HPAK 8569A Microwave Spectrum Analyzer: front panel repair/re-build

The easy of use by the three-knob operation mentioned before – it has a downside: a very complex front plate assembly. And this might be the reason why more modern units almost exclusively use single-axis rotary encoders, often only one single encoder to operate all functions, per unit. The multi-turn-coupled-limited-synchronized-geared movements, just too complex and too expensive, too heavy, too laborious to maintain.

The front panel controls – as delivered.
8569a front panel rhs as delivered

The 8569A, 8569B and 8565A all use very similar front assemblies – only seem to differ in the print and in the exact shade of the label colors. And these assemblies use plastic parts – knobs, wheels, and so on. After 30+ years, no wonder, aging has visible effect, and parts become brittle. Common defects are broken-off sliding contacts. Not a big thing, can be fixed (re-attach the contact with M1.2 screws).

The attenuator and “0 dB” warning indicator – these are located on the same rotary cam, same contact pair. And, you bet, the corresponding wiper contact – broken off-missing.

Well, having rebuild some of these assemblies for other repair jobs before – the only way to fix this will be to take the assembly apart, almost fully. Actually, if all screws are kept well-sorted, not really such a big effort as is sounds. Some hours, but then it will be working again.

8569a rebuilding the from assembly

In addition to just the repair, all contacts and surfaces were thoroughly cleaned, by various means: DeOxit D5, a good eraser, isopropyl alcohol. For the painted surfaces and parts, use 40-50% isopropyl alcohol only, and a rather soft or medium-hard brush, to avoid damage to the paintwork.

The replacement cam (on the right) has two wiper contacts, the defective part, only one – the plastic fingers holding it down became brittle, and it eventually broke off.

8569a attenuator selector cam

Here, the result: the rebuild assembly. From earlier repairs, the remaining contacts should last – somehow, no all are susceptible to breaking-off; or it is just the amount of use they get – at least, I have some units fixed with small screws holding down the contacts, and there have since been working better than before. Therefore, I am not too worried about more contacts failing in the near future.

8569a rebuild front panel assy

Note: as it turned out, also the 12 dB vernier attenuator encoder had a partially broken off contact – can be fixed with some epoxy glue, but I rather replaced the encoder, with a spare from the parts unit.

HPAK 8569A Microwave Spectrum Analyzer: display fix, A6 assembly

No display – this can be bad news, because spare CRTs are hard to come by. Let’s see what we can do here.

Overview of the assembly – red: XYZ driver boards; green: high voltage (and other supplies) for CRT; blue: digital circuits-video signal generator.
8569a display unit

The digital part and XYZ driver – seem to be fine, can get a nice video signal when probing around.

Well, the HV power supply – no signs of life. You can easy check, just switch off the unit – there are two neon bulbs that discharge the CRT – but in the current case, all remains dark.

Removed the HV power supply assembly (A6) – fuse is blown! A quick look at the schematic:

8569a partial schematic a6 assy

Only three options, either the 26 V filter, the transformer, or the Q1 driver transistor. Driver transistors are a common fault – high voltage transients can damage them. But, Q1 is testing fine.

The transformer, looking good.

The filter – well, there we go, a classic fault, a defective cap.

8569a hv power supply

This is a 50 µF, 50 V, Sprague (now: Vishay), 30D series. Date code: 1980, 26th week! “Made in USA”

8565 30d cap

This exact series – still available today, after nearly 35 years! These don’t fail so easily, but the A6 assembly, there will be some stress on these parts. The failed part is capacitor C2 – the part that is doing most of the filtering. Maybe it would be a good idea to put a 1 µF foil capacitor in parallel with the electrolytic caps, to absorb the spikes…

For the time being, just replaced the defective cap, and ordered some 105°C rated long-life caps. 30D series caps aren’t cheap. So I settled for a Kemet brand type. Will be good enough.

After some soldering, powered the thing on:
8569a working display

We have a working display!! And, it is actually a very good CRT – no signs of wear! Most likely, sitting in some lab at Iowa State University, as one of the labels on the case suggests, and then, stored away in a nasty shed or garage for years.

Now, repair can move forward – without a display, at would be only half as useful.

