DigiThin - Pre-production Info

The place for discussion about the RPi add-on RB-TV modulator card developed by G4EWJ and supported by the BATC Shop
G4EWJ
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Re: DigiThin - Pre-production Info

Post by G4EWJ » Thu Aug 20, 2015 4:18 pm

Top and bottom with slightly better lighting than my last.

Brian
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g0mjw
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Joined: Sat Sep 20, 2014 9:15 am

Re: DigiThin - Pre-production Info

Post by g0mjw » Wed Aug 26, 2015 6:19 pm

I have it nearly built, having had to wait for parts. I don't have a 3.3V zener and no PIC socket yet, though I only have a pickit2 it claims to be capable of programming the device. Once I get the zener I might give it a go.

I am puzzling over what to do with the 2.2M multiturn POT supplied with the kit. It doesn't appear on the schematic or the parts list. I am assuming it is something to do with the si570 board.

Mike

G4EWJ
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Re: DigiThin - Pre-production Info

Post by G4EWJ » Wed Aug 26, 2015 7:33 pm

The pot was an addition to investigate the possibility of improving the LO suppression by changing the biasing of the IQ inputs, but I don't have a technique for doing that yet.

Brian

g0mjw
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Re: DigiThin - Pre-production Info

Post by g0mjw » Mon Aug 31, 2015 11:26 am

G4EWJ wrote:The pot was an addition to investigate the possibility of improving the LO suppression by changing the biasing of the IQ inputs, but I don't have a technique for doing that yet.

Brian
OK - I have it completed now and a few observations on this remarkable board and software.

Firstly my output was disappointing at -30dBm. After checking practically everything else first, It turned out my modamp was dead. Not sure why. I didn't have a spare so I replaced it with one of the "Not MAR-6" devices from Kent. This worked well but had a bit too much gain on 2m, developing -3dBm output but IMD shoulders only -40dB, so I looked at the attenuator following the modulator with the aim of reducing the gain by a few dB.

That is where I had an issue. The 6dB pad uses two 110 Ohm resistors and a 22 ohm in a pi configuration. That's not a 50 ohm 6 dB attenuator, so not sure why those values were used. Presumably some other matching? Anyway, with a view to replacing as few parts as necessary I replaced the 22 ohm with a 62 ohm which makes a 9 dB attenuator. As expected the output fell about 3dB. All good.

Next I looked at the spectrum. The digital signal itself is good, but there are some unwanted spurii a few MHz away and quite high levels of harmonics. Definitely needs a filter. The LO was am HP sig gen so I don't think it was responsible for the spurii. There is a fair amount of digital interference on the 5V line. It might be worth a separate supply for the analogue signals. The constellation is OK, if slightly skewed. I grounded the bard to the ethernet connector but possibly it would be more stable without the on-board modamp.

Mike

G4EWJ
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Re: DