Common-Mode Leakage, cal shift, and filters

[abrush1]

This post is to share the document we have regarding performance of Type IV bridge balancers with thermistor mounts that "leak" RF energy from their bias leads. There are two attachments to this post:

• The TEGAM RF LEAK Memo, which outlines the problem, impacts, and solutions for TEGAM products.
• A paper presented by NIST at the 2007 MSC meeting that gives NIST findings on the same issue.

NIST describes a different filter as the solution than TEGAM supplies. This is because extensive testing at TEGAM showed that the leakage is common-mode, and therefore we applied a single-pole common-mode choke. The NIST filter will provide some attenuation, but will not be as effective against the common-mode leakage. One such NIST filter in EACH LEG of the Bias, referred to the RF return of the thermistor mount, would probably be very effective.

After writing the memo, we noticed that the installation of the filter is not very clear. The TEGAM filter is to be installed as close to the mount bias terminals as possible, preferably by plugging directly into the mount. If a 4-wire connection to an 1806 is desired, we recommend connecting the filter to the mount and both source and sense leads to the filter. Alternately, separate filters at the 1806 could be used for source and sense. The TEGAM 1000027 filter has a series resistance of a few milli-ohms, and therefore will not impact readings at frequencies where leakage was not impacting results before.

The 1000027 filter is designed to keep RF energy out of the Type IV (and Type II) circuits and therefore will have negligible impact if installed in the voltmeter leads.

[lhiga]

Were all purchasers of affected devices notified?

[abrush1]

The answer to Larry's question is, "Yes." Although TEGAM does not view the leakage as a product defect, as the affected products are derivatives of historical Weinschel products, we take the possibility of a customer getting incorrect results from our lab or using our equipment very seriously, and therefore worked hard to get this information out using multiple channels.

All customers who had purchased the affected products from TEGAM were notified by mail or e-mail at their address of record. Additionally all contacts in our CRM database who were identified as points of technical contact or as users of these items were notified by e-mail. The US military Primary Standards Labs RF personnel we have contact information for were notified. The NIST RF power lab in Boulder was notified. Customers who send these products to the TEGAM lab for calibration are also notified. We have discussed this issue at our RF Power Calibration tutorial at MSC in the last two years. By making this knowlege as public as possible, TEGAM hopes that if any users' contact information has changed, that they will find out through normal channels of technical discussion.

Some of the affected products are "legacy" products mostly produced and delivered by Weinschel prior to the designs being purchased by TEGAM. This is the case with products such as 1807A, F1109, 1805B, and 1806. TEGAM does not have the contact information for all purchasers who acquired their equipment from Weinschel, and could not directly advise them of our findings and corrective action. We believe that publicizing our findings, for example to NIST, to the military PSL's and at MSC, has resulted in most operators of old equipment becoming aware of the issue.