PICOWATT: Support - Questions and Answers

Revised: 22 July 2005
To the opening screen

Questions and Answers

AVS-47

Picobus primary computer interface
Square-wave excitation
Preamplifier cable
Can the AVS-47 be used for dc biased measuments
Can the measuring range of the AVS-47 be extended?

AVS-47A

Grounding of the I- sensor leads

AVS47-IB

The Start/Print button does not work always
Is it necessary to move the AVS-47 jumpers?
The AVS47-IB refused to start any more

LabView

Comma as the System Decimal Point creates problems

PICOLINK

Can we use our own RS232 fibre link?
Can we upgrade our old AVS-47 with the Picolink?

AVS-47

You say that the AVS-47 has a primary interface. What does this "primary" mean?

The IEEE-488 interface has been divided in two parts. The first part, the primary interface, is located inside the AVS-47 whereas the secondary part is external to the instrument. The primary part was designed to be very quiet, whereas the secondary unit contains actually a PC computer and has its own power supply. One can place the secondary unit, and also the noisy IEEE-488 bus, meters away from the AVS-47 and so reduce the danger of heating the sensor by EMI.

You also call the primary interface "Picobus serial interface", which can make use the computer's serial port. Then you say somewhere that it is not RS232. I feel this confusing. What is it actually?

The Picobus is our proprietary, but not secret, synchronous protocol. The RS232 deals with asynchronous communications, where the data is sent using a fixed speed, called baudrate, so that the receiving device can synchronise itself with the data. In the minimum configuration, only tree lines are required: ground, transmit and receive. The data is synchronised by means of start and stop bits. The RS232 is commonly used, but it is difficult, though possible, to build an RS232 interface without using a microprocessor. Because we wanted to implement the AVS-47 EMI-safely without a uP, the RS232 was not a solution for us.
We needed a simple way to interface the AVS-47 with a computer without a adding a microprocessor for that purpose. Fortunately, the COM: ports of the "IBM-type" PC:s have some hardware handshake signals that are suitable for building a synchronised interface. One of these signals can work as the clock, one as data flowing out from the PC, the third as data coming from the instrument to the PC, and the fourth is a sort of "service request".

The Picobus is almost foolproof. You can switch the AVS-47 off and on, and the system continues as if nothing had happened. You can filter the Picobus lines, if desired, very effectively, and for that purpose you can reduce the speed of the interface as much as you need.
As a conclusion, Picobus is serial, because the data is sent serially along one wire, one bit after the other. It is synchronised by using a separate clock signal, whereas RS232 is asynchronous, having no clock signal. Neither the high-level programming languages nor communication programs support the Picobus, but some special program routines are needed to make the handshake lines to work as signals.

With my old computer, I sometimes used the Picobus interface together with your test programs. Recently, I got a new modern computer, and I had difficulties when I tried to use your old programs. Should I upgrade them or what?

Sorry. The Picobus was designed when DOS was still the most common operating system, and Windows 3.1 was just making its coming. The Picobus accesses computer hardware directly from the high-level programming language, and this is exactly what the new operating systems like WindowsNT do not want the users to do. We have the feeling that direct interfacing with a PC by using the primary interface alone will soon be but a theoretical possibility, and that the IEEE-488 is the method that everyone should use. Fortunately, IEEE-488 instruments are now so common that hardly anyone has to buy an IEEE-488 controller card for his/her computer only for the AVS-47.

Square-wave excitation

Your AVS-47 feeds a square-wave current to the sensor. The spectrum of such a waveform contains a lot of harmonics. Isn't it dangerous to use square-wave as far as feedthrough to other sensitive devices in the cryostat is concerned?

No, it is not. The operating frequency of the AVS-47 is 12.5 or 15Hz (Europe/USA). The fundamental frequency has the highest amplitude of all the frequency components of the waveform, and the amplitude of the fundamental is approximately equal to the low excitation voltage of the AVS-47. All the other components are the smaller the farther they are from the fundamental. Even with a fast square-wave (tr < 1us) the power spectrum of the second component has gone down by 10dB. the 80th harmonic of 12.5Hz has an approximately 20 dB lower power. A frequency of 10 kHz is the 800th harmonic. It is still within the audio rather than radio-frequency band, and usually feedthrough at 10kHz is not yet a problem.

Okay, but isn't square-wave more sensitive to stray capacitance in the sensor leads than a sine wave. Why do you use square instead of sine in the first place?

The fast edges of the square wave are sensitive, but the flat parts are even less sensitive to capacitance than a sine wave. The effect comes from the fact that the preamplifier compares two waveforms: one, almost ideal square, coming from the feedback, and the other coming from the sensor. If the second waveform is shunted by a stray capacitance across the sensor, the comparison yields sharp spikes which are the difference between the sensor and feedback signals. We simply disable the phase-sensitive detectors for a time which is long enough to let the transient spikes die out completely.
The only reasonable possibility to generate a good low-frequency sinusoidal waveform, which has a linearly and accurately variable amplitude, is to use a D/A-converter driven by a microprocessor that continuously calculates the digital input. On the contrary, a similar square-wave can be generated by using only two analogue switches and a couple of operational amplifiers. The lower cost is a benefit for the user, too.

