INTRODUCTION In simulcast radio systems, VF channels need to be matched to provide clear transmission in RF overlap areas. In these areas, received signal is the sum of signals coming from multiple transmitters. Therefore it is crucial that all voice frequencies be maintained near the same phase and amplitude so that the composite signal is a true replica of the parts. This is required for both human intelligibility and modem accuracy.

The objective is 0.2 dB of amplitude match, and 30 degree phase match (or better) between 300 and 3,000 Hz. 30 degrees equates to a delay difference of about 28 microseconds at 3,000 Hz.

Delay and amplitude response are controlled by the Convex 2042 Simulcast Equalizer. The 2042 has frequency selective control needed to shape the relative delay and amplitude response. "Bulk Delay" and "Flat Gain functions adjust the absolute delay and level of the channel. Measurements required to align the equalizers are made with a Convex 806A or 806AR Test Set.

VF TESTING Tests are run using a test receiver with one transmitter at a time. The 806A (generator) test signal is applied to the active transmitter, picked-up by the test receiver, and returned to the 806A. Measurements are stored as PC files.

CURVE MATCHING involves determining equalizer settings that will adjust each channel to a common response curve. Convex C40CSS Control Set Software is used for this process. It accepts measurement files produced by the test set and outputs equalizer settings files.

TIME MATCHING - Last, the 806A is used to measure the round trip time delay of each channel. This data is used to set the Bulk Delay on the 2042 Simulcast Equalizers so that all channels are delayed as much as the slowest channel.

PROCEDURE There are four main steps to matching process. Before starting, ctcss must be turned off.

1 Measure the response of each channel - with the equalizers patched out of the circuit.

2 Compute equalizer settings for a flat channel response. Download setting files to equalizers.

3 Verify the response of each channel by re-measuring with the equalizers in the circuit.

4 Measure, adjust, and verify round trip delay of all channel with the equalizers in the circuit.

RESPONSE MEASUREMENT Control the Convex 806A Test Set with a PC. Run the VF Auto-Test in Delay Mode. Save the measured data as PC files - one file per channel per site.

SETTING CALCULATIONS Run C40CSS Control Set Software. Input the measured response files and solve for equalizer settings which provide flat delay and amplitude response. Download the setting files to the equalizers. Set the Bulk Delay on all equalizers to 200 microseconds to provide some downward range.

ROUND TRIP DELAY Put the 806A Test Set in Delay Mode, set the Filter to 15 kHz. Set the send frequency to 1804 Hz.

ZERO - Open the normal I/O connections on the test set. Connect the test set send port directly to the receive. Zero the delay reading when the display settles. Remove the direct connection and reconnect the normal I/O lines.

Measure and record the round trip delay of each channel for each site. Reestablish the above "zero" connection to verify that the zero reference remained.

Adjust the "Bulk Delay" of each equalizer so that all channels from all sites have the same round trip delay to the center of their RF overlap area.

Related Products:
Audio: Equalize voice land lines or analog mux-voice channels

Last update: 5/4/06

Simulcast Solutions®LLC 18 Port Meadow Trail, Fairport, NY, USA, 14450 TEL: (585) 223-4927, FAX: (585) 223-3255 info@simulcastsolutions.com