The culprit, and the dead fuse:
8569a hv pwr supply culprits

HPAK (HP Agilent Keysight) 8569A Microwave Spectrum Analyzer: a great find, 10 MHz to 22 GHz, and some spare parts at hand

Two weeks ago I spotted something on xbay, a 8569A spectrum analyzer, for parts or repair (“powers up, no display”), at a very reasonable price – just a bit above USD 300. This is not much more than the parts value of the 70 dB attenuator of this unit, so, a great deal in any case. The pictures shows a really beaten-up unit, black dirt all over it (mould?), and corrosion. At least the connectors seemed to have caps on them, fair enough. And it has option 1, a 100 MHz comb generator, good out to 20 GHz.

Unit as offered… doesn’t look very appealing.
8569a offer

Good news: I have a severely damaged 8565A here (broken frame, but still full of good parts), as a donor unit – looks like it is a unit that had been buried under tons of heavy stuff, or had a truck driving over it, a heavy truck. And, it doesn’t have a CRT.

8565a donor unit

The 8565A, 8569A and 8569B analyzers – these have a really easy to use design, three-knob operation, and are the best analyzers, from my point of view, for any general service and adjustment work, at any frequency above a few MHz. With resolution bandwidth down to 100 Hz, also up for some more special tasks. Sure, there are the 8568 and 8566 units (I also own a 8568B), but these are bulky, and you always need to push buttons AND turn knobs – more like for a cal lab or really high precision characterization work.

Plenty more modern analyzers exist, with LCD display, and all kinds of software features. They come at a price, and once you have experienced the responsiveness and easy of operation of the 8569 series, fancy LCD screens and color buttons can’t really be the reason to consider anything else. Good units, fully functional-refurbished, go for about USD 2-3k.
For general-purpose, low budget work – if you are looking for a 20+ GHz analyzer, there isn’t much that goes for less than 10k used, or 30-50k new. Some of the later, semi-portable 856x series might be an alternative. However, these have a rather small screen, just about 3.8″ diagonal, for the spectrum display, while the 8569B has 5.5″, and an electrostatic CRT, of excellent brilliance.
Also, keep in mind that the down-converted output can be easily converted to digital data by SDR, to recover even the most complex modulation schemes. No need to pay extra dollars to get custom plugins for modern analyzers. Such things pay off for the big corporations, but if you are up for special tasks, there is no way around dealing with some software, anyway.

Currently, I use a 8569B in the workshop back in Germany, and here, in the US, I rely on a 8565A – which is essentially the same RF hardware, but has an analog storage display (variable persistence), rather than a digital display, like the 8659B. Also, the 8565A doesn’t have GPIB or other means of data bus, all computer-controlled measurments require the use of a A/D D/A converter unit.
So, it would be good to get the 8569A working, and eventually, sell-off the 8565A.

Historically, it seems the 8565A first appeared in about 1978. Looking up in some old HP catalogs – the 8569A appeared first in the 1982 edition (marked as “new”); quite surprising, because the unit I have here has a much earlier serial (1980) – and also the parts data back to end of 1980.

8569a serial
No doubt, seems to be a rather early unit (take the first two digits of the serial, and add 1960 = year of manufacture).

HP-Cat-1982-8569a

HP-Cat-1983-8569b

In the 1983 edition, HP already has the 8569B, with virtually identical specs. Quite interestingly, the 8565A (with the analog storage display!) remained available, in parallel with the digital-display 8569B, for many years – up until 1988 (at USD 34k, including option 1). The price difference was rather small, just about 10% (about USD 3k) more for the 8569B, with the much better display, and digital/automated control capabilities.

If you study the inner workings of the 856x in detail, it is also evident that a good part of the circuits were taken from 8555A analyzer, introduced in about 1970….

After all this, the analyzer arrived – well packaged. And first downside of it, it is 30 kg, 65 pounds, plus packaging. But hey, who needs to carry spectrum analyzers of this kind around – it will rest on the microwave test bench, along with generators, counters and so on, some 100s of pounds anyway.

Well, plugged it in – the display is really dead, but the frequency display is on. Powder supply is good. And, the X-Y output – when connected to a scope – you can actually get some signals, and it is sweeping. There is hope.

After some probing and basic tests, these are the items found so far:

(1) No display. However, there is a display signal on the deflection plates, and it is looking sound, so the digital display generator seems to work just fine. Just the CRT isn’t. Might be hard to fix, if the CRT is at fault – I can’t see the cathode glowing….

(2) The attenuator at 60-70 dB, and “0 dB” attenuator light at the front panel – not working (missing 20 dB attenuation; light off even when attenuation is set to 0 dB.

