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Old 23rd Jan 2019, 12:23 pm   #1
Wendymott
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Default Homebrew SSB Transceiver

Hi Peeps...... Now the Exciter and receiver sections are complete... I am turning my attention to the PA. I bought a 150W Linear kit from Ebay... and its still to be constructed...however one or two questions have now cropped up.
1. Meter. At present my meter is monitoring the RF level to the input of the second mixer... i.e post 9 mhz filter..thus I get an idea of signal strength on receive.. and on TX the modulation or drive "if you like" . Should I monitor the PA Current ? or what other points do you suggest.

2. Power switching. I have bought a SMPS with an output of 12V @ 20A. On transmit would you switch the DC Output or the AC input ? Obviously the DC end would require BIG contacts... however the relay is connected it will have a delay of 1 second to allow the aerial and other relays to have operated.

The exciter has a dedicated psu...

Your thoughts would be appreciated
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Old 23rd Jan 2019, 12:36 pm   #2
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Default Re: Homebrew SSB Transceiver

With an SSB rig, if you're not talking the PA is only drawing bias current so 99.9% of the time you wont be switching large currents - DC side switching is fine.

10amp relay with two sets of contacts paralleled.

Metering - nobody meters anything except SWR and sometimes power out the spout these days.
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Old 23rd Jan 2019, 3:33 pm   #3
merlinmaxwell
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Default Re: Homebrew SSB Transceiver

Quote:
Obviously the DC end would require BIG contacts
Or a medium sized MOSFET.
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Old 23rd Jan 2019, 10:51 pm   #4
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Default Re: Homebrew SSB Transceiver

If you're going to use a MOSFET watch out for a little gotcha. If there's a big cap on the output of the power supply and one on the input of the amp, the current dump from the former to the latter via the MOSFET can exceed Ids of a lot of "ballpark" MOSFETs and let the magic smoke out.

I would just leave the PA with standing current running and just switch the RF in on TX. I'm doing this with my slightly lower wick 25W CW amplifier. Idle current is about 100mA though the PA IRF510.
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Old 23rd Jan 2019, 11:17 pm   #5
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Default Re: Homebrew SSB Transceiver

Modern rigs just switch the bias and lower level Tx paths and leave full +V on the PA these days.

Actually, I think a lot of older rigs did as well.
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Old 23rd Jan 2019, 11:40 pm   #6
Wendymott
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Default Re: Homebrew SSB Transceiver

Hmmmm... Great food for thought peeps... Many thank Mr Bungle.. Terry, and Merlin..The SMP is a commercial LED PSU..Thats it then..... No DC switch/relay....Leave it idling.. and just switch the drive... I will still need an antenna relay.. but thats no problem..re metering..Just "sniff" the output.
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Old 24th Jan 2019, 10:49 am   #7
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Default Re: Homebrew SSB Transceiver

The 150w spec on the linear may also be a bit pie in the sky, depending on what the output transistor/FET is.

Especially as the frequency goes up.
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Old 25th Jan 2019, 2:23 pm   #8
Wendymott
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Default Re: Homebrew SSB Transceiver

I agree Terry... I am not intending using full power.. whatever that is anyway.. as my antenna is a half 5rV dipole and on 80m is a bit lacking.. I have a KW antenna match.. but I realise its not optimum.. so I will limit the power anyway The transceiver is 80 / 40 / 20 .. Onward and upward.
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Old 25th Jan 2019, 9:30 pm   #9
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Default Re: Homebrew SSB Transceiver

Another thing to watch out for is the potential for HF PSU switching noise on the output of the PSU powering the PA.

As you're using a high current LED supply the output might not be adequately filtered for radio purposes which could lead to spurious generation of an unacceptably high level centred around your main signal.
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Old 26th Jan 2019, 11:42 am   #10
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Default Re: Homebrew SSB Transceiver

That's a good point. When I looked at mine under ~4A load on a scope there was 100mV p-p noise which was pretty bad. I couldn't tune it on my receiver however which was probably good. The final build has a Schaffner mains filter and then a fairly hefty pi filter on the output of the power supply. The pi filter is a bit of FR4 with a couple of low ESR electrolytic caps paralleled with 3 100nF ceramics and a hand wound FT82-43 with about 50 turns on it ~ 1mH. All glued down and then conformal coated. Can't see anything at 5mV/div on the scope
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Old 26th Jan 2019, 1:04 pm   #11
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Default Re: Homebrew SSB Transceiver

Hi Wendy,

May I enquire: what method of SSB generation are you using?

I plan on doing some bench experiments (no radiations!) with SSB and the Weaver method struck me as about the easiest to implement.
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Old 26th Jan 2019, 5:37 pm   #12
Wendymott
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Default Re: Homebrew SSB Transceiver

Hi Karen... I used Double balanced modulators.. the MC1496.... with a 9 mhz (ish) carrier and a 9.00128 filter. The ( mhz ish) oscillator is switched with caps to drag the frequency +/-
I would be interested in your results....
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Old 26th Jan 2019, 5:41 pm   #13
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Default Re: Homebrew SSB Transceiver

Hi Mr Bungle.....and Tim.... Grrrrr... Many thanks for the info guys... I will load up the outputs and scope the voltage... THANKS both for your inputs..As it happens I have enough space on the top of the chassis to fit RF Bandpass filters, the SMP and now.. it seems a probable DC filter..
Nothing is EVER simple LOL.
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