Gerry, I use my IC 756 as a cheapie spectrum analyzer on a regular basis. It has proved to be reasonably reliable and accurate, using a piece of wire on a coax center pin as a "sniffer." It also works to accurately align local oscillators, etc. I have never seen it introduce or display oddball, spurious sidebands as described.
I do run into AC hum with some of my other rigs used to monitor locally, but the 756 actually avoids that problem, somehow - I use it to check signal quality with the test transmitter on a dummy load - sounds like I do on the air, but no overloading. There are funny things that can happen, of course. One is picking up signals other than the unit under test, but I have seen that on high end HP analyzers, also; an IC 756 is sensitive enough to pick up all kinds of local signals with practically no antenna at all. I wonder if the spurs go away when the carrier is off. All of that said, since the reported "spurs" were so far off the carrier frequency, something does sound fishy. Listening on a known good receiver in the shack for the spurs or asking a local to listen for them is not a bad idea. One thing to keep in mind is that the T4XB carrier is relatively weak (only a "few watts") compared to a classic AM signal with respect to the peak modulation level. The noted spurs should not be half its value, however, unless the carrier itself is WAY down from its nominal level (possible with an SSB rig, of course). 73, Rocco N6KN ----- Original Message ----- From: "Gerry" <[EMAIL PROTECTED]> To: "'John_K7FD'" <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]> Sent: Tuesday, November 04, 2003 11:23 PM Subject: RE: [drakelist] T-4XB AM question > > The 756 band scope was intended to be used as a visual receive indicator I > don't think it was meant to be used as a piece of test instrument such as a > spectrum analyzer. Maybe it can be used as such but I wouldn't rely too > heavily on the results. Maybe the T-4XB overloaded the front end of the > receiver. Yes, I know the 756 front end is bullet proof but there are limits > to everything. Was the T-4XB defective (usually a bad tube or contact > oxidation), was it tuned properly into a dummy load, etc...? Certainly what > you observed is not good but I question the method and the tools used. The > only sure way to find out would be to put it on a test bench with a spectrum > analyzer. Lacking that I would enlist the help of a fellow ham some distance > away to listen to my signal and give me a report as he tuned roughly +/- > 20kHz. Maybe he could give you an audio recording of your test transmission. > Forgive my skepticism but strange things can happen when you try and use a > sensitive local receiver to monitor your own signal. For example, my > otherwise fine SPR-4 picks up a loud AC hum while trying to monitor my own > signal without an antenna plugged into it. If I insert an RCA plug with a > short to ground, the hum goes away. That's on HF. For 6 meters I use a > converter ahead of it and I don't have a hum. > > 73, Gerry W1ID > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of John_K7FD > Sent: Monday, November 03, 2003 11:57 PM > To: [EMAIL PROTECTED] > Subject: [drakelist] T-4XB AM question > > > Hello, > > When I keyed my T-4XB (no modulation) on AM at 3.871 MHz, I took a 'look' at > the signal on my IC-756ProII band scope. What I saw has me a bit concerned: > I see my signal has two smaller carriers on each side -- one at 3.851 MHz > and one at 3.889 MHz. It appears the relative amplitude of the side signals > are about half the strength of the carrier on the intended transmit > frequency... > > ...has anyone come across this before? > > Thanks, > > John K7FD > > ---------------------------------------------------------------------- > Submissions: [EMAIL PROTECTED] > Subscribe: [EMAIL PROTECTED] - subscribe drakelist in body > Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in > body > Hopelessly Lost: [EMAIL PROTECTED] - help in body of message > Brought to you courtesy of TLCHost.net http://www.tlchost.net/ > ---------------------------------------------------------------------- > > > > ---------------------------------------------------------------------- > Submissions: [EMAIL PROTECTED] > Subscribe: [EMAIL PROTECTED] - subscribe drakelist in body > Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body > Hopelessly Lost: [EMAIL PROTECTED] - help in body of message > Brought to you courtesy of TLCHost.net http://www.tlchost.net/ > ---------------------------------------------------------------------- > ---------------------------------------------------------------------- Submissions: [EMAIL PROTECTED] Subscribe: [EMAIL PROTECTED] - subscribe drakelist in body Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Brought to you courtesy of TLCHost.net http://www.tlchost.net/ ----------------------------------------------------------------------
