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Electronics

Noise Meters and Noise Figures

HP 8970A Noise Figure Meter: voltmeter assy (A7) defect

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Not so good news today, after characterizing all kinds of noise sources, the 8970A stopped working. Can’t get it to calibrate properly, or to show any reasonable noise power values. A quick check revealed that the detector output (voltage proportional to the noise power measured) is good. But no proper display when activating the 8970A-internal volt meter (special functions 80, 81).

Checking various traces and signals – the issue seems to reside with the A7 assembly, voltmeter.
8907a a7 assy

Red – input voltage section; green – reference voltage section (about 1.2 V); blue – auto-zero section.
8970a a7 assy schematic

How it works, quite well-established dual-slope integration with autozero – a capacitor, initially at zero volts, is charged first from the input voltage, then from a (negative) reference voltage, until zero is reached again. The time it takes to do this directly relates to the input voltage.

See here, working example (sorry a bit dim- see the triangular shape in the lower left hand corner of the scope screen).
working trace

Here, non-working condition – integrator not working.
non-working trace

After checking various FETs, and timing signals – the TL072 integrator opamp appears to be the faulty device. It is a strange, intermittent fault – not triggered by vibration, but appears to be intermittent with no direct external cause – maybe a defective output stage of the opamp? Removed it from the circuit; unfortunately, all spare back at the main workshop in Germany, but fair enough will get some TL072s in soon.

tl072 defective

…. once repair is done, noise source project will continue asap!

Noise Meters and Noise Figures

TWS-N15 Noise Source: checking out some design alternatives

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So far, we have mainly been discussing series type noise sources, i.e., noise sources where neither anode nor cathode are connected to ground. Another common design is shown here – the shunt configuration (one port of the noise generation element grounded).

noise source bfr93a shunt

The assembly, more or less just a little blob of solder with a few tiny parts inside… mostly, 0603 SMD format. The output attenuator (not shown) is a 14.5 dB(!), 18 GHz coaxial attenuator.

noise source bfr93a shunt assy

Some quick measurements, at bias currents of 2.5, 5 and 7 mA…. still, there seems to be a lot of 1/f noise (increase of noise power at lower frequencies). This is model #1, with a 22 nF capacitor (see schematic)

noise bfr93a shunt configuration 1

Don’t really see any advantage over the series variant of the noise source. But will test further.

…Progress on another front, ordered a set of PCBs – they can be used for various noise source configurations. Not yet a “prototype”, but need to see what kind of GHz performance is available from such design, and how reproducible it is. No current source yet on this PCB – will add later, or on a separate board – to limit shielding to the RF section.

noise source pcb 150827-2

Noise Meters and Noise Figures

TWS-N15 Noise Source: some RF transistors as noise generating devices

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After testing some Zener diodes and regular transistors (see earlier posts), some attempts with high frequency transistors, to generate white noise (noise power constant with frequency).

So far we have found that Zener diodes generate high noise power, and are rather flat out to 1.5+ GHz (if proper package and mounting is chosen). However, there is appreciable 1/f noise (increase of noise power) below 100 MHz, and this is difficult the equilize with just plain R-C networks.

Another attempt, with regular tansistors – they don’t have enough noise power at high frequencies, past a few 100 MHz.

Now, finally, I have received some 6 GHz BFR93A and 22 GHz BFG410W transistors, from my stock of parts back home in Germany, and have put these to the test. Same circuit is used like before, with positive current fed into the emitter, and the base grounded via some resistors (transistor is run in emitter-base breakdown condition to generate noise).

These are the parts concerned, some general notes – the BRF93A is a very useful part for all kinds of RF applications, and available at low cost.

noise bfr93a

The BFG410W, it is also quite remarkable and I use it a lot for LNA (low-noise amplifier) designs – hard to beat at their cost, delivering considerable gain, at low power. Unbelievable what the semiconductor folks have been able to achieve, a 22 GHz transistor, for a few cents each!

noise bfg410w

Here, the ENR results, vs. bias current, in mA.

BFR93A
noise enr vs bias bfr93a
-note that the ENR increases at low bias current!

