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Author Topic: Sluggish VHF/UHF amplifiers  (Read 777 times)
G4AON
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Posts: 1376




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« on: May 11, 2019, 06:10:50 AM »

It looks like much of the world above 50MHz has very slow switching amplifiers. I recently bought a Linear Amp Gemini VHF amp and it uses low loss, but slow, Tohtsu RF relays, c/o circa 20mS. The control board is by W6PQL.

RX to TX timing is in the region of 40~50mS, a figure mentioned elsewhere for mast head pre-amp and linear sequencing. Given that most modern transceivers only cater for 8~25mS keying delay, and nowhere approaching these long time delays, do we all put up with missing first “dits” on CW, or suffer hot switching of the input relay?

I will experiment with the adjustable delay control on the W6PQL board and see what I can do to get things running faster, having a 4 Channel 150MHz oscilloscope in the shack helps.

Are other VHF/UHF amps this slow?

73 Dave
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AD5X
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Posts: 1619




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« Reply #1 on: May 11, 2019, 06:22:51 AM »

If you can set your transceiver delay to greater than 20ms, you won't hot switch. The 40-50ms drop out time also ensures you won't hot-switch since the RF will terminate much sooner than this.

Phil - ADA5X
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G4AON
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« Reply #2 on: May 11, 2019, 06:30:19 AM »

Hi Phil, it’s the RX to TX time that’s the issue. It chops the first character, often a “dit” is completely missing. I’m used to Elecraft K3/KPA500 switch times, this amp is Stone Age by comparison. I accept that in order to minimise loss, relays tend to be better than diodes at these frequencies, but there is a practical limitation as to how much you can delay the start of a transmission on CW.

I’ll have a close look at the timing this next week.

73 Dave
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W1VT
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« Reply #3 on: May 11, 2019, 06:41:24 AM »

As someone who has done a ton of microwave operating operating I sort of expect to run into slow T/R changeover from time to time.

I recall guest operating a VHF station in which the 2M changeover was almost like a small firecracker, it was so loud!
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AD5X
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« Reply #4 on: May 11, 2019, 08:17:31 AM »

Hi Phil, it’s the RX to TX time that’s the issue. It chops the first character, often a “dit” is completely missing. ...73 Dave

Yes, you're right. I guess in the interim you could use an external amp-control input, like a foot switch, to get the amp switched before you start transmitting.

Phil - AD5X
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G4AON
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« Reply #5 on: May 13, 2019, 05:20:43 AM »

I checked the amp timing this morning, it was 35mS... with a K3 as the source on 6m, the amp relays chop the first character with the K3 set to its max delay of 20mS (no surprise there). By turning the delay pot (VR3) on the amp control board it was easy to adjust the amp relay switch time to 21mS.

I didn’t want to risk hot switching the output side of the amp by setting it to 20mS (the max switch time in the o/p relay spec sheet). As I mostly use a TS480 to drive a transverter, and the 480 can be set to 25mS, there should be sufficient timing courtesy of the 480.

73 Dave
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