[time-nuts] HP 58540A Phase Noise Improvements

[Paul Christensen]

Wow, great set of responses from everyone...

As it turns out, ICOM is aware of the noisy 2nd L.O. and has been offering 
an upgrade program for the past several weeks.  Accordingly, I thought it 
may be a prudent time to pay attention to the 10 MHz reference oscillator 
phase noise.  However, given the responses so far, it appears that any PN 
performance gained at this section of the circuit will almost surely be 
masked by other limiting factors, including the GPS receiver and perhaps 
even the new 2nd L.O. from ICOM.

I also didn't take all the other factors into consideration when I was 
contemplating a swap from the MTI 230 series to the 260 series OCXO.  I was 
aware of D.C. current limitations, but had no idea those other factors were 
so critical to performance.  So at the end of the day, I may win the battle 
but loose the war: the result may be a  better 10 MHz oscillator -- only to 
have the performance ruined by imperfect matching of other electrical 
parameters -- and no easy way for me to evaluate performance without sending 
the HP 58540A out for measurements.

It's been an interesting discussion for me, to say the least.

-Paul, W9AC

----- Original Message ----- 
From: <SAIDJACK at aol.com>
To: <time-nuts at febo.com>
Sent: Friday, July 07, 2006 3:53 PM
Subject: Re: [time-nuts] HP 58540A Phase Noise Improvements


> In a message dated 7/7/2006 09:15:28 Pacific Daylight Time,
> time.bandit at btinternet.com writes:
>
> Hi  Paul,
>
> While there are certainly oscillators out there approaching the  -160dBc 
> at
> 1KHz (OSA BVAs spring to mind), some of the phase noise  performance of 
> the
> OCXO will be degraded by the GPS receiver itself. Not  sure from my
> experience that this sort of figure would be achievable in  practice.
>
> I'm interested to see what other comments you  get.
>
> Good luck.
> Hello Paul, Rob,
>
> yes you are right, the phase noise also depends very much on the design of
> the GPSDO itself, not just the oscillator. But it can be done!
>
> Some weeks ago I sent a phase noise measurement of our new Fury GPSDO unit
> to this message board (let me know if you like to get a copy), in this you 
> can
> see that our output has -155dBc phase noise at 1KHz offset, and we achieve
> this with an MTI 230 oscillator, an innovative power supply, filtering, 
> and
> buffering circuitry. There are also no visible spurs above 20Hz carrier 
> offset.
>
> Keeping in mind that the circuitry matters, it may not be straight forward
> to replace the 230 OCXO - you have to make sure the 260 unit you want to 
> use
> is compatible in Vdd, VEFC, control slope, output power/type/frequency, 
> power
> consumption (very unlikely) etc. Also, HP probably measured the thermal
> behavior of the entire unit at the factory (they mention this in some of 
> their
> literature) and stored these calibration values in EEPROM - these  values 
> won't
> work well for a different OCXO.
>
> Also, to get the best performing 260 unit, you have to get a 5MHz one, and
> the HP unit probably uses a 10MHz one.
>
> There is also another alternative you can use besides aquiring one of our
> $750 Fury units :)
>
> This alternative is to use the 260 Oscillator (you may already have one) 
> in
> a very simple, and highly effective PLL loop slaved to the HP GPSDO:
>
> You can use a simple TI/Philips 74LVC86 Exor gate as the phase comparator
> between the outputs of the two OCXO's, then low-pass filter the output of 
> the
> Exor (with a time constant of <1s), and feed this control voltage to the 
> 260
> OCXO. Since the PLL phase comparison frequency will be 5 or 10MHz 
> (depenging on
> the 260 OCXO), you can use simple RC, or two cascaded RC circuits to
> low-pass  filter the output of the Exor Gate. The Exor gate will now 
> control the
> phase of  the 260 unit to follow the HP GPSDO with 90 degree phase shift.
>
> You may have to buffer or add a DC offset to the sine wave outputs from 
> the
> GPSDO and 260 OCXO before feeding them to the Exor gate.
>
> Run the Exor gate with a clean 5V power supply to get 0V - 5V EFC control
> voltage (2.5V being the desired steady state EFC voltage).  The 260 OCXO 
> will
> require a very clean supply as well (no switching regulators etc). For the
> loop filter, preferrably use Polyester caps, and make sure not to load the 
> gate
> too heavily (use a 3.3K Metal Film resistor with a 47uF + 10nF cap for
> example).
>
> The output of the 260 will now be perfectly aligned to the GPS reference,
> but any noise from the HP unit above the loop filter bandwidth (1Hz or 
> less)
> will be completely filtered out. This includes all spurs etc that the HP 
> unit
> is generating.
>
> Besides the 260 OCXO, all you need is <$2 in components to make this  PLL.
>
> Hope this helps,
> bye,
> Said
>
>
>
>
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