BFG410W
noise enr vs bias bfg410w

As can be seen, and don’t ask me why, the BFG410W generates much less noise. Some quick change of the attenuator pad – 4 dB less attenuation. Just to check if this has any effect (besides increasing output power) – all seems well behaved and power is increased without changing any of the general characteristics.

BFG410W – lower 3 traces are 390 ohms parallel, upper 3 traces are 130 ohms parallel output attentuator (390 ohm 0603 pad resistor, paralleled with 390 or 130 ohm 0603 resistor)
noise enr vs bias bfg410w 130 ohm pad

The BFG410W appears to have the best white noise characteristics so far, note that the measurements are still not too accurate, mainly for screening of parts. With proper bias current selection, flatness, 100 to 1000 MHz, <0.2 dB should be possible. Will do some more experimentation, and fine-tuning of the filter/equilization components; ideally, the noise power should be a bit higher, to be able to use a larger, well-matched attenuator, giving good output SWR. Also, I think it is now about time to fabricate some better HF boards (still using FR4, but precision made), to get a reproducible assembly, and to have several TWS-15N prototypes made and characterized.

Various

TMS 2532 EPROM adapter: one byte every 50 ms…..

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EPROM progammers seem like a thing of the past, still, they are very popular for test equipment repair, arcade games, and all kinds of other occasions where small amounts of data need to be stored in a bulky, fancy package.
Such programmers, mostly copies of the “Willem” design, are widely available, Made in China, and generally, these work pretty well. Well, as luck would have it, most of the ancient pieces of equipment use 2532 EPROMs, and just this kind is not supported by the common programmers, which support the 2732….. same capacity, different pin layout.

2732 pinout

tms2532-45

tms2532jl-45

To adapt the 2532/2532A (these only differ by their programming voltage, 25 V vs. 21 V – make sure the set it correctly) to the common 2732 programmers, the only thing you need is a small adapter, with a most complicated schematic (the only pins that change are 18, 20 and 21). Most of these EPROMs require programming pulse widths of about 50 ms, but often program OK with just 10 ms, or less.

2532 eprom adapter for programming schematic

2532 programming adapter view 2

2532 programming adapter view 1

Noise Meters and Noise Figures

TWS-N15 Noise Source: noise generating elements

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Some trials with various low-cost noise generating circuit elements:

(1) Zener diodes
(2) Transistors B-E junctions in break-down mode
(3) Noise diodes – these are not being considered, not low cost.

For (1), a BZV55-12 diode was used, directly soldered on the traces of the noise source circuit described earlier.
For (2), as a first try, a BC238B transistor was used (with legs cut to very short length). Sure, I will try some RF transistors, but these are all back in the main workshop in Germany and will come over in a couple of weeks.

noise bc238b lin

noise bc238b log

The output, measured with a HP 8970A noise figure meter and some GPIB software to do this efficiently, it shows quite interesting behavior.

For the Zener diode, there is appreciable 1/f (pink) noise at <30 MHz, but the output is pretty much flat at higher frequencies. The transistor, well, it is working fine at lower frequencies, at 10 mA bias, the noise is flat-white up to about 300 MHz. But not enough noise at higher frequencies - maybe just not the right part for this purpose. These are just a few of the components tested, stay tuned.

Noise Meters and Noise Figures

TWS-N15 General Purpose Noise Source: some progress on the design, and some tests

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Many design of amateur noise sources have been published, they all have there benefits and shortcomings, but it is mostly the lack of a calibrated test source that makes it difficult for the hobbyist to employ a cheap, home-made noise source for the ever-so-critical noise figure measurements of the amplifiers and mixers he may build.
Having recently acquired a good calibrated source, HP 346B, I have decided to make available a noise source design and build so many, and calibrate them, in order to make calibrated sources, up to, say 1.5 or 2 GHz, available for everyone at a really low price. Thinking about USD 45 per piece which should be fine for everyone dealing with such measurements.
Requirements are pretty simple, it should be a ‘low-frequency’ replacement of the HP 346B, about 15 dB ENR output, flatness, preferably, within 0.5 dB, starting from 10 MHz. It should also have high output return loss (low SWR), which can’t change significantly from on to off state. Last but not least, powered by the ubiquitous 28 VDC noise source drive signal common to most noise figure analyzers.

