Tag Archives: phase noise

A13 30 MHz Reference Oscillator: a reasonably quiet oscillator, and a noise cable

A nice little oscillator assembly came my way, supposed to generate about 17 dBm at 30 MHz. Nothing special at first glance, but after checking out its internals, it appeared to be worth a more careful look.

a13 ref osc

A hand-made box, and even more labor intensive assembly work inside. All build by point-to-point wiring, using only the best components available, glass trimmer caps, filters, mica caps – most of these parts are still available today – about 100 USD bom, at least.

a13 upper side

a13 lower side

After a bit of reverse engineering, here the schematic, a modified Colpitts oscillator. Note: base resistor of 2N5109 is 150 Ohms.

a13 schematic

To measure phase noise, connected it to my HP 3585A spectrum analyzer (this is really a great piece of equipment, a bit heavy, but still best of class noise performance and holding this title for the last 35 years….). Connected the oscillator via a 6 dB attenuator, to provide a clean load to the output, rather than dealing with the imperfections of cables, adapters, and the analyzer input.

30 mhz ref osc floor0

Quite shocking, all this noise. The green trace shows the analyzer noise floor. Check, and re-check, still a lot of noise. Too much to be true. After 3 hours of tests, found the issue: a defective BNC cable. Center connector was fine, but both shields were non-connected.

a13 bnc plug

A bit more examination of these cable shows their lousy construction. Not bad for 2 dollars a piece, but you get what you pay for…. the shield is not even reaching to the plug – there is a 5 mm gap from the screen end, to the actual plug. So even if all would have been connected fine, the would still be a lot of leaking, from inside out, and outside in.

a13 rg-58u cable

Notice the BNC plugs – these have a somewhat uncommon construction, the dielectric is covered at the front… not quite according to BNC standard.

a13 bnc cable assy

Clearly visible, the cold solder joint…. Turns out, both ends were open-circuit at the shield.

a13 bnc cold solde

Finally, using a good quality BNC cable (also, using LMR-195 double-screened cable). Looking much better. Noise is down -115 dBc at 10 kHz from carrier. It’s good, but not great. I think one could do better, especially, considering all the pricy parts, and high-quality construction. A good target for a Colpitts osciallator would be better than -130 dBc, at 10 kHz separation.

30 mhz ref osc recheck1

Note the pink trace – this is the bad cable, terminated with a 50 Ohm resistor (with the shield broken at both sides, it is actually a 1 meter wire antenna, with an open-circuit 50 ohm resistor at the end).

3047A Phase Noise Test Set: getting closer…

Quick update on the 3047A software:

(1) Implemented the beat note search for low frequencies – using the 3562A analyzer. This is now working just great, using a zero crossings algorithm.

(2) Implemented the loop correction (including the “Difcorrection” method).

(3) Implemented the phase detector slope measurement for low frequencies – using the 3562A analyzer.

(4) Implemented loop characterization for below 20 Hz. This utilizes the noise source of the 3562A, and both channels, to measure the transfer function. Actually, sounds easy, but took quite some effort to get it to work properly.

That’s the latest version of the source.
main – 150329 backup

A quick test with the 8782B – and, it seems to work!
8782B PN test loop corr

Will put it to a test with a very narrowband PLL soon.

HP 3047A Phase Noise Measurement: progress on the low frequency part, and a GTK interface.

Quite some progress on the 3047A software – while I don’t really need a lot of phase noise data below a few kHz, still good to have things complete and working. As mentioned earlier, the 3047A test system used the 3582A FFT analyzer – which is very much outdated, apart from the fact that I don’t have one around. So a little bit of adaption, to incorporate a 3562a into the system.

To test the setup, three generators were tested, vs. a 8662A reference:

A 8782B at 28.800000 MHz (ref: 8662A DC FM, 5 kHz/V, scope: 50 mV/div):
pn 8782 50 mv per div

A 8782B at 28.800013 MHz (ref: 8662A DC FM, 5 kHz/V, scope: 50 mV/div):
pn 8782 50 mv per div at 28800013
Note the spurious content, seems to be related to a fractional divider in the 8782B.

A Micro-Tel SG 811, which is a free-running generator (ref: 8662A DC FM, 100 kHz/V, scope: 1 V/div):
pn mtl 1 v per div

And, as the “gold standard”, a 8645A (no picture, scope shows just a flat line, with a few mVpp).

pn test

A few items – firstly, the 40 dB LNA of the 35601A interface is working fine – see the tests with and without – very little offset, except in the 1 kHz region, where such offset can be expected due to noise levels/parasitic noise of the setup that can only be overcome when using the LNA. Secondly, see the sharp drop at just above 10 kHz for the Micro-Tel measurement – this is corresponding to the loop bandwidth of the PLL. Note that the lower frequency data were only collected for test purposes – they are within the loop bandwidth and more or less invalid at below 10 kHz for the Micro-Tel, and below ~1 kHz for the others.
Above a few kHz, the system reaches the noise level of the 8662A – except for the spurs, and the Micro-Tel.

Afterwards, a quick reproducibility check:

pn reproducibility

The transition from the 3562A to the 3585A analyzers occurs at 25 kHz – sometimes, there is a little step. Need to check this further – maybe related to the disabled auto-recalibration of the 3562A.

Remaining items:

(1) Beatnote measurement for low frequency, using the 3562A – this is needed to test using very narrow bandwidth PLLs, like with voltage control inputs (ECF) of stable crystal oscillators.

