phase-noise above 100Hz offset is the instrument floor significantly below the result for the distribution amplifier.

Update: Here's a 14 hour run of the residual phase noise, ADEV, and TDEV, as measured through the frequency distribution amplifier. Instrument is a Microsemi 3120A phase-meter.

Aivon has now produced around 16 of these distribution amplifiers.

There's a new op-amp which might be interesting to try for the FDA board: THS3491. It has a grounded pad under the op-amp, so we need a new EP-variant of the PCB.


Aivon LTD makes FDA/PDA ohwr-design

My open-hardware design for a 1:8 pulse and frequency distribution amplifier is now available from Aivon LTD.


Distribution Amplifier

Assembled another distribution amplifier today. S/N 004. Measurements to follow at some point.

Electronics Time and Frequency

Frequency Distribution Amplifier 2017.01

1:8 frequency distribution amplifier based on LMH6702 and LMH6609 op-amps.

In particular the power-supply section using a common-mode choke, a Murata BNX025 filter, and low-noise regulators LT1963 and LT3015 seems to work quite well. I also used ferrites (2 kOhm @ 100 MHz) as well as an RC-filter on all supply pins. Perhaps overkill? Performance with the intended AC/DC brick is still to be verified.

Measurements around 10 MHz show a 1 dB compression at over 14 dBm and an IP3 of around 27 to 30 dBm. The gain extends beyond 100 MHz with some gain-peaking.

Some measurements of residual phase-noise with a 3120A phase-meter, at 10 MHz. My earlier distribution amplifier required shielding with aluminium foil as well as powering from a lead-acid battery to achieve a reasonably quiet phase-noise spectrum. These measurements were done with lab power-supplies for +/-12 V to the board and without any shielding.

Finally some measurements of gain vs. frequency with a Rigol spectrum analyzer.


Electronics Time and Frequency

Distribution Amplifier 2017.01

A new distribution amplifier design featuring a 1PPS pulse distribution amplifier (PDA) and a 5/10 MHz frequency distribution amplifier (FDA).

1U 150mm deep rack-enclosure from Schaeffer. Prototype PCBs without soldermask or silkscreen from Prinel. Both the FDA and PDA boards have 1:8 fan-out with 9 BNC (optionally SMA) connectors spaced 16mm apart. The boards fit comfortably side-by side on a 19" rack panel. Some funky BNC-cables with unusually large connectors may not fit side-by-side 16mm apart - a price to pay for the compact design. The plan is to use an +/-12 AC/DC brick power-supply (not shown) which fits in the back of the enclosure.

Detailed posts on the PDA and FDA boards to follow.

Time and Frequency

Ettus OctoClock distribution amplifier

I measured the phase-noise of an Ettus OctoClock distribution amplifier. These plots compare it to earlier measurements on an SRS FS710 and a Symmetricom 6502 as well as my own TADD-1 inspired AD8055 prototype.

OctoClock schematic here:

10 MHz clock-distribution chip:

For time-nuts kind of stuff (H-masers!) the 10 MHz phase-noise doesn't look that great, and there is something funky going on in the AM noise!? The box is powered by a +6 VDC wall-wart PSU which is probably the cause of all those spikes...

The PPS-channels are based on 7404 hex-inverters and look OK. 200ps of skew is equivalent to 4 cm of trace-length (?) - which seems like a lot if there was an effort to minimize it... For a 1 V/ns rise-time pulse 200ps is equivalent to 200mV of DC-offset in either the signal or the counter trigger-level which also seems like a lot?


Frequency Distribution Amplifier, v2 simulations

I wasn't entirely happy with my frequency distribution amplifier prototype measurements, so I decided to do some SPICE simulations.

Here is a circuit close to the original TADD-1 design, configured for a voltage gain of 2, which when loaded with 50R corresponds to unity gain or 0 dB.


The main contribution to the noise floor at 10 MHz is by the AD8055 op-amp (roughly 3/4ths), with the gain-resistors R4 and R5 also contributing (roughly 1/4th). The simulation gives an output-referred noise-floor of 17.4 nV/sqrt(Hz) between around 100kHz and 20MHz. At low frequencies the 1/f noise of the op-amp dominates. The large 47k bias-resistors R2 and R3 are not bypassed/filtered in this design and they contribute significantly at low frequencies.

17.4nV/sqrt(Hz) is -142 dBm/Hz. This is a one-sided spectrum so we subtract 3 dB to get a single-sideband number, and then another 3 dB since noise is divided equally into AM and PN. This gives a best-case PN of -148 dBc/Hz for a 0 dB input/output power. In my measurements I got about -157 dBc/Hz with +7 dBm output.


The v2 design uses the ADA4899-1 op-amp instead of the AD8055. This improves the op-amp input voltage noise floor from 6 nV/sqrt(Hz) to 1 nV/sqrt(Hz)  while also reducing the near-DC voltage noise by more than ten-fold.


The simulation for the ADA4899-1 design shows an output-referred noise floor of 4 nV/sqrt(Hz) from 10 kHz to 20 MHz. This corresponds to about -155 dBm/Hz, a 12.7 dB improvement over the original design. The SPICE model for the ADA4899-1 does not include 1/f noise so I have estimated it with a dashed line. I have tried to minimize the resistor noise with reduced resistance values for the gain-setting resistors R4, R5, and a bypassed (C5) 'T'-circuit for the DC-bias (R2, R3, R9).

The theoretical PN floor with 0 dBm signal is now -161 dBc/Hz (again 13 dB better than for the original design).

Here is a figure that compares the two simulations:


These figures show an AC sweep response for the SPICE simulations:

fda_v1_ac_sweep  fda_v2_ac_sweep

Further ideas and ToDo:

  • What is the limit for reducing values of R4 and R5? Power-dissipation, current-draw from the op-amp?
  • Reduce value of R7 - do we even need it.. (improves isolation between output stages?)
  • Replace R9 with an inductor - BUT it creates a resonance with C5 that needs to be damped - probably not worth it.
  • Improve on the powersupply schematic in the prototype. Spurs were big with a SMPS +12VDC supply.
  • Do PSRR simulations? Does that give different optimization goals for the DC-bias circuit?
  • Find an even better op-amp?
  • Where do we find a good SPICE model for ADA4899-1? The one I am using has a realistic AC gain response but unrealistic noise model near DC. There is an alternative on the website with realistic 1/f noise behaviour but infinite AC gain bandwidth!!??

Constructive comments are welcome!

Electronics Time and Frequency

Frequency Distribution Amplifier - first tests

Update 2015-12-18: Things improved quite a lot by simply wrapping the board in aluminium foil!

The amplifier phase noise floor is now at around -156 dBc/Hz while the 6502 is at -163 dBc/Hz. The AM noise numbers are similar.

Original post 2015-12-17: I put together a first prototype (only one output channel) of my TADD-1 inspired frequency distribution amplifier. Preliminary schematic here.

I compared the prototype board to two commercial distribution amplifiers: an SRS FS710 (quite awful) and a Symmetricom 6502 (very good). I also compared my new data with John Ackermann's measurements from 2007.

The new board showed ugly spurs at 50 Hz and harmonics using an el-cheapo wall-wart 12 VDC SMPS, so I also tried it with an "ultra-low noise DC-source" a.k.a 12 V lead-acid battery.