DCF77 – Tell me, how far do these waves travel?

2014/08/27 Simon Leave a comment

For those of you who don’t know, Germany has a time signal and frequency standard station, a 77.5 kHz carrier, emitted at a place near Frankfurt. About 50 kW of power – enough to provide most of Western Europe with perfectly accurate time. Radio controlled clocks have become the de-facto standard, in most of these places, and are available for a few Euros, amazingly cheap.

While this is all common knowledge, it was definitely news to me that this signal can be picked up in the US, at least at the East Coast – New York area, where I currently reside. These news came from a very much trustworthy fellow German, just a few miles away – he carried a radio controlled alarm clock over from Germany. And one day it started receiving a signal, and set itself back to German time.

Definitely, time for some experimentation.

The setup:

(1) A well-tuned ferrite antenna (same as is used in alarm clocks), with a little J-FET preamp. Additionally, a long wire (about 10 m/30 ft), connected to the hot end of the tuned circuit.
(if you need a schematic, just ask, works with a J310 J-FET)

(2) A long coaxial cable to get the antenna out of near field interferences.

(3) A xtal filter and amplifier/driver – to provide adequate signal levels.
(if you need a schematic, just ask, uses a NPN transistor, and an OPA703 CMOS rail-to-rail opamp)

(4) Monitoring of the signal, via PC soundcard, and Spectrum Lab software.

Current status:

So far, not a trace of the 77.5 kHz carrier has been received, even using a most sensitive HPAK 3585A spectrum analyzer – maybe, I just need to wait for better propagation conditions, to get these long waves over the ocean.

To be continued…

NOAA APT - you want to see the clouds from above?

Some little improvements…

2014/08/27 Simon Leave a comment

Over the last days, more and more imagery has been received, with the NOAA satellites on their appointed rounds. A few things were found that might help you to get things working better:

(1) A high sample rate of the SDR (like 3.2 MSPS) is neither necessary, nor advantageous, in fact, I discovered this to be the main reason for some interrupted frames – the data is flowing via USB at 3.2 MSPS, but not all the time. Also, the S/N seems to be better, at lower sample rates, like 0.9 MSPS.

(2) The bandwidth. Provided that it is heated-up and left running, the little crystal in the SDR USB stick is actually pretty stable. I need to use a correction value of about 17 kHz at the 137 MHz frequency, but once entered, no need to change so far. Once the frequency is corrected, there is no need to set the FM bandwidth any wider than it needs to be, about 38 kHz seems to do the trick.

(3) Volume control. Easy to set, about 70% on the WXtoIMG works fine – I didn’t look at this too carefully at the start – make sure to re-adjust the volume after coming back from other tasks with the SDR USB stick.

NOAA-19 (2014-08-27)

NOAA-15 (2014-08-27)

Not quite perfect, but I can see improvement!

Wavetek 907A

Leveler calibration PROM, now: EPROM

2014/08/27 Simon Leave a comment

Fortunately, the manual of the Wavetek has some detail on the leveler correction PROM. Essentially, it is fed with 8 bits representing the frequency, 255 (0xff) for 7.0 GHz, 0 (0x00) for 12.4 GHz. For each of the 256 steps, it has a correction byte stored in a Texas Instruments TBP28L22N 256×8 PROM. This is programmed at the factory, to match each individual RF deck.

Getting this exact device and programming tools ready was out of question, some of these PROMS might still around, with datecodes of the 80s, but really not worth the effort and cost.

Step-by-step

Step (1) – A little test rig was set up, with the recently repaired HPAK 8904A as a DC source (can source -10 to +10 Volts, in very fine steps), and the DC voltage connected to the leveler correction control voltage line. The Wavetek is designed for servicability, and there is a nice jumper to disengage the actual level correction DAC, and the feed an external voltage instead. An EIP 545A was used as a frequency counter and power meter. Cable and EIP 545A power meter accuracy was tested with my best calibrated source at hand, and found to be within +-0.5 dB over the 7 to 12.4 GHz band.