The bill of materials, combined with the desired target price, won’t allow the use of a custom-made noise diode. But this is not really a disadvantage for the intended frequency range and purpose.

Three main tasks will have to be solved, to get this thing (called, the TWS-N15 Noise Source) to work:

(1) First and foremost, we need a noise generator circuit; these require a bias current, typically 5-15 mA.

(2) We need a temperature-compensated current source; currently experimenting with discrete Zener and TL431 based circuits. Current should not vary significantly, if input voltage is changing from, say, 27.5 to 28.5 VDC.

(3) Mechanical package. This is quite important, because the noise source needs to be well shielded, and put in a sturdy case – these sources tend to float around labs, and are often dropped, or dragged down from the bench by the heavy cables attached. Output connector will be SMA, because I have a large supply of really nice quality SMA print connectors.

The draft schematic (current source not shown, cirucit works, but still need better characterization, and possibly, some improvement). Note that the output attenuator will set the ENR level. Might need to adjust this a bit; for the time being, anything from 10 to 15 dB ENR will be fine.

tws noise source schematic

This is the small cirucit, build on a 0.6 mm FR4 board. Traces were simply carved out with a knive…..

2 ghz noise source

Here a few plots from the VNA characterization. Not bad for a start. Input return loss is at least 15 dB. This will still be improved. Using 0603 SMD resistors for the critical section.
tws noise smith chart

ON and OFF traces are shown – virtually, no effect on the return loss.
tws noise rfl loss on off

tws noise swr

Some tests of the noise output, really much better than I thought. Bias current has some impact on flatness; low frequency end is determined by the first capacitor (between diode and attenuator). This will need some further tweaking to get it as flat as possible, but no rocket science.
To measure the noise output flatness and level quickly, there is no also an automated test rig here, which measures bias current and on-off noise figure differences.

noise source test1

noise source test1 log

Next steps… will need to decide if the 0.6 mm board is the best option (these boards allow rather narrow 50 Ohm traces; most of them, I machine using a router mill – no etched traces), or if a commercially made board of regular dimensions would be the better option. Cost ist not really a concern, because the noise source only needs little space. Stay tuned.

R820T/RTL2832U USB SDR: Various hacks and measurements

USB RTL SDR 28.8 MHz Reference: VCO design established

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A quick update on an earlier post, USB RTL SDR 28.8 MHz Reference: VCXO, development of a low-noise 28.8 MHz is now complete. A BB159 varicap has been selected for the voltage tuning control, and it works nicely, even within a 10 V tuning range. This is a UHF varicap diode, C_28V of about 2.1 pF, 9:1 ratio.

bb159 varicap

The updated schematic – the test setup still shown here still uses a few through-hole parts, put most of the RF caps are SMD 0805. J310 J-FETs are used, both for the oscillator, and the amplifier – mainly because of their low cost, and they are widely available.

28800 vco schematic

This is the test setup – test signal provided by a 8904A, and frequency measured by a 5372A Time and Frequency Analyzer, 4 s gate time.
28800 vco test

That’s the tuning curve, it happened to be at 28.800000 MHz, for about 5.5 Volt, nicely centered within the tuning range. No need to worry about the deviation from linear slope, the PLL will have a very long time constant, and the non-linearity of the tuning curve won’t have any impact on the VCO performance.
28800 vco tuning curve

A nice additional feature – this VCO circuit works with the default 28.8 MHz crystal (which is actually no so bad, it is quite stable over temperature, at least the samples I have tested here, taken from cheap USB RTL SDR sticks). +-700 Hz range, +-25 ppm, should be pretty much sufficient to keep the VCO locked over a wide range of temperatures. If not (for other 28.8 MHz that might be around out there), two of the varicaps can be used in parallel, and the C* capacitor adjusted a bit; pullability of the xtal is pretty good, +-1.8 kHz is easily possible. It is best not to use a trimmer cap, but to solder-in suitable capacitors, to keep noise down, and stability up.

Noise Meters and Noise Figures

HP (Agilent Keysight) 346B Noise Source: finally, a calibrated ENR standard, and a temperature compensated current source

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Today, a rather ordinary envelope arrived, still it feels a bit like xmas, because of the contents….
346b envelope

…. a HP (Agilent Keysight) 346B Noise source, with nominal 15 dB ENR. I have long been looking for one, at a reasonable price, and finally scored this unit on xbay.

