W7SOE
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« on: February 18, 2015, 01:03:43 PM » |
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I am working on a frequency counter project fro my TR-4CW. I need to sample the PTO oscillator as well as the crystal oscillator. These two signals will be brought out via small coax. I found an instruction manual for a frequency counter (attached) that directed the user to grab the crystal oscillator at pin 7 of V1. I tried this but as soon as I added the length of coax it killed the oscillation. I am guessing this is due to the added capacitance of the cable. I tried adding a series cap, I tried 10pF and 82pF. This allowed the oscillator to keep running but the signal at the end of the cable was not usable.
The signal at pin 7 is connected directly to the crystal and it seems likely that it is easily disrupted.
I wonder how this worked for that frequency counter design?
I am not proficient in this type of circuit analysis, I appeal to you gurus.
Thanks
Rich
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Pete, WA2CWA
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« Reply #1 on: February 18, 2015, 01:40:36 PM » |
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You didn't mention it but are you using the original cables that came with the counter?
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
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W7SOE
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« Reply #2 on: February 18, 2015, 01:47:12 PM » |
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You didn't mention it but are you using the original cables that came with the counter?
No, I am just using it's manual for reference. Rich |
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Pete, WA2CWA
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« Reply #3 on: February 18, 2015, 01:52:32 PM » |
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So what type of coax and length are you using because both might be critical?
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
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W7SOE
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« Reply #4 on: February 18, 2015, 01:56:18 PM » |
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So what type of coax and length are you using because both might be critical?
I am using some small diameter coax of unknown type. ;-) I just learned that later revisions of that products manual had the user put a 100 ohm resistor in series so i will try that next. Rich |
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Pete, WA2CWA
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« Reply #5 on: February 18, 2015, 02:10:26 PM » |
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When I added a counter to one of my receivers years ago, I used RG-62A coax and the length was no more then about 24 inches long.
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
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N2DTS
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« Reply #6 on: February 18, 2015, 03:35:17 PM » |
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I wonder if "almost all digital electronics" makes one for the drakes.
They have a preamp built in and need no direct connection.
I just get a wire close on my homebrew receivers and it picks up plenty of signal, no connection at all.
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WD8BIL
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« Reply #7 on: February 18, 2015, 04:49:16 PM » |
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Rich........If you are gonna mix these 2 signals, why not just sample the output of the converter (V1) or the Cathode follower feeding the mixers (V3a). The mix products are already there and you won't load down the oscillators.
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W7SOE
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« Reply #8 on: February 18, 2015, 05:31:50 PM » |
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Rich........If you are gonna mix these 2 signals, why not just sample the output of the converter (V1) or the Cathode follower feeding the mixers (V3a). The mix products are already there and you won't load down the oscillators.
I was planning on using the same scheme as that commercial counter, measuring the PTO and xtal frequency and then doing the math according to what band you are on. 80m - 9.0MHz - PTO 20m - 9.0MHz + PTO 10, 15, and 40m (xtal - PTO) - 9.0MHz I could try to sample where you suggest, it looks like it would be at C196, or the junction of C29 and R25. Is this what you were referring to? Rich |
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WD8BIL
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« Reply #9 on: February 19, 2015, 08:48:39 AM » |
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Yes, that's where I am reefering to. You might need an MPF102 (or similar) for a buffer but the level there should be usable.
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W7SOE
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« Reply #10 on: February 19, 2015, 12:15:58 PM » |
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Yes, that's where I am refering to. You might need an MPF102 (or similar) for a buffer but the level there should be usable.
Yup, the signal looks good there. It also means I don't need to use the PTO signal. I think, by looking at the measured frequency and maybe bringing one wire out from the bandswitch, I can make the counter automatically swap bands. No switch required. Thanks Rich |
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WD8BIL
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« Reply #11 on: February 19, 2015, 12:47:42 PM » |
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Good deal, Rich.
It also means you won't be bothering the PTO which will help maintain stability!
Good luck.
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W7SOE
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« Reply #12 on: February 20, 2015, 12:26:39 PM » |
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I still need to do some debugging, maybe add some gain to the signal and put it in a box, but the frequency counter is alive. LCD displays are functional but are a shame to match to a tube radio so I went with Nixies! Hopefully they compensate somewhat for the digitalness....... Rich https://www.youtube.com/watch?feature=player_detailpage&v=ethB_GODHWc |
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KB2WIG
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« Reply #13 on: February 20, 2015, 11:27:04 PM » |
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nixies?? we don't need no stinking nixies!
Do it old school.
klc
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What? Me worry?
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KA2DZT
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« Reply #14 on: February 20, 2015, 11:59:37 PM » |
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nixies?? we don't need no stinking nixies!
Do it old school.
klc
I took a few of those old HP counters apart. Still have those meters that were the last digit. I used that model back in the late 60s at Westinghouse Electric. |
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