Step (2) – Data were collected by setting the Wavetek to various frequencies, mostly in 0.5 GHz steps, and the control voltage was adjusted for the power meter to read about 0 dBm. The data were then used to calculate the coefficients of a forth degree polynomial, and converted to the 256×8 bit format. The Wavetek uses a DAC0800LCN DAC, and the output voltage (after the on-board opamp, a LM307N) was found to be very close to 10.00 V with 0xff input, and nearly -10.00 V, for 0x00.

wavetek 907 cal

Step (3) – The tricky part. How to get a replacement for the 28L22 PROM? There are mainly two choices, one option would be to use a little microcontroller, that can easily function as a pseudo-memory, or use something more permanent, in this case, an EPROM. I had some 2532 around, therefore, not much effort. Only a small fraction of the 2532 will be used, 2 kbit of a total of 32 kbit. What a waste!
Unfortunately, the 2532 isn’t close in size to the 28L22, nor are the pins arranged in a similar fashion – but this can be solved with a little adapter board, and a few wires. Not the most beautiful solution, but who cares, it works!

Wavetek 907A

Leveler – Missing part!

2014/08/27 Simon Leave a comment

As it turns out, somebody must have opened up the Wavetek 907A before, and it is missing a crucial part – the detector, for the leveling circuit! Checked out with the manual – there used to be a 6 dB pad (to get better SWR for the detector), and a positive-type tunnel diode detector (SMA input, SMA output). Nothing I have around here surplus.

Wait – there is a spare HP Schottky negative-type detector around in a drawer back home in Germany, and coincidence allowed to have it carried over to the US. This litte device as SMA input, SMC output, and with a little SMC to SMA adapter, at least a mechanical fit.

A little mod will is required to get this going, changing the polarity of the input amplifier. No big deal:

Next step – there are correction coefficients stored in the Wavetek (in ROM), to compensate for signal losses along the RF chain, and to keep the output calibrated. This will require some more thought, to be continued.

HP 8904A Multifunction Synthesizer

HPAK 8904A – Power supply repair

2014/08/27 Simon Leave a comment

With the failed parts identified, we need a replacement for the FES8DT diodes: 200 V, 8 A, 0.95 V drop, 35 ns recovery time, 125 A surge current.

Something more rugged – because already on the datasheet, it says “Ultrafast, rugged” – what do you want more – are the BYV79E-200 diodes.
These are 200 V, 14 A, below 0.9 V drop, 30 ns, 150 A. These should last for another 20+ years, and come in the same packaging, SOD59, aka TO220AC.

Fortunately, I had some spares around from other activities, but you know – the best source is not xbay, but just some junk switchmode supplies that should be sitting around in any good electronics workshop – these are always keepers, and have come in handy as a source for parts, for more urgent (commercially relevant) repair jobs, rather than for the HPAK 8904A…

For the 8904A – one important thing! Never short out the RAM backup power on the main board – you will need to go through an activation routine, otherwise, your instrument will be rendered non-working. Don’t now why HPAK didn’t use EEPROM at the time, maybe, these were not yet invented… Following the instructions in the user manual (Thank You, HPAK, for providing all these manuals free of charge on the web!!!), the backup battery was replaced with a new Lithium cell. In the end, it might be a good idea anyway, to open-up the case every 10 years, get the the dust out, and a new battery in.

With new Schottkys, new RAM backup, new serial (and new code), the instrument is now fully functional, option 001 and 002, and will provide good service, hopefully, for decades to come!

Anyone with a broken 8904A – feel free to contact me!

HP 8904A Multifunction Synthesizer

HPAK (HP Agilent Keysight) 8904A Multifunction Synthesizer

2014/08/27 Simon 3 Comments

The HPAK 8904A – you will find the detailled specs elsewhere on the web, it is a one-of-its-kind instrument, an early type of arbitrary waveform generator, capable of frequencies up to 600 kHz, but its mainstay is the audio region.
Why is it so unique, well, it has a complex (some say, difficult to use, but are there any easier ways?) user interface that let’s you program all kinds of test signals without any time lost for generating complex waveform bit by bit, uploading it to a modern arbitrary generator, and so on. The HPAK 8904A support all the common modulation features, AM, FM, Phase, and additionally, DSBSC (double sideband supressed carrier) and Pulse modulation. It can gernerate FM stereo test signals, various other types of composite signals, DTMF tones, just to name a few. In fact, HPAK has published a catalog, with some of the common waveform (will try some of them later).