(3) Mechanical defect of the span knob – general mechanical soundness of the front panel. Will need very through cleaning, aligment, etc.

(4) Something is wrong with the tuning stabilizer, engaged at below 100 kHz span – when it is activated, it sets the span to zero, and never comes back, unless the stabilizer is de-activated.

HPAK (HP Agilent Keysight) 8662A Synthesized Signal Generator: ref sum loop adjustment, output amp/peak detector repair

Introduced in about 1980, the 8662A is a marvelous machine, it is an ultra-clean signal source, with very low close-in noise. This is the kind of oscillator used as a reference for phase noise tests, narrow channel receiver testing, and so on. Just a quick glance at it inner working, and it is clear that only the most brilliant engineers must have been working on this apparatus in those days. Sure enough, this did not come cheap, about 30 k$ in 1981, something like 75+ k$ nowadays…

Even more interesting, these machines are still in use today, and are still valued for the same reason – hardly any synthesized generator exists that has similar close-in noise.

The block diagram – essentially, there are two ultra low noise switched inductor osciallators that form the reference and output sum loops, together with with some fractional and n divider circuitry, to allow for the fine resolution. The output sections has a heterodyne band, and other bands derived by doubling or dividing the 320-640 MHz fundamental output of the output sum loop oscillator.
8662a block diagram

As with all wonders and complex machine, they sometimes stop working. This one had two issues reported, inaccurate output power (and error #10B – ALC error for some frequencies/settings), and a ref sum loop unlock condition at certain frequencies. The latter issue had been persisting for some time, but the ownder didn’t use it at the critical frequencies, the ALC loop error just recently seems to have come up, for now external reasons, apparently, during a measurement.

(1) The output sum loop A6A5, error #06. This turned out to be a drift issue, of the low noise REF oscillator. This had drifted out of pretune span, and needed just very slight adjustment of the tuning screw (under a heavy metal cover, rear panel of the instrument). Such adjustment is not mentioned in the manual, but easy enough, set the generator to 640 MHz, check that the pretune is properly adjusted (adjustment is done for -3.75 V at 320 MHz), introduce a -34 V tuning voltage signal, and adjust for a frequency a bit above 640 MHz, say, 642.5 MHz, to give the phase loop a bit of room to operate.

08662-60417 oscillator

Great care must be exercised, because the adjustment uses a copper (!) screw – red frame in the above picture, most likely, acting as a capacitor vs. a metal surface inside of the unit (don’t open up these oscillators – they are wraped in Mu metal or similar exotic material). Don’t overtighten the retaining nut, copper is pretty soft.

After that, the ref loop is stable again, even when sweeping through the full band, for some hours.

(2) The more tricky part, as it turns out, the ALC loop. This is implemented on assemblies A4A1 (which has the peak detector), and A4A7, the ALC control, this has a DAC, to set the target voltage, and a regulator, to keep the output signal at the level set by the DAC output.
8662a a4a7 probing
After some time of probing around, and substituting the input of the A4A1 assembly by a calibrated 100 MHz signal, it is clear that the A4A1 peak detector delivers insufficient voltage, and that the voltage is frequency dependent.

A quick glance at the schematic:

8662a output amp

Blue frame – the bandwidth limiting circuit, enganged at below 10 MHz – first guess: something is at fault, in this circuit. Well, unfortunately,after even more probing, and even swapping some transistor (they tested good but you never know) – the focus shifted to the part in the red box. The peak detector diode! This is really bad news, because it is part of the output amp microcircuit, part number 08662-67008. Carefully desoldered, and checked – the detector diode has 0.3 V forward, which is fine, and 0.7 V backwards – this is a far too low reverse breakdown voltage. Something must have happened with this diode, maybe, it is just age. So, I openend up the circuit (the lid can be lifted off with a knife, glued on with silver epoxy), and inspected with a microscope – no obvious defect, all kind of nicely wire-bonded parts on sapphire(?) substrate – nothing I can fix with the tools at hand.

08662-67008

But, as luck comes along, found two of these circuits on ebay. Very mysterious. They look like old and used and ripped out of the boards (with a screwdriver, not by desoldering), and I suspected that the two might just be damaged parts – but 14 days right of return were offered, so not an issue. Even then, it doesn’t really make sense to rip the out of the board, every reasonably skilled and knowledgeable person would rather desolder them carefully, and fit a spare. However, these spare still had the through hold plated vias on the connectors, from the board. Glad the feed-throughs at the bottom of these microcircuits are so sturdy! Well, the only explanation I have – someone saw the golden parts, on some odd circuit boards, and only wanted to keep the gold, but not the boards. Fair enough.

