In a previous posting, I alleged that cheap portable radios are prone to having CTCSS tones that have significant distortion. Now I can offer some proof.
My test setup comprises a Motorola R2600D service monitor feeding a Kenwood HM-250 distortion analyzer from its DEMOD output. The service monitor was set for a 5 to 300 Hz bandwidth. To get a baseline reading, I fed a pure 131.8 Hz tone from a Wavetek 188 audio generator to the external modulation input of an HP 8656B signal generator. The 8656B was set for a 146.000 MHz carrier modulated at 750 Hz deviation of the 131.8 Hz tone, and cabled to the RF input of the service monitor. With this baseline signal, my HM-250 indicated 0.32% distortion. I then tested four portable radios, with the following distortion measurements: Motorola HT1250: 1.2% Motorola GP68: 1.3% Alinco DJ-S11: 4.0% Puxing PX-777: 7.8% As one might expect, the cheaper the radio, the poorer the signal quality. It's not surprising that the Puxing radio produces a raspy buzz when transmitting through a repeater. 73, Eric Lemmon WB6FLY
