Dear Treggers, On the topic of taxometer filters for Germany, I have some additional technical comments to make:
The problem as I see it lies mainly with an active DC holding circuit, where used, leaving its linear region during the taxometer pulse peaks. A DC holding circuit is needed whenever a "dry" isolation transformer is used. (A transformer that cannot pass the line current used for signaling an off-hook status is known as a "dry" transformer, whereas one that performs the additional function of passing line current is known as a "wet" transformer.) DC holding circuits may be active or passive. In my experience, active circuits are more commonly used, and these are designed to simulate the behavior of an inductor, by presenting a low, defined resistance to DC, while appearing as a very high impedance (tens of kilohms or more) to AC signals in the voice band. The active circuit is biased so that it will not enter cut-off or saturation for the largest AC levels expected on the telephone line in the off-hook state -- maximum DTMF signaling power, or maximum superimposed transmitted modem signal plus received modem signal on the line. However, the taxometer pulses as defined by BAPT are close to 10 volts RMS, which is about an order of magnitude greater than voice-band signals, and the active circuit cannot be biased to cope with these. Therefore, as I see it, the only options are: a. Use a wet transformer - generally a problem because it is hard to achieve high-speed data modem performance (easy for a fax modem) with a wet transformer; b. Use a passive holding circuit - I haven't explored this option but am aware that designs are available that implement this; c. Use an active current hold circuit, but add a taxometer pulse filter to attenuate the 16 kHz sine wave bursts before they reach the holding circuit, to a level that is sufficiently low that the holding circuit will not depart form its linear region, when the maximum taxometer signal is superimposed on the maximum voice-band signal level. It appears to me that to be quite certain of meeting the requirements for worst-case component values (5% inductors and capacitors in the filter) and worst-case 16 kHz limits from the telephone exchange, a 2-stage filter is necessary. Since the first stage has the inductor in series with the line (to ensure that the subscriber's equipment presents an appropriately high impedance to the network at the taxometer frequency), this part needs to have the correct inductance while passing DC levels of up to around 60 mA, which is the maximum nominal line current in the German network (60-volt battery with 1kilohm feed). The second stage inductor need not be capable of passing DC because it is connected in series with a capacitor across the line. Incidentally, if the equipment you're designing is series-connected before other telephone equipment, DO NOT place the taxometer filter in the series path! I have seen this done in a design that someone was selling for good money. However, your equipment would have a huge insertion loss at the taxometer frequency, which is not what the Deutsche Telekom people have in mind at all, and would prevent a taxometer from being located beyond your equipment! One more point: remember to take into account both the DC resistance of the filter inductor, and its AC behavior in the voice-band, when designing for return loss and trans-hybrid loss. Also, don't place the filter after the polarity protection bridge (the bridge rectifier that ensures that the active holding circuit always receives the same DC polarity whichever way the line is connected). When the pulses are huge, the line voltage may actually reverse during the peaks, and the 16 kHz sinusoid will longer be sinusoidal. The result will be an impressively distorted waveform and, I imagine, loads of problems with data reception! I would be very interested to hear experiences of others who may have passed worst-case BAPT conditions using an active circuit with fewer than two filter stages, or who have used or encountered arrangements based on the other two alternatives (wet transformer or passive holding circuit) and are happy with V.34 performance, cost and size. I believe information of this kind would be enlightening to quite a few of us. Finally, as I mentioned, it is easy to design for a fax-only modem, because fax data transmission is half-duplex and the distortion requirements much reduced. In this case, wet transformers are suitable, and I would recommend the Midcom 671-8001 for the North American market (where taxometer signals are not a problem) or 671-8067 for the European market (where they are). The difference between these transfomers are, of course, associated with the widely differing safety requirements of these two regions. Both of these Midcom transformers represent cost-effective solutions. I hope this information is useful and look forward to further comments on this topic and the associated topics of THL and return loss optimization. All these topics are linked and need to be considered together when designing a DAA circuit that meets the regulatory requirements and functions well for a large proportion of short and long lines, and lines with inductors added (for improved high-frequency response), and lines with worst-case taxometer signals. Best regards, David Drori --------------------------------------------------------------------------- ------------------ Novarex Enterprises Ltd., POB 2833, Jerusalem 91028, Israel. Tel: +972 2 581 0995 Fax: +972 2 581 3750 Mobile: +972 5 067 8686 E-mail: [email protected] --------------------------------------------------------------------------- ------------------