Preamplifier Cable

The length of the preamplifier cable is 5 meters. What happens if we need a longer cable?

The division of the AVS-47 circuitry into two parts, namely the preamplifier and the main unit, is a somewhat violent operation. Therefore, some of the signals in the cable are rather sensitive to disturbancies and to changes in the wire length. One thing is certain: the longer is the cable the more sensitive the system will be to a possible ground current flowing via the the cable .
Anyway, reasonable cable lengths work well: we have tested 10 meter cables and no degradation of performance can be seen. Recalibration of the bridge offset and scale are necessary whenever the cable is changed.

We would like to keep the bridge outside the shielded room and the preamplifier inside. Can we break and filter the preamplifier cable at the room wall?

You can break the cable and use 25-way 1-to-1 connectors to implement the feedthrough. But do not filter the lines in the cable. Filtering would destroy the square waveforms. The ground connection between the two parts must be as good as possible (=low impedance), and this excludes filtering of the ground line. Easier than breaking the existing screened round flat cable and inserting new connectors would be just to buy a second standard cable from us. Then make only the feedthrough yourselves.
If you are afraid of RF-heating in the sensor, consider filtering the input lines to the sensor. The same rule applies also here: do not filter the ground lead.

Measurement of a DC-biased resistance

Is it possible to use the AVS-47 for measuring a resistive sample if there is a voltage across it?

Unfortunately not. The AVS-47 was designed for measurements of passive resistors only, and a bias voltage would immediately saturate the DC-coupled preamplifier.
We have been developing a new type of instrument, called "Differential Resistance Bridge". In addition to measurements of nonlinear resistors by applying a DC bias to them, this new instrument will be suitable also for measurements of other DC-biased resistors, including active elements. The voltage range will be limited to a couple of volts, however, and there will also be restrictions regarding the measuring range. Read the article "Measuring Nonlinear Resistors" and the description of the DVS-10, and contact us if you need more information.

Measuring ranges

The range of the AVS-47 extends to 2 Megohms. Is there any way to exceed this limit, or is the measurement of samples in excess of 2 Megohms just an impossibility?

There are two solutions.
You can use the deviation display mode, and set the deviation reference to some convenient value which is below 19999. Then your resistance reading is the difference between the sample's resistance and the reference. For example, if the reference is set to 10000, a sample resistance of 2.5 Megohms would be shown as 1.5 Megohms.
Another possibility, more suitable for permanent use, is to modify the AVS-47 by adding a new 20 Megohm range. This new range will be selectable by a switch, and if selected, it will replace the original 2 Megohm range. In other words, the number of ranges is not increased, but the bridge will then jump directly from the 200kOhm range to the 20Megohm range. Any software written for the unmodified bridge, has to be changed so that it can interpret the new readings correctly. Ask for more information if you might need this modification.

AVS-47A

Is the input of the new AVS-47A different from that of the older AVS-47? It seems that the I- leads of those channels, that have the new jumpers inserted, are connected to ground all the time. With the AVS-47, an I- lead was grounded only when that channel was selected.

This is true and the input is different. Those channels that have the short-circuit piece inserted have their I- leads connected to the preamplifier ground at the sensor side of the multiplexer relays and therefore this connection depends only on the jumper. With the older AVS-47, connection to ground was made after the relays.
It was not possible to implement the alternative grounding options without making this change. We did not see anything preventing the new configuration. If your application works with an old AVS-47 but not with the new "A" version, please let us know. It is not difficult to restore the preamplifier's old behaviour.

AVS47-IB

Sometimes, when I have tried to use the direct printing feature of the AVS47-IB, the START/PRINT button refuses to start anything. In other words, it does not work. But I have also managed to use it some other time. What could be wrong?

You can encounter this problem only with AVS47-IB units prior to serial number 332.
     In normal operation as a single instrument, the AVS-47 sets a special digital output bit after each A/D conversion in order to show that a new conversion result is available. If the result is not read by a computer, this bit remains set all the time.
     When the AVS47-IB boots up, after it has been powered on, it will initialize the AVS-47 into a state where this output bit remains permanently reset, and only the activation of the START/PRINT switch can set it again. The interface unit is waiting for this bit, and after having seen it, starts a measurement which ends up with printing the result.
     A correct order of powering the instruments is therefore necessary. The bridge must be powered first, the Picobus cable between the AVS47-IB and the bridge has to be in place, and the interface unit must be powered as the last item. Then it should work.
     Starting from s/n 332 this problem has been eliminated by an improvement in the firmware.

