Audio test attachments for load isolation and matching

To connect a 600_audio circuit to a 50_or 75_circuit or test instrument, an impedance matching circuit is required. Or a transformer is needed to isolate the circuit. Both methods have advantages and disadvantages. Common transformers can achieve impedance matching with typical losses as low as 1.5 dB, provide DC isolation, and can be used in balanced or unbalanced 600_primary circuits. The passband of a high-quality transformer can be adapted to the range of 300 MHz to 15 kHz audio with minimal amplitude variation. However, transformers that match 600 to 50 or 75 are not readily available and may be expensive.

A lossy, fixed value impedance matching circuit (or attenuator pad) can provide a constant frequency audio impedance conversion with only two resistors. Although a pad can provide useful impedance matching functions, its insertion loss is considerable, being 14.8 dB for 600_~75_conversion and 16.6 dB for 600_~50_conversion, both of which cause unacceptable loss in the dynamic range.

As part of a set of test accessories, this low-cost, switchable dual impedance conversion circuit consists of a common transformer and two pads with minimal loss (Reference 1). A low-cost common transformer can reduce the primary input impedance of 600_to a medium impedance level of 100_ (Fig.1). Switch S1 selects the minimum loss pad of 100~50 or 100~75. The device is constructed using less stringent point-to-point connections, although this design uses a Hammond 1590LB stamped aluminum shell for shielding and sealing to support the installation of three Amphenol (www.amphenolrf.com) RFX series BNC panels, insulated frame input and output sockets. T1 is a 42TM031 audio transformer for Mouser Electronics (www.mouser.com), with a resistor of 0.5W and a tolerance of +1%. At volume discounts, the overall material cost is less than $20.

To validate the frequency response and attenuation in a 600_test device, two independent components are connected back to back through 50_or 75_terminals. By halving the transmission loss measurements from 600_to 600_, the measurement data for a single unit can be obtained (Table 1).

The insertion loss of 100_~50_loss pad is calculated to be 7.7 dB, while that of 100_~75_loss pad is 4.8 dB. By subtracting these values from the measured loss, the central bandwidth loss of the transformer is 1.3 dB ~ 1.5 dB. Insertion loss due to parasitic coupling from selected output port to unused output port is greater than 40 dB. By combining a common transformer with two loss pads, you can take advantage of both technologies.

Low cost transformers have moderate insertion loss and provide DC isolation and good frequency response. In addition, low-frequency roll-off of transformers can help reduce 60 Hz AC sound and low-frequency noise. An electrically insulated input jack connects the transformer input to a balanced or grounded 600 signal source.

Reference:

[1].BNCdatasheethttp://www.dzsc.com/datasheet/BNC_2319479.html.