I suspect the accuracy of the deviation measurement depends on how the scope is calibrated. I too have for many years used a scanner as a pseudo service monitor. I have used both a calibrated FM signal generator and the local public service channels for reference. The DC coupling of the scope was adjusted to give a center line reference for frequency from the scanner's discriminator and the vertical gain controls were adjusted for a convenient deviation scale. Haven't measured it, but suspect the IF bandwidth on my scanner is fairly broad.
Using a calibrated service monitor to compare this scheme indicated no difference in readings. Assuming we can get over that hurdle, the next one would be how to make an RF signal generator from the scanner. Could we use the scanner's image through a broad band amplifier block into a rudimentary attenuator? 73 de Jack - N7OO ----- Original Message ----- From: [EMAIL PROTECTED] To: [email protected] Sent: Saturday, April 21, 2007 8:24 PM Subject: Re: [Repeater-Builder] Service Monitors At 4/21/2007 14:55, you wrote: >Cheep Service Monitor - Take a receiver and an oscilloscope and calibrate >the scope to measure transmitter deviation. This doesn't quite work. The reason is that the IF filters in NBFM receivers lop off a bit of the incoming signal, resulting in an inaccurate deviation measurement. To get around this, you can replace the 455 kHz final IF filter with a coupling capacitor. I did this with an old, expendable Regency M100 scanner. The IF passband widens to ~30 kHz or so, which is enough to pass all of the NBFM signal to be measured. Now you know why the IF filtering in your service monitor seems a bit wider than your radio. Bob NO6B