And, this is almost the end of the story, the spares arrived within a few days, and I cleaned them up, and fitted the most “used” looking 08662-67008, and, quite to my surprise – working just fine.

The only thing that remained was the amp bias adjustment, the offset adjustement (both on A4A1) and the CW power adjustment (A4A7) – also checked the other alignments of A4A7, but they were all still fine.

In the end, still one output amp in the box, for the next 8662A, and the current one, back alive.

AIOM – Analog Input Output Module: the low-voltage low-current SMU

Quite frequently, I encounter rather large stacks of system power supplies and multimeters to test various types of equipment or circuit prototypes. Often, voltages are just in the 0..10 V range, sometimes, up to 40 V, at small currents. The response of the ciruit is then measured, often, after conversion to a voltage, or as a frequency, etc, with a counter.
This all works, but consumes space, a lot of power, heavy lifting, etc.

Therefore, what is needed, is a kind of Swiss Army Knife, a ADC-DAC unit, with some capable software, a small box that replaces at least two power supplies, and two voltmeters. Preliminary name: AIOM

Essentially, a small brother of the now very common source measure units, aka, “SMU”s.

Possible uses include:

(1) Test of diodes etc – trace recorder.

(2) Control of sweep generators, synthesizers, analog spectrum analyzers (most of these devices have a 0..10 V tuning-frequency control input, and often, a chart recorder-analog output, +-5 V, +-10 V, +-1 V, or similar).

(3) Test of VCO tuning curves, YIG current drivers, YTF circuits. Special VCO analyzers exist, but no need to block a whole lot of space just for some tuning test, and are too expensive to have multiple units at hand all the time. Often VCO tests need to be done at various temperatures, and over considerable time, to ensure performance of a circuit – multiple small test units will allow parallel testing, at very resonable cost.

aiom scheme draft

Features:

(1) Two 0..10 V outputs, 16 bit resolution, low drift, better than 0.05%; source and sink. Needs to have reasonable low noise.

(1b) Optional, to be added, voltage to current converter (to control current, rather than voltage, 0..10 mA, source and sink).

(2) Two inputs, one referenced to ground, one fully differential +-10 V.

(3) Ramp generator and similar features (rough sine, triangle, square/PWM), in hardware, to allow fast sweeps or steps or PWM-related tests (something like a simple arbitrary function gen).

(4) While the basic design will be kept strictly unipolar, there will be a simple switching matrix, to allow polarity reversal. This will also ensure fully symmetrical tests, e.g., for charge counting-battery test, charge-discharge efficiency tests.

The whole little thing has two parts: the AD/DA converter board (prototype build), and a switching materix (still waiting for some parts).

aiom board

First tests successful, stay posted!

aiom test setup

8645A Agile Signal Generator: when disaster strikes

For a long time I have been looking for a reasonably prices 8645A, and finally, I found one – with the note “doesn’t power up”. Well, most likely, a defective power supply. The 8645A and some related generators (not the 8643A) use linear supplies, because these are really low noise devices, and a switching supply just doesn’t seem to do the trick. Interesting, because for the very quiet 8662A/8663A signal generators, HP was relaying on switches supplies…. maybe they just could not fit anything else.

Back to the 8645A – this is no less than a miracle, a marvelous apparatus. No idea how many manhours (man-decades) of engineering went into it. It’s complexity, and subtle detail, nothing short of a moon landing vehicle, made for an electronic test lab. They way HP designed the shielding, and implemented a rigorous low leakage approach, this alone is worth special admiration.

Even better, the unit discussed here has option 1, which is an OCXO high stability reference, and it has a build-in doubler, extending the frequency range to above 2 GHz. About USD $50k in 1990, nowadays, nobody can afford such build quality anymore.

Well, all these are good reasons for looking forward to soon doing some repairs on such kind of unit, and make it “power up again”. Well, that was the thought.

This issue: while many hours of hard work went into fabricating this thing, not more than a few seconds were spent, to consider adequate packaging, to ship a box, 80 pounds.

