[time-nuts] Re: Phase noise of HP8663A 640 MHz reference? {External}

[Jim Muehlberg]

Here are some PN plots from the PN9000.(mixer version, not the xcor version)

The upper plots are the 8663's v. the PN9000 synthesizer.  The lower 
plot is the two 8663's, one is the VCO and the other is the DUT.  PN 
data of the lower plot is attached.  I've got 2 more 8663s I can 
measure, once my hernia belt is back from the cleaners.

Jim

On 2022-08-21 5:52 PM, ed breya via time-nuts wrote:
> Tobias wrote
>
> "Hi Ed, could you share some info about your own 640 MHz source?
> I would be very interested.
> I do have an 8663A, but I have no idea about how good it actually is and
> currently, I don't have a possibility to test it because I still lack an
> 11729C carrier noise test set (but I would like to make something 
> similar myself some day)."
>
>
> The plan was to do the doubling and amplifying similar to the 
> 8662A/8663A (which I think are about the same in this respect), except 
> that I don't have anything like the mentioned 40 MHz and 160 MHz 
> crystal filters. Starting from the higher OCXO frequency should help 
> some, depending on its noise performance.
>
> The filters I do have can't fix anything close-in, but should work 
> very well on spurious content from almost any multiplier scheme. The 
> first one especially is a single unit 160 MHz BPF made from two 
> tubular coaxial ones cascaded. I didn't make it - it came this way as 
> a commercial product. It has enormous stop-band suppression maybe 200 
> dB (theoretical) by +/- 10 MHz away from fc, but large insertion loss 
> about 10 dB. The plan was to double the 80 MHz OCXO into this filter, 
> which should eradicate all spurious from below. The 320 and 640 MHz 
> BPFs are more conventional.
>
> The 80 MHz OCXO was apparently quite common around 20-30 years ago. 
> Despite this, I couldn't find any specs twelve years ago, and can't 
> find any today, even though there seems to be plenty of them still 
> around and for sale. The ones I have are Vectron 229-9237, and 
> 229-5657-1, apparently the same except for mechanical construction.
>
> I have a bunch of similar units, mostly oddball frequencies in the 100 
> MHz range, and I did have to take some apart (soldered shut cans) over 
> the years to modify for particular projects. I found they all used 
> half-frequency crystals and built in doublers. The 80 MHz is no 
> exception - a quick look on the SA shows it's a 40 MHz OCXO that's 
> doubled up internally, so it's really only starting with a four times 
> frequency versus multiplying a 10 MHz reference.
>
> Another thing I noticed is that the 640 MHz SAW BPF in the 11729C may 
> not be for closer in spurious content, but mostly wide cleanup, and 
> optimized to form a good oscillator when used for that mode. The 
> manual says that the purpose of the filter is to reduce 120 MHz, 520 
> MHz, and 760 MHz spurs. These and others naturally come from the 
> rather complicated 8662A/8663A reference generator/multiplier system, 
> and the 640 MHz output does not appear to have very much isolation 
> from all this activity.
>
> So, if you make your own 640 MHz "clean and simple" by direct 
> multiplication, with no side deals for other frequencies, the result 
> will not include the extra stuff that would be coming from the 
> generator. If you also start with a good HF OCXO with known specs, and 
> do careful multiplication, filtering, and PLLing, I think it can beat 
> the noise performance of the 8662A/8663A's 640 MHz source. How much? I 
> dunno, but suspect that the hump from around 10 Hz to 10 kHz may be 
> due in part to all the reference making and synthesizing action going 
> on in there, that's somehow included in the 640 MHz output. That is, 
> presuming the 10 MHz internal reference has no such hump. If it does, 
> then it could be simply the result of the multiplication factor, and 
> unavoidable.
>
> Ed
>
>
>
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-- 

Jim Muehlberg
Senior Engineer

National Radio Astronomy Observatory
ngVLA Local Oscillator Lead

1180 Boxwood Estates Rd B-111
Charlottesville, VA 22903-4602
P 434.296.0270
C 434.422.2017


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