As indicated last time, there are two key parties with a vested interest in the outcome of the upcoming Medical RF immunity revision:
- The test house manager who has to justify an upgrade to his test system to the people holding the purse strings.
- His customer, the company wishing to sell medical equipment compliant with the revised standard.
I shall add a third party to this discussion:
- The EMC community conversant with current test practices. This third party (you and I) will question the rationale behind the current upgrade route versus the actual threats faced by medical equipment.
But first, the test-house manager’s perspective:
The test-house manager needs to justify the expense involved in the upgrade before being granted the necessary funds. The argument justifying the outlay will be laid out in the form of a business plan. As a minimum, the business plan will comprise of:
- Cost of the upgrade
- Current and future demand for the new test capability
- Predicted break-even point
- Disruption to current test programs
- Risk analysis
Ballpark Price of the Upgrade
The greatest expenditure by far (~90%) will be the cost of higher power RF amplifiers. So to get a rough idea of the size of the problem faced, let us see if we can estimate the cost of replacing the existing 26-1000MHz 200W amplifier. Note, we can add the cost of extending to 2.5GHz and 6GHz capability later, and it will also be fun to look at the more complex factors involved with a real life upgrade (e.g., antennas cannot handle the increased power, etc).
Let’s assume that a typical test set-up provides 10v/m@3m from 26MHz – 2.5GHz and uses a 200W amplifier for the 26-1000MHz portion of the band and a 50W amplifier for the 1000-2500MHz portion. Let us also assume that a new product standard based on the newly revised generic standard calls for 20v/m to 380MHz and 30v/m at select frequencies over 380MHz to 2.5GHz. What amplifier power level will be required for the 26-1000MHz part of the test band, and at what price?
It is a cruel fact in physics that doubling the field strength requires a fourfold increase in power, and worse still, tripling the field strength to 30v/m requires a ninefold increase in the power. So if the current power amplifier covering 26-1000MHz is rated at 200W, the new power amplifier will need to be of the order of 800W for 20v/m and 1800W for 30v/m.
A web search shows that a single amplifier covering 26-1000MHz at 800W is not available. This is bad news in that two amplifiers will be required to replace the single 200W amplifier. Compounding this is the fact that the amplifier covering the higher end of the band must be capable of creating 30v/m. However, this will be offset by the higher gain available from the antenna at frequencies above 380MHz.
A ballpark price for a 1000W amplifier covering 26-250MHz is $100,000.
Let us assume that compared to the gain at 26MHz, the antenna gain is twice as good at 380MHz – 1000MHz. This still means we need 4.5 times the existing power over 380MHz to 1000MHz, so basically we need 1000W.
A 1000W amplifier covering 250-1000MHz is $180,000.
At over a quarter of a million dollars just to replace the existing 200W amplifier covering 26-1000MHz, the test manager will need to come up with a very convincing case before being awarded the funds for the upgrade.
To me (and I daresay to you), this is commercial madness, so now is a good time to kick in with the point of view of the EMC community. The current test distance is 3 meters. A strong case can be made that the real threats to medical equipment will likely be from electronic equipment placed closer than this (for instance a visitor to the hospital room placing a smart phone on the equipment, etc). If the test distance is reduced to 1 meter, the 800W/1000W requirement above would be reduced to around 300W. Any detractors out there?
-Tom Mullineaux
RF Engineer
Presenter
Freelance Author