Most importantly, your plain 8904A might not be able to do all these things, because all the nice and special functionality is only available with option 001, which is, believe it or not, a software option (to my best knowledge, one of the earliest occurrences of a really powerful software option feature for HPAK equipment).

The unit I’m using came with option 002, two channels, but not with option 001 – well, it’s software only, all you need is a serial and key, and some intructions – which I will be happy to provide to you, when needed.

But before we start: the unit discussed here came from an undisclosed source, and failed power on test (no noise or other signs of functionality). Once opened up, big surprise, HPAK was using a Computer Products switchmode supply, 90 Watts, Model XL51-5601. Fair enough, this is a quality supply, but somethings must have gone wrong – no power.

Troubleshooting a switchmode supply, with my somewhat limited workshop here in the US – well, it’s worth a try. No schematic, so hoping that it failed because of one of the more obvious defect modes. And, fair enough, here are the cuprits: FES8DT Schottky diodes, the secondary rectifiers – 2 of 3 were dead.

The FES8DT are 200 V, 8 A, 85 pF, 0.95 V drop devices. 35 ns recovery time. 125 A surge current. Seems they don’t fully hold up to the requirements.

Attenuator calibrator (objective: a really precise and accurate apparatus)

Remotely controlling the Micro-Tel SG-811

2014/08/25 Simon Leave a comment

The SG-811 comes with various option – mine didn’t come with the IEEE-488 remote control option. At least, it has a BCD type TTL interface. All the essential functions (band, operation mode, attenuators, and in particular, external frequency control-phase lock input enable) can be controlled via no less than 23 signals, plus ground.

All the signals are available at the rear of the instrument, via a 50-pin Centronics connector (similar to the old-fashioned SCSI connectors).

Several steps were taken to make sure that the ancient but still valuable SG-811 will carefully listen to the commands of a modern area microcontroller:

(1) Fabricate a suitable connector cable. Centronics 50 to D-sub 25. Starting from a pre-assembled D-sub 25 1:1 cable, cut in half, the Centronics connector was soldered on. Quite an effort! Turned out that the 1:1 cable uses pretty thin wire – they are saving on copper, over there, in China!

(2) A little shift register, 3×8 bits (3x 74LS164) – a total of 24 wires that can be controlled. 3 of these wires will be used to select the band of the 1295 receiver (via optocouplers, PC817), the reminder, via direct TTL connection, for the SG-811. The shift registers will later be set by a microcontroller, just using 2 outputs to set 24 wires.

Wavetek 907A

The Wavetek 907A – initial assessment

2014/08/25 Simon Leave a comment

A Wavetek 907A – this apparatus generates microwaves from about 7 to 12.4 GHz. All the typical modulation capabilities are provided, FM, AM, pulse. Found it on xbay, USD 45. That’s less than the value of the two precision bulkhead SMA to N connectors.

Quick look inside:

RF unit:

You can see the YIG oscillator on the top, followed by an isolator, leveler, isolator, two attenuators, isolator. There is a side chain, starting at the leveler, and with an additional isolator: this is a non-attenuated reference signal, which is available as an auxilliary output at the front panel – quite handy to use as as a signal for PLL stabilization, or other purposes.

Some items that need attention:

(1) Power LCD display doesn’t work properly, not showing the power reading

(2) Attenuator works, but leveler doesn’t seem to work (always at maximum, unleveled power)

(3) Only have a manual with partial, cut off schematics!

(4) Some TLC required, de-dusting – done.

(5) Two pushbuttons are missing, but switches work – more a cosmetic issue, will replace, once everything else has been fixed

(6) Some switches don’t work properly – fixed with the help of DeoxIT D5.

Attenuator calibrator (objective: a really precise and accurate apparatus)

The Microwave PLLs: stabilizing the YIGs

2014/08/25 Simon Leave a comment

The Micro-Tel SG-811 and 1295 are great units, however, they lack PLL control. Even at their time, in the late 70s, early 80s, government labs required PLL control – and Micro-Tel offered PLL controlled frequency stabilizers for these units. Stabilizers that are now virtually impossible to source (if you have two spare Micro-Tel FS1000, please let me know!).

