Re: TG501 2ns & 1ns operation
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The schematic depiction of switches in Tek schematics isn't always clear to me.
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Yes, I was clear on the 5, 2, and 1 ns circuits - really the 5ns and 1/2ns circuits, and that K450 switches between them. Thus my confusion regarding K450 DE-activating in 1ns mode. I further missed that in NOT 5ns mode the base of Q460 is pulled up by the disconnection of the 5ns pushbutton (S450A) and R451. And checking the 2ns and 5ns output I now understand why the 5ns mode jumps up in DC offset compared to the 2ns output. Incremental understanding. So the 2ns Filter output is disconnected from MARKER OUT, Q460 is off, anode of CR468 is pulled down by R465, and MARKER OUT is floating. That explains is susceptibility to noise. Recall I'm the guy who has Sutro Tower looming over my house blasting me with 100MHz broadcast frequencies. Still, the proximity of the 500MHz/2ns signal out of the 2ns Filter is plenty to appear on the MARKER OUT. I had (in the pictures) the 2ns level C515 set for a large output. Adjusting it to a minimum 2ns level results in a significantly smaller "noise" level on MARKER OUT in 1ns mode. No surprise. There also seems to be a balance between maximum 2ns level and maximum 1ns level. As this is all a tuned resonant type circuit, I'm not surprised. I'm working on rebuilding my small signal analysis chops. And again, I'll say I'm closer to full understanding. But it's always a closer approximation - never quite fully there. As an aside, since you brought it up: I've been looking for a suitable non-capacitive trimming screwdriver. I just tried making one out of hard plastic, but these aged devices tend to be kind of sticky. It can't stand up to the torque needed to turn some of the caps. So I have to resort to a metal trimmer to get the caps turned. I can do incremental adjustments and get the effects I need. I've been wary of buying plastic non-capacitive trimmers online. I suspect they'll be no better than I've been able to make myself. You mentioned ceramic. Do you have a particular brand or source for such? An Amazon link perhaps? Thanks.Dave On Thursday, December 2, 2021, 04:18:29 PM PST, Raymond Domp Frank <hewpatek@...> wrote:
On Fri, Dec? 3, 2021 at 12:41 AM, Dave Peterson wrote: I'm not sure if you realize that 2ns and 1ns aren't generated by the 5ns+ circuits; they are separated by K450-S1, which selects either all 5ns+ signals or both the 1ns and 2ns signals. The fact that you can see 1,2 ns and 5ns+ signals indicates that K450-S1 does its selection job. For 1ns and 2ns, everything "above" K450-S1, i.e. from P468, doesn't count. The 2ns signal is the bandwidth-filtered result of a dirty signal going into the 2ns filter (CL510, C/L511, C/L512, CL513). CR515 "sharpens" that signal, creating harmonics, which are then bandwidth-filtered out by C/L517, C/L518, C/L519 and C/L5120. the result is a "soft" signal with a frequency of 1GHz, presented at the "1ns only" BNC socket. The irregular 2ns signal in your picture looks like the result of mixing, caused by leakage: Bad screening by the aluminium cage covering the 1ns and 2ns etc. circuits may cause this. It should be a pretty tight and complete fit. Just the screws holding it in place and making contact isn't enough. If you can make a setup where you can put pressure on the cage with power on, I wouldn't be surprised if the signal shape of 1ns and 2ns signals visibly varies, depending on pressure exerted on the cage cover. If that is so, you may want to remove the cage lid and bend its sides together, even to a slightly smaller than 90 degree angle. Use some tool to fit the lid on and put in the screws. Next, perform the adjustment for the 2ns and 1ns signals (C/L510 ...C/L520), unless everything looks perfect already. Preferably use a ceramic screwdriver, definitely not one with a metal tip. Simple peaking of the signal and watching for a regular signal on your scope will do the job; first 2ns, then 1ns, and repeat until no more changes. This usually is enough to achieve excellent results. The BW of your 'scope is only important in so far as it is able to show and trigger on the signal. Correct amplitude is not important. Raymond |
Dave Peterson