346b

The calibration sticker shows good flatness, especially, in the 0.01-1 GHz region, which I need most, it is perfectly flat.

346b enr cal

Prior to having it re-calibrated, a good opportunity to look inside. There are two sections: the current source, and the RF noise source assembly (which is hermetically sealed, and you better don’t touch!).

346b parts

These are some close-ups of the 00346-60001 power supply and current regulator board.

346b top side

346b bottom side

The current regulator inside of the 346B has always been a big mystery to me, because no schematic has been published by HP, in any of the service manuals. How it works, check out the schematic. The incomming 28 V (which is provided by the noise figure meter) is converted to a square wave, about 7-8 kHz, using a LM311H comparator. This is then converted to a negative voltage, about -20 V, absolute value varies a bit with loading condition.
The negative voltage is then used to sink current from the noise source assembly. The current setting of my unit is about 18.7 mA, programmed by the “10 Ohm” resistor.
Why the negative voltage? To simplify the design of the noise diode.

00346-60001 346B noise source schematic

Why did HP use a 5V6 Zener for the current reference, well, this is fairly obvious, when looking at the datasheet of such diodes.

bzx85 zener data

Around 5.6 V, the temperature coefficient virtually vanishes (this is why such voltages are also used for voltage reference circuits). The other Zener diode, unfortunately, I was not able to identify. It has 24 V drop, fair enough, any regular Zener should to the job to keep the base bias of the output constant.

For a quick burn-in, the source has now been connected to a 8970A Noise Figure Meter, and output appears to be very steady, less then 0.1 dB drift over a few hours. More noise measurements and calibration tasks to come, let me know if you need any noise sources measured, might be able to help.

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

Latching Coax Relais Driver: useful for all kinds of HP/Agilent/Keysight attenuators and switches

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Driving a latching relais doesn’t sound all that complicated, and it isn’t, still, some thought is needed to get it working properly. This is what the circuit should do

(1) Accept one 5 V TTL input signal, LO signal will result in the relais switched to the rest position, HI signal to the activited position (“dash” position), the switching signals are called A and A’, for relais A.

(2) There should be an output-enable signal for all relais, to be able to de-energize the relais, because otherwise, the circuit can suffer if the relais is disconnected while powered; these latching relais only need a few 10s of Milliseconds to switch, power will be turned off, by the ATmega32L, 1 sec or so, after each switching event, just to avoid any unintentional short circuit or spike, and the positive supply line will carry a fuse.

(3) The circuit should be able to handle any common relais voltage, 5, 12, 15, 18, 24 and 28 V. The relais I am using here are 24 V rated, but work fine at 18-20 V, and up to 28 V.

(4) It must be avoided, at all cost that both the A and A’ signals are active at the same time. This can cause nasty relais oscillation, and damage/wear of the expensive relais. We don’t want to rely on software control here, because things can go wrong, during startup, program freeze, and so on.

coax relais driver schematic

The circuit is almost self-explanatory. The LS04 generates the complementary control signals, from the digital input. The LS373 is configured as a transparent latch, in fact, the latch function is not used, but the output control function is. If the OC input goes LO (and thus, the neg_OC high), the outputs of the LS373 will tri-state High-Z.

The output driver, a Toshiba ULN2803APG is used. These are very reliable circuits, Darlington transistor arrays with build-in diodes.

uln2803apg

Make sure to put a good quality capacitor close to the COM pin, the return path for the snubber diodes, otherwise, the inductance of the traces will deteriorate the action of the diodes. Actually, the HP coax relais have build-in diodes, so I don’t expect and issues with voltage spikes here.

Note that the inputs of the ULN2803 have adequate resistors to ensure LO state when connected to a High-Z output; if other transistor arrays are used, the circuit might need some ~10 kOhm pull-down resistors to ensure noise immunity and defined signal levels.
The 15 kOhm input resistors serve the same purpose, they keep the relais de-energized if no logic input is connected.