So I decided to build some very broadband PLL circuits that can handle 2 to 18 GHz, at reasonable frequency resolution. 10 kHz, or 100 kHz resolution seems to be perfectly adequate; mostly, the attenuator calibrator will be used in 2 GHz steps anyway.

Both units have two inputs:

(1) A frequency control input – a voltage controlled input, 0 to 10 V, that sets the frequency roughly, within the given band. Bands are: 2-4, 4-8, 8-12, 12-18 GHz. There is some thermal drift, but preliminary test shows that a 16 bit DAC would be most suitable for this kind of “coarse” frequency control.

(2) A phase lock input. This has a sensitivity of a few MHz per Volt. 0 to 10 V input, for the 1295 – and -3 to 3 V for the SG-811, as it turns out. Accordingly, with the coarse control set to the right value, the phase lock voltage should be somewhere around 3-7 Volts, for the 1295, and close to 0 V for the SG-811.

Now, the tricky part, how to get a phase comparator running, for the 2-18 GHz range? Traditionally, this requires a broadband harmonic generator, locking to a certain harmonic, and so on. All possible, has been done before, but a lot of work to get it working.

There comes the rescue, from Analog Devices: a truely remarkable little thing called ADF41020. It is a full 18 GHz PLL circuit, works with more or less any reference (10 MHz will be used here), and has pretty high input sensitivity, all that is needed are about -10 dBm to drive it over the full band.

After some tricky soldering, in dead-bug style, and some auxilliary circuitry, with 16 bit DAC, reference voltage supply, very clean and stable supplies for the PLL, all the typical loop filters (0.5 KHz bandwidth) – and an ATMega32L – this is the current setup, for the 1295. Believe me, it is working just fine, and even has an auto-track feature, to keep the phase lock voltage mid-range – so it won’t un-lock with drift.

Upper left hand corner: ADF41020
Lower left hand corner: PLL loop filter
Center: Low noise voltage regulators, reference and DAC
Other parts: ATMega32L board (16 MHz, USB interface), LCD display (just for troubleshooting)

Attenuator calibrator (objective: a really precise and accurate apparatus)

Equipment selection: switching matrix

2014/08/25 Simon Leave a comment

There are quite a few coaxial switches around – I figured that I need two transfer switches to accomplish the task of “through” calibration, and reflection/insertion loss measurement.
Any unused ports should be automatically terminated with 50 Ohms, when switched out.

Looking around, I found that the HP/Agilent/Keysight (will call it HPAK from now on, and add further letters, with next name change of this wonderful company) HPAK 8763B transfer switch, offers really good data, especially on repeatability. 0.03 dB – for millions of cycles.
Determining this switching reproducibility will be the first task for the attenuation calibrator!

They go for USD 813 each (August 2014), but you can find them much cheaper elsewhere. Preferably, get a unit that doesn’t have 10 million+ cycles yet!

These are of latching type – so we will have to device some drive circuitry to switch them, 24 V positive supply. Won’t be too difficult.

Interconnections will all be rigid coax, and precision SMA to N test cables to connect to source/receiver.

Note the Sage 0.5-18 GHz coupler, left of the switches. This will be used to get a sample of the SG-811 signal – stay tuned.
For this coupler – this item was found on xbay, quite reasonably prices for its bandwidth. However, the coupled port has a little damage of the SMA connector – rendering it non-usable for its original destiny, but will now be very handy for this project.

To the outside world, the interface is a pair of HPAK SMA (3.5 mm) to precision N panelmount transitions. These are the best and most reliable know to me to date.

SimonsDialogs

DCF77 – Tell me, how far do these waves travel?

Some little improvements…

Leveler calibration PROM, now: EPROM

Leveler – Missing part!

HPAK 8904A – Power supply repair

HPAK (HP Agilent Keysight) 8904A Multifunction Synthesizer

Remotely controlling the Micro-Tel SG-811

The Wavetek 907A – initial assessment

The Microwave PLLs: stabilizing the YIGs

Equipment selection: switching matrix

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