8645a damage

8645a damage 2

8645a damage 3

8643a damage 4

Such kind of damage, not seen before. Except for the little bit of Instapak, no other protecting foam or anything – nothing to hold it in place in the box. The result – a badly damaged front panel, broken input connector, and even the front frame, damaged beyond repair.
Just the single front end connector (which has an internal airline, gold plated) – USD 200+; the machine, it seems beyond repair.

The only good news – the seller (who did not package it himself) seems to be a very resonable person, so we will work something out. To be continued.

Micro-Tel MSR-904A Microwave Receiver: reducing phase noise – phase detector frequency

Like with most PLL build, there a quite a few things that can go wrong – the result: a lot of phase noise. For the current setup, all precautions had been taken to avoid bad surprises – low noise supplies, well-proven loop filter amplifier, low noise DAC, adequate cables. And, phase lock was quickly achieved (see last post).
For more detailed analysis, both the 160 MHz and the 21.4 MHz IF signals of the MSR-904A are fed to analyzers. For the 160 MHz, to a RTL SDR stick, just for the rough picture, and the 21.4 MHz, to a 3585A analyzer. The 3585A has very low noise, ideally suited to look at phase noise, except if you are working the ultra low noise segment.

Initial finding – phase noise is down at about 60 dBc at >10 kHz offset, dropping off as expected, but the close-in noise is really bad. Close in noise often related to the phase detector, or the reference. Substituting the 10 MHz reference from the EIP545A by a really low noise HP 10 MHz OCXO didn’t change much. So to high noise level must be connected to phase detector.

With the detector set to 1.25 MHz (:8 reference divider), there we can gain quite a few dB of noise supression, by increasing the detector frequency (within limits, doubling the detector frequency lowers the associated noise contribution by about 3 dB). And, even more, we can check out the reference doubler, which is a build-in feature of the ADF4157. With the doubler in use, it needs to be ensured that the duty cycle of the reference is close to 50%, but this is ensured by the OCXO anyway.

The ADF4157 can handle phase detector frequencies of up to 32 MHz, no issue at all with 20 MHz. The only downside – more fractional-N spurs – channel spacing for integer only dividers is now 160 MHz, rather than 10 MHz….

msr pll phase noise

msr pll phase noise averaged

Red and green traces – you can see, the PLL is completely detector noise saturated within the bandwidth.

Other traces – all with a phase detector frequency of 20 MHz – and at different charge pump currents (CPC). A CPC of 15 corresponds to a 5 mA current. This has direct impact on the phase loop cut-off frequency. There is some peaking, at 2 kHz (dark blue trace, CPC 1), and at about 7 kHz, light blue trace, CPC 10.

Comparing the yellow and magenta traces – these differ by the 10 MHz reference signal source only (yellow uses an HP 10811 OXCO, magenta uses the EIP 545A build-in reference which is pretty stable, but rather noisy). In the curent setup, both references yield very similar results – accordingly, the noise within the PLL bandwidth is dominated by the PLL cirucit itself, and the phase detector, not the reference source.

There are some mains-related spurs at 60 and 180 Hz, but these might just be due to the temporary cabling and lack of a proper case. The circuit is fully exposed, tranformers closeby, etc. For the final setup, all cables will need to be as short as possible, especially for the pretune voltage (which is about 2 MHz per Volt – 2 kHz noise for 1 mV!).

Credits go to KE5FX for the great PN.EXE phase noise measurement tool, invaluable for any such work!

Micro-Tel MSR-904A Microwave Receiver: phase lock test, YIG driver bandwidth modification

Some final parts added to the MSR-904A digital interface/PLL: the actual PLL circuit (frontend), an Analog Devices ADF4157 fractional-N PLL, together with an ADF5002 8:1 prescaler. The phase detector is set at 1.25 MHz, to allow 10 MHz integer-only steps. Some experimentation with other phase detector frequencies might follow later.

Here – the schematic of the PLL frontend. The circit is wired point-to-point, sure enough, with VERY short wires, soldered using a microscoped – hope you have a steady hand. After a quick test (using the MUX output of the ADF4157), the wires and the very tiny ADF gadgets, all sealed with a few drops of epoxy.

msr pll adf5002 adf4157 schematic

On the main board, the PLL loop filter. Build around the remaining half of the already installed OPA2703 (other half used for the DAC output buffer).

msr pll loop filter

With all these parts now put together, to do some basic tests on the PLL – a Gigatronics 605 Microwave Synthesizer was connected to the MSR-904A input, and the LO sample output of the MSR-904A connected to PLL. A sample of the “LO sample” taken by a broadband -10 dB coupler is used to monitor the frequency, using an EIP 545A. The 10 MHz reference output of the EIP is used as the ADF4157 reference.

msr pll phase lock test setup
msr pll test setup 2

The MSR-904A down-converts the signal to a first 250 MHz IF (by fundamental LO), the 250 MHz IF is then mixed with 410 MHz (this can be locked to a 5 MHz signal – not locked at the moment, but the signal is very clean and stable anyway).

