US2266496A - Carrier frequency system - Google Patents

Description

Dec. 16, 1941. w HAGEN CARRIER FREQUENCY SYSTEM Filed June 23, 1939 2 Sheets-Sheet 1 u UMU lllllllll lnpuf Chan/Zei- U D] 1 U Carr/er fieyuenc/es Channels- Patented Dec. 16, 1941 CARRIER FREQUENCY SYSTEM Wolfgang Hagen, Berlin, Germany, assignor to C. Lorenz Aktiengesellschaft, Berlin-Tempelhof,

Germany, a company Application June 23, 1939,Serial N0.280,7151

In G

1 Claim.

The invention relates to carrier frequency systems of the kind comprising a number of frequency channels and it consists in certain features of novelty which will appear from the following description and be particularly pointed out in the appended claims, reference being had to the accompanying drawings, in which Figs. 1, 2, 3 are graphic representationsthat serve to explain the theory underlying prior arrangements While Fig. 4 is a diagram which serves to describe one embodiment of the invention by way of example. 7

When assembling carrier frequency channels or separating them from each other difiiculties arise which are the greater the closer the adjacency of the channels in relation to frequency. The filters of such arrangements have to meet the following requirements: In the transmission range they must have the least possible attenuation whereas in the ranges of the neighbouring channels the impedance of the filters as viewed in backward direction must have either high ohmic impedance or very low ohmic impedance according to whether the filters are connected in parallel or in series.

In multi-channel systems, such as a system comprising ten channels, for instance, a twofold modulation is employed in order to reduce the demand on the filters, as will be'understood from Figs. 1 and 2. The suppressing attenuation intended to afiect the neighbouring channel is produced with the aid of means located in the output filter A of the premodulating stage A while the output filter B of the second modulating stage B has to suppress by attenuation the second sideband only. The output filters A are then assembled in a manner so that all the chaner many July 1, 1938 (o nap-44) nels are amplified by an amplifier V common to them. However, the fact that less attenuation is here demanded of the output filters B is inconsistent with the strict requirements regarding the impedance of the neighbouring channels. The filters here employed have the property that the impedance, before assuming high values in the suppression range, passes through a resonance point of the impedance zero, as will ap pear from Fig. 3. With filters of such type this point would be contained in the neighbouring channel. The attenuation characteristic in this channel will therefore be irregular to such extent that it will be diflicult to correct or equalize it.

Th invention aims to overcome all these disadvantages. To such end the even numbered ones on the other hand are assembled to form frequency mixtures and these are conveyed to a fourwire connection where they join each other. In th case of ten channels, for instance, the channels numbered I, 3, 5, 1, etc. and those numbered 2, 4, 6, etc. are assembled to join each other in a four-wire connection, as shown in Fig. 4 by way of example.

To terminals 6, ,f, Fig. 4, of a bridge transformer Ul forming part of a four-wire connection, channels Kl, K3, K5, K1, K9 are connected while to terminals g, h of a transformer U2, channels K2, K4, K6, K8, KID are joined. Connected to terminals 0, d is the input circuit of a sending amplifier V of the system. A simulation network that corresponds to the input resistance of the amplifier V is attached to terminals a, b. Since owing to the four-wire connection the frequency of the odd numbered channels does not reach the transformer U2 the filters need not be of high ohmic impedance for any adjacent channel. They must only be of high ohmic impedance for any channel separated from them by another channel, that is to say, the characteristic of channel K3, for instance, may be of so fiat a course as not to present the requisite high ohmic impedance characteristic until the commencement of channel K5 or KI, as the case may be. Within the intermediary range, which includes the channels K2, K4, the characteristic of the filter impedance is of no effect upon the operation of the system. It therefore entails no risk to have the impedance pass here through the zero value. Similar conditions apply for the filters of the even numbered channels. The demands on the filters are considerably reduced in this way. By use of the proposed arrangement the requirements with respect to the impedance characteristic need not be more severe than with respect to the attenuation characteristic. In the foregoing, the filter output circuits of the channels are joined in parallel connection to the transformers, that is to say, the impedances of the filters must be of high ohmic impedance in the suppression range. The aforesaid particulars, however, also apply for series connection of the channel filters, provided that these are so calculated that within the range of attenuation their impedances approach the zero value.

The described arrangement entails an additional attenuation of about 3 decibels. This, however, does not represent a loss greater than if the channels are comprised without making use ofthe novel arrangement, because as stated channels on the one hand and the odd numbered 55 the filters in consequence of their simple construction do not attain either a high impedance or a low one, in the suppression range for the neighbouring channels, causing the neighbouring channels to undergo losses due to attenuation. Such attenuation losses also have the disadvantage of depending on frequency, whereby the channels require special correcting or equalizing measures to be provided. In the novel arrangement, however, such correction may in general be dispensed with. If nevertheless an equalization is provided then this will have to correct only the so-called corner attenuation of the appertaining channel filter that is the attenuation in the neighborhood of the cut-offfrequencies, thus being independent of the filters belong.- ing to the neighbouring channels.

The frequencies of the even numbered chan-- nels are prevented by the respective bridge tramsformer from passing into the output circuits of the odd numbered channels. As a result; the suppression of an intermodulation of the channels is considerably simplified. This is of particular value since behind thesecond modulators numbered channels with a common carrier line over a balanced bridge to prevent interaction between said even numbered and said odd numbered channels, channel band pass filters in the individual channels of each carrier frequency, and means operatively associated with each filter for compensating attenuation only in the neighborhood of the cut-ofi frequency for each channel filter.

WOLFGANG HAGEN.

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