(2) Phase slope measurements for small offsets, using the 3562A – currently, using the DC FM tuning input of the 8662A with a few kHz deviation – but this extension is needed for narrow loop BW/small tuning range as well.

(3) Some general cleanup of the code, and full incorporation of the PLL loop suppression correction (section “Difcorrection”). Nevertheless, that’s the current status, with some bugs fixed over the earlier version (LF filter settings F0, F1, F2, and some other minor things).
pnt_ main _150322

(4) Finally, adding some user-friendliness. Quite a bit of progress on this front, with a GTK based interface (need to implement the hardware control layer). Here a first glimpse.
tws pn1 gtk

3047A Phase Noise System: the revival

For quite some years I have been dealing with temporary setups for phase noise measurement, mixers, amplifiers, analyzers, of all kinds. For the more noise sources, direct measurement with a good spectrum analyzer is rather straightforward, but in most cases, dealing with rather low noise sources here, and often, at rather high GHz frequencies.

Various techniques exist for reasonably accurate phase noise measurement of low noise sources, lately, at least in the <100 MHz region, correlation type (sampling) methods are used, and these instruments are great, really fast, and a good choice, if you have the funds and want something new and fancy. With all these choices nowadays, for many years, the HP (Agilent, Keysight) 3047A and 3048A were the gold standard for phase noise measurements (along with some Wenzel instruments). And, even better, the software at the time was written in BASIC, with all the source code available - so at least you know what it does and how it works. 3047a code

After scoring two of the 35601A units for a handful of dollars, time for a revival. With the old software no longer usable on current systems, some hours were spent to adapt it modern times. In the meantime, most of this software, at least the essential parts, have been implemented in C (still needs to be cleaned up, if you are working on a similar thing and need it urgently, let me know).

This is the general setup:
3047a scheme

The noise floor, depends on the mixer (the 35601A has two mixers, one for the 5 MHz to 1.6 GHz range, and a second mixer for up to 18 GHz – the later one has about 10 dB reduced sensitivity).
3047a noise floor

So far, the system is running with a 3585A analyzer only, with temporary software, at single frequencies. The 3047A employs the 3585A analyzer from 25 kHz onwards, and a 3582A FFT analyzer for the lower frequencies; this will be replaced by a 3562A. The 3585A is also used to measure the correction factors of the interface, which is rather straightforward because of the build-in tracking generator.

Now, after some more programming, some first measurements, of a few generators I have around here (8645A, 8782B, and a Micro-Tel SG811). As a reference the 8662A is used, so the 8645A can’t perform to its full specs – the measurement is limited by the reference.

3047a 3585a analyzer test

Next steps: improving the 3562A functionality, for lower frequencies – this requires some more work on the HP code, which was written for a 3582A analyzer. And, finally, making it a bit more easy and intuitive to use.

Controlling the 35601A Spectrum Analyzer Interface

Amongst other heavy boxes, the HPAK (HP Agilent now Keysight) 35601A is one of the instruments that form the venerable 3047A phase noise measurement system.
It contains two phase detectors (for up to 1.6 GHz, and for up to 18 GHz), a 40 db low noise amplifier, and other useful components to form a PLL (lead lag network, lock detector, a DAC, and a PLL to drive a low frequency signal from the 3585A IF).

35601a block diagram

This is the block diagram, sorry for the bad quality, it is the best I have, and you can read it with a magnifying glass. The 35601A has all kinds of switches that are GPIB controlled. The original HP software requires a particular dialect of BASIC, no longer used these days.

Therefore, the little program below might be helpful, written in C (compiles with GCC, and most other C compilers). Sorry, not particularly user friendly or anything, but those having ordinary skill in the art will know how to use it. The binary is quite useful for a quick check of the 35601A. Just connect a 10 MHz reference, and a 10 MHz VCO with about 10 kHz per V tuning range (like a 8662A with DC FM), and you can watch to PLL close.

PLL unlocked (see the upper yellow LED illuminated, indicating an open phase lock loop):
35601a not locked

PLL locked (the upper LED is dark!):
35601a locked

The program files. Working with a National Instruments GPIB interface card, but can be adapted to other cards with little effort.
sw35601 program 150227

Please, no unfavorable comments about the Tektronix 2215. It is still a very good scope, despite the obvious lack of a LCD screen and gigabytes worth of sample memory (it doesn’t have any memory).

11729B Low Noise Down Converter: a small box, with very low noise

In an effort to upgrade my phase noise measurement system, look what I found, for less than 1 cent, for the earlier list price in dollars: a quite amazing HP (Agilent, now: Keysight) 11729B, equipped with all bands from 0 to 18 GHz.

The 11729B works best with a 8662A generator, to supply a 640 MHz low noise signal, and a variable 0-1280 MHz to move the IF to the right position.

11729b front

11729b inner workings

11729b scheme

The 640 MHz low noise signal from the 8662A is filtered by a quite massive Wavetek filter, amplified to about 30 dBm (1 Watt), driving a HP 33004A-H18 comb generator. The right line is selected by a filter assembly, using multiple 18 GHz coax relais. HP did not save on parts here….

11729b comb gen and filter

11729b mixer and if amp

The “brain”, a MC68B09P, a trusty 8 bit CPU, or as Motorola calls it, a “Microprocessing Unit”.

11729-60008 cpu ram rom

Just in case you need it, the ROM file, stored on a 2764 EPROM. Version 2.1!

11729b v2.1