The 160 MHz 2nd IF is available at the rear panel, and connected to a R820T RTL SDR. This is a very handy method to monitor noise, and do some basic adjustments on the PLL. Using headphones – and the human ear as a phase noise meter… more quantitative analysis to follow.

Here, the transition from manually controlled CW mode, to PLL controlled mode.
msr-904a locked at 7250 mhz lo

A close-up:
msr-904a locked at 7250 mhz lo 2

For these tests, the LO was locked at 7.25 GHz, receiving a signal at 7.0 GHz (SDR offset set to about 160 MHz).

Note – same as for the Micro-Tel 1295, and the SG-811 – the YIG driver has a bandwidth limit (by a 100 uF Tantalum capacitor – and a 499 k resistor!) that is controlled by a reed relais on the YIG driver. This doesn’t allow low phase noise operation, even with the best PLL. Well, 100 uF is a bit too much. Therefore, a 100 n capacitor was added – this is enough to suppress most of the noise of the YIG driver stage, and still the circuit remains fast enough for full band sweeps at moderate scan rates. Might modify this later, by adding a bit of logic that adds the 100 n capacitor only when the external frequency control is active, but disconnects it during full band sweep, etc.
msr-904a YIG driver board

Micro-Tel MSR-904A Interface/PLL: low noise power supplies for PLL and pretune circuits

Recently, some pretune DAC and microcontroller circuitry has been build, see earlier post. This is now basically functional, however, we need to confirm that is is really working as desired. Never trust any circuits just build – especially when it comes to “unpredictable” aspects like noise. The parts used, they will most likely perform up to their specification, but there can be all kinds of hidden issues that will later on lead to lengthy troubleshooting of phase noise or spur issues.
From experience, power supply related noise is one of the most severe and troublesome item, if not taken care of at an early stage of design, or prototype construction.

Many articles exist on how to characterize power supply noise, especially at very low levels. This is not really what we need here, because we are talking about a real-world circuit that will later work with a mains power supply, in a reasonably well shielded case. So, our standard will be the lowest noise analyzer I have around here, an HPAK (HP Agilent, now: Keysight) 3585A Spectrum analyzer. This has pretty low noise anyway, down to -137 dBm for a 3 Hz bandwidth.

msr pll 3585a noise floor

The only downside of the 3585A, it is about 80 pounds – you will need a sturdy bench and a strong assistant to lift it.
msr pll 3585a analyzer
As a side note: The instrument on my bench, it has an interesting sticker, formerly owned by ST (STMicroelectronics, formerly known as SGS-Thomson as printed on the cal label). ST does a pretty massive amount of R&D in the field of semiconductors, and has a long-standing history of inventions. Well, fair enough, I got this instrument in bad shape, seems to have passed through many hands since ST time, but it is now fully repaired and calibrated, providing great service.

Step (1) – analysis of the circuits already build. Just some 0-25 kHz spectra.

Noise floor, probe grounded at AGND.
msr pll agnd floor 25k

317 regulator output (11.4 V)
msr pll 317 noise 25k

– well, much worse than expected! More than 30 dB above the noise level!!

Well, after scratching my head for a while – and doing some measurements around the not-too-complicated 317 circuit, one 22 uF cap was added, to the adjustment input. Ideally, for best frequency response, use a low ESR cap, with wide response, like a tantalum or multilayer ceramic. I could not be bothered, just used a plain electrolytic.

Improved schematic:
msr pll power supply updated
(red frame shows additional cap)

The result:
msr pll 317 noise 22 uf ref bypass 25k
A 20 dB improvement, fair enough!

Step (2) – PLL low noise power supplies (2x 3.1 V)

The PLL (an ADF4157 fractional-N synthesizer with ADF5002 prescaler) requires a +3.1 V power supply (2.7 to 3.3 V for the ADF4157, 3.0 to 3.6 for the ADF5002 – so I decided on 3.1 V for both devices). Also, we need a charge pump supply, for the ADF4157. This can be up to 5.5 V, but for simplicity of design, and to follow earlier (rather commercial) designs I did fully using 3.3 V technology, another independent supply is required, for 3.1 V.

