--- In Repeater-Builder@yahoogroups.com, "Don Kupferschmidt" <[EMAIL PROTECTED]> wrote: > > Nate, > > I'm wondering if you can point me to a link that would further explain lo / > hi injection. Hate to admit, but I'm not all that familiar about the > subject, especially in repeater operation. > > TIA, > > Don, KD9PT > > (Snip)
Hi Don, The normal MASTR II receiver uses a low-side Local Oscillator injection for their mixer. The IF is 11.2 MHz, so you deduct 11.2 MHz from the operating frequency. The normal operating frequency for the 88 series MASTR II radios is 450 to 470, so the LO (Local Oscillator) is engineered to run between 438.8 and 458.8 MHz (450 11.2 = 438.8; and 470 11.2 = 458.8). When we in the amateur radio service order a low-side injection crystal, we are essentially asking the LO to operate between 428.8 and 438.8 MHz, worst case being 10 MHz below the engineered operating frequency range (440 11.2 = 428.8, and 450 11.2 = 438.8) This is stretching the original design of the local oscillator 10 MHz beyond the design parameters. If we order high-side injection crystals, we add 11.2 MHz to our operating frequency, rather than subtract it. So the LO will runs between 451.2 and 461.2 MHz (440 + 11.2 = 451.2, and 450 +11.2 = 462.2). If you are operating in an area of the country where you run low-in, high-out split repeaters, your receivers will operate between 440 and 445 MHz, and the highest possible LO frequency will be 456.2 MHz (445 + 11.2 = 456.2), or right in the sweet spot for the LO range (438.8 and 458.8). If you are operating in an area of the country where you run high- in, low-out split repeaters, and your receiver will operate between 445 and 450 MHz, and the highest possible LO frequency will be 450 + 11.2 = 462.2, or only 3.4 MHz higher than the designed frequency of the LO. The cross-over frequency seems to be 448.300 MHz, which is 1.7 MHz higher at a high-side injection than designed, and a low-side injection is 1.7 MHz lower than designed Local Oscillator. So, maybe a good rule of thumb would be to order high-side injection on operating frequencies below 448.300, and low-side injection on receive frequencies above 448.300. That way, the worst case scenario is your being either 1.7 MHz higher or lower than the original engineered design on the local oscillator. You can do the same math for high-band radios. Same IF. Hope that helps.