This is the little board, with all space well used. Sure the logic could build using more modern devices, and I would definitely do so for a commercial device, or anything build in series, but LS logic is still popular, and it is cheap, at hand, and reliable.

coax relais board

That’s the setup, with the two 8673B transfer switches, and a 0.3 Hz test signal fed to the digital inputs by the brown wire shown above (removed for the picture below).

coax switch setup

The signal at one of the contacts, 24 V drive level, 1 V per vertical DIV, 10 ms per horizontal DIV. No spikes, high voltages, or similar undesirable features are present.

coax switching

After about 2000 switching events, two events per 3.3 seconds, everything is still working well, and all circuits, in particular, the ULN2803 stays cool.
As I said, not a very complex circuit, but may be useful for other purposes, and is a very cost-effective, reliable solution.

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

Tripple PLL Controller, for Micro-Tel SG-811/1295/1295: converting a ratsnest to a shiny box

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Comming back to an earlier project, the frequency stabilizers/PLL controls for the Micro-Tel 1295 receiver (2 pcs, one for through, and one for reflected power), and the Micro-Tel SG-811 generator (0.01-18 GHz).
The setup has been working well, but it is an awful mess of cables, as a result of the development process.

18 ghz pll wire mess

Now is a good time to finalize the circuit, and to put it into a nice case.

pll controller case

This is the front panel – the back panel has numerous BNC connectors, for the “PLL Phase Lock” voltage, and the “Frequency Control”, i.e., coarse tune voltage. The latter voltages, 3x 0-10 Volts, are generated by a DAC board, which is now ready and tested. Not a thing of beauty, but it works, and no reason to assemble another one just to clean things up a bit.

dac board

The circuit is reasonably straightforward, a 2.5 V precision reference is used, and three MAX541 16 bit DACs, followed by OP284 amplifier to convert the signal to the 0-10 V range.
The MAX541, it’s a really great part, full 16 bits, +-1 bit integral nonlinearity, and very stable over temperature and time. Highly recommended for any precision application.
One of the channels has an additional TLC2201 opamp, which is there more for historic reasons than for anything else.

dac board schematic

The PLLs use three ADF41020 18 GHz chips, following the approach discussed earlier. This will give 100 kHz frequency resolution, which is more than sufficient for the intended purpose of measuring gain/attenuation and SWR over the full range of microwave frequencies up to 18 GHz, or 40 GHz, if no SWR measurement is required.

The 10 MHz reference circuit has also been completed, and is working well, converting a 10 MHz input signal (of more or less arbitray shape and amplitude) to 3x 10 MHz REF signals for the ADF41020, and two 5 MHz outputs, for the 1295 receivers, just in case I need to work with external downconversion-the 5 MHz signal is only affecting the 0.01-2 GHz range of frequencies. Well, why not having everything phase locked, if we can.

10 mhz 5 mhz ref circuit schematic

The 10 MHz circuit has been modified slighly, to accept 1 kOhm impedance 10 MHz input signals, which are rather common for instrumentation purposes. Some caps and protection circuitry has been added, output levels are just about 0 dBm. No need to go to stronger signals, it will only cause spurious response, and other trouble, because shielding inside of the PLL case won’t be very strong.

Some remaining work, before we can call this project complete:

(1) A power supply regulator, +5, +3, +3 (Vp low noise), +15, +20 supply has been build, but need to be mounted to the case, for heat sinking. This will need to wait for a few weeks, need to wait for some travel to Germany coming up, where the metal working machinery resides.

(2) Digital interface and cables for band control of the SG-811, and the two 1295s are complete, just need to be mounted and tested.

(3) Relais control module needs to be designed and build – there are two 18 GHz transfer switches (one is needed for basic functionality, a second one for the various unexpected test configurations). The transfer switches are HP 8763B, see earlier post. These are wonderfully precise and repeatable, albeit, list price is about USD 1k, each.

(4) Need to route the two serial signals (dB reading, signal strength) from the Micro-Tel 1295 to the ATMega32L, which only one USART input – no problem, because this is all TTL-level USART signals, but need to get a multiplexer/a few NAND gates to set this up. Currently, only one 1295 can be recorded at a time, which means, either gain or SWR measurement.

(5) Sure, the software, both firmware and instrument control software will need to be refined and consolidated. All PLL calculations and adjustments are done within the ATMega32L, still there is a need for a more convenient user interface than just command line.

… to be continued!