These supplies need to very low noise, supply line noise will end up at the charge pump output, increasing phase noise. Glitches on the PLL supply lines can cause all kinds of issues, even the reliability of the circuit might be compromised (miscounting of the prescaler, etc.).

Quite a few more recent parts exist to provide about 3 V regulated output (see TI, Analog, LT), but these devices are non too widespread, and not much better, if not even worse than a trusty old part: the LM723 (aka µA723) regulator. This has a low noise reference build in, and should provide much better performance than any 3-pin regulator.

The schematic – main DC input, and 3.1 V low noise power supply section:
msr pll low noise power supply 3 volt

That’t the little board, during test:
msr pll test setup

And here, we have the results – all tests now using 1.2 KHz stop frequency (not much going on at higher frequencies), 10 Hz resolution bandwidth, 3 Hz video bandwidth, and, using the noise measurement function of the 3585A – this directly measures and calculates the noise level, at a given frequency, for a 1 Hz bandwidth. Very handy for conversion to nV/sqrt-Hz (nV divided by square-root Hertz is a common way of expressing power supply noise).

Noise floor:
msr pll noise floor 1k2 grounded at agnd

The 317 output (11.4 V) – supply of the pretune DAC circuit and amplifiers, and for the PLL active loop filter
msr pll 317 noise 1k2

The 7805 output (5 V) – digital supply, DAC supply
msr pll 7805 noise 1k2

The 723 output (3.1 V) – Vdd section
msr pll 732 vdd 1k2

The 723 output (3.1 V) – Vp section (charge pump supply)
msr pll 732 vp 1k2

A converter worksheet, to relate the dBm numbers, to nV/sqrt-Hz (calculation also has provisions to convert from other bandwidth – not considering a few extra dB to account for the averaging nature of the detector, etc. – we rely on the 1 Hz normalized value of the 3585A anyway, just in case you need to convert from other BW, please keep dectector response related offset correction, if the task requires such levels of accuracy).
power supply noise calc 3585a

Converted values
-141 dBm – about 20 nV/sqrt-Hz (Vp supply) – very close to noise floor of the setup, the LM723 still seems good enough!
-122 dBm – about 180 nV/sqrt-Hz (5 V, 11.4 V supply)

Also, quick look at the DAC pretune output – at the OPA2703 scaling amplifier output:

msr pll dac amp vtune output 1k2
Virtually, below noise floor.

Note: there are some litte contributions at 60 Hz/180 Hz from mains. These are due to the test setup/signals picked-up by the test cables – don’t seem to originate from the circuit itself.

Micro-Tel MSR-904A Remote Interface: pretune DAC, precision reference, and some auxilliary circuit

Not a very exciting circuit today, but definitely, a very important one: the pretune DAC for the MSR-904A. This DAC will drive the 0..10 V input of the MSR-904A, to set the frequency for a given band. The frequency needs to be set to about 1 MHz or better, and the DAC needs to be virtually free of noise – any noise will be converted to phase noise, and cause a lot of hazzle for the PLL circuit to be added later.

Rather than a dual supply, the intention is to use a single +18 V supply for the whole remote control circuit. Therefore, we need a few linear regulators, to derive the +5 V for the digital circuits, including the ATmega8-16, and a positive voltage of about 12 V, for the analog circuit. The output driver for the pretune (0..10 V) uses an OPA2703 rail-to-rail opamp. So, I decided on a 11.4 V positive supply, for convenience of resistor values available – 270 Ohm, and 2k2, for a LM317 regulator.

The DAC, a Texas instrument DAC8831. A highly linear device – 1 LSB of INL error. Low noise, low power. The DAC is connected to a +5 V precision reference, a MAX6350. This is a pretty stable and low noise reference, very much recommended for 16 bit converters.

Well, not much more to say, here is the schematic:

msr-904a interface pretune circuit and power supply

And, a quick glance at the board:

msr-904a interface pretune and digital control

There is some space left on the board – for an ADC (to monitor signal strength), and for the PLL power supply (needs 3x 3 V, UA723 – for low noise), and a 10 MHz/5 MHz distribution circuit.

SimonsDialogs – A wild collection of random thoughts, observations and learnings. Presented by Simon.