There are some jumpers on the AVS-47 circuit board and the manual says that they should be removed when using the bridge with the AVS47-IB interface unit. Otherwise "the optical isolation will not be enabled". But I do not have any need for the isolation, all my grounds are connected together anyway. Everything seems to work even if the jumpers are in place. Can I just forget this instruction?

Please do not! After having read this, go and take the jumpers away, otherwise you will probably burn the AVS-47 mains transformer sooner or later.
If the AVS-47 is interfaced with a computer using only the primary Picobus interface, the bus needs to get its power from the bridge. Then the jumpers must be in place. But the AVS47-IB interface has its own 5V power supply, and it draws a substantial current from it. Suppose that you have the jumpers in place and disconnect the AVS47-IB from the mains. The power supply of the bridge will try to supply the 5V power for the interface. You can see the green READY led lighting and everything can even work properly, which makes the error difficult to realize. But the AVS-47 power supply will be highly overloaded, which will finally lead to a transformer failure.
The opposing situation is not as dangerous: the AVS-47 will not work properly if only the interface is powered, and from the very dim status lights you will soon see that something is wrong.

I had a strange problem with my rather new AVS47-IB interface. It just stopped and refused to start again. I sent it to you for repair, but there was actually no fault. You said that the setup memory had gone crazy and that you just reprogrammed it for correct settings. Now it works well, but I am afraid this will happen again. Can I do anything to prevent such a failure in the future?

We are still puzzled with these failures. No definite reason has been found so far and we ourselves have been unable to reproduce the fault.
     The setup of the computer module is stored in an EEPROM memory. The memory is read and written by using bios extension routines supplied by computer manufacturer. Writing occurs once during manufacturing the interface, and after it only when either of the two commands is used: IBAx and PBDx (set the GPIB device address and set the Picobus delay factor). None of these commands is expected to be used frequently.
     The write count into the EEPROM is limited to about 10000 times. After that, the memory can turn unreliable or writing may no longer be succesful. One must therefore avoid exceeding this limit. During writing the EEPROM, the computer is vulnerable. If a power failure takes place, the results will be unpredictable. Writing is a slow operation. If it occurs frequently, the likelihood of a power failure during writing is no longer negligible.
     Try to avoid these failures by checking that your GPIB control program does not repeat either the IBA or the PBD commands. Remove them from a possible initialization section, and put somewhere so that they are available if needed.
     If you use a LabView driver that has not been written by us, check that it does not repeat these commands anywhere. If the source code of the driver is not available, you may be able to use a "spy" program supplied at least by National Instruments together with their GPIB boards. Our own LabView driver has been designed so that frequent writing to the EEPROM cannot be made by mistake.
    We have added to firmware versions starting from 1R5 the capability to print the write count if a printer is connected, and starting from 2R0 also the eeprom write count query. Please contact us for a new version if you have this problem.

LabView

Question. I had to define comma (,) as the Windows System Decimal Point in my computer. After that, your LabView driver stopped to work properly. Following your advice, I downloaded the newer version 1.1. In the beginning, everything seemed to be OK, but when I tried to use the TS-530A temperature controller with your "Application example 1.vi", the heater power display got crazy. Is something still wrong?

In driver version 1.1 we repaired those VI's that read data from the AVS47-IB. This data, when it is in floating point format, uses always the period as the decimal separator. In the "Application Example 1.vi", the heater output power and voltage readings are multiplied by constants, which depend on the power range. LabView uses the system decimal point, which you have defined to be a comma, for these constants, and period for the readings that come from the AVS47-IB. The multiplication will then yield unwanted results.
If your other LabView applications allow, you can most easily overcome this problem as follows: From the Edit menu select Preferences. Select Front Panel and uncheck the box "Use localized decimal point". Then LabView will then use period as the decimal separator for all your applications.

PICOLINK

Q.	We have in our laboratory an unused optical link for the RS232. Can we use it for isolating the AVS-47 and the computer?
A.	Unfortunately not. The hard-wired primary interface in the AVS-47 is based on a synchronous, serial protocol which requires four signal lines. The RS232, on the other hand, needs only two lines if one uses software handshaking. The AVS-47 cannot do that because it has no microprocessor. Therefore, Picobus has four cheap fibres, transmitters and receivers, but no intelligence.

Can we upgrade our existing AVS-47 and the AVS47-IB units by installing the new Picolink?

Installation of the Picolink requires that the rear panels of both units are replaced by new panels, which means some work that is critical in respect of electrical safety. The transmitters and receivers must be installed and a few wires must be soldered to the circuit boards. Finally, the whole thing must be tested for proper operation. This option is by far easiest to install in new AVS-47+AVS47-IB pairs. Upgrading requires that both the bridge and the interface be shipped to us. The charge will include, besides the Picolink, also the new rear panels, work for the upgrade, and return shipping. In order to make things easier, we have started to accept some credit cards (see Sales & Contact us). In fact we favour using them for small orders like upgrades and repairs.