SRN 1283A

APC - 672, 749

Date: October 1999

From: Radio Product Group / CGISS

Subject: HT750 and HT1250 Buzz or Noise when Receiving a TPL Code

An internal frequency used in the receiver circuitry (the base band offset frequency) interacts with the PL of the incoming signal. The radio software changes the offset frequency to eliminate this interaction for TPL decode for the TPL programmed for that receive personality. When the radio is not programmed to decode the TPL, i.e. in the monitor mode, the offset frequency may interact if the TPL is between 95 and 135 Hz. Other known cases of Base band Offset noise occur during multiple TPL/DPL conditions, such as community repeater operation, and when using the radios for TPL decode along with QCII or MDC signaling.

The base band offset frequency must be changed to a frequency more than 20 Hz away from the incoming TPL. This offset frequency can be changed by use of the Tuner contained in Customer Programming software version R02.00.02. If the radio having the offset changed contains firmware version R02.00.01 or later, the base band offset tuner in this CPS must be used. The previous tuner version R01.01.03 referenced in SRN1283 is not compatible with the new firmware. This new tuner is compatible with all firmware versions.

Instructions for using the tuner appear in this bulletin.

Base band offset frequency adjustments may be required to prevent PL hum in some customer radios. The most common reasons for this is the customer is listening to radio transmitting TPL and the customer radio is programmed for CSQ operation, or the customer is receiving TPL with either MDC or QCII signaling. Both of these situations will require BBO adjustment.

There are six different BBO frequencies offsets: CSQ, DPL and four TPL. You will rarely have to adjust the TPL offsets. In most cases you will be adjusting the CSQ and/or DPL offset frequencies. The most common adjustment you will be making is for when a carrier with a TPL is being received on a mode set for CSQ. Please refer to chart 1 and you will see 115 Hz. is the default BBO frequency for CSQ receive. If the TPL being received is within 20 Hz of 115 Hz, then the buzz will occur. 110.9 Hz TPL is one example. In a case like this you would most likely find that a BBO frequency of 86 Hz will improve the receive audio. However, if the customer's radio is using a TPL close to 85 Hz (such as 85.4 Hz) on another mode, the receiver will now buzz on that mode. Make a list of the different TPL frequencies used in the radio on modes set for CSQ receive. Try to select a BBO offset frequency from chart 1 that is at least 20 Hz. away from all the TPL's that will be received in the radio.

The other case is if the radio is receiving TPL on a mode programmed for MDC or QCII signaling. In this case the BBO frequency being used is NOT the TPL BBO. The BBO offset used for MDC and QCII signaling is the DPL offset. Refer to chart one and you will see the DPL default is 646 Hz. If buzzing is heard on a signaling mode you must change the DPL offset to correct the problem. Choose a BBO frequency from chart 1 and change the DPL offset from 646 Hz to 57, 86, or 115 Hz, which are the defaults for TPL.

The last thing you may encounter is a signal with DPL even though the mode is programmed for DPL receive. Choose a DPL BBO frequency other than 646 Hz., such as 86 Hz, and try that offset.

The Global Tuner has 3 selections for Bandwidth: 12.5, 20, and 25/30 kHz. It is recommended that any adjustment to BBO be made for all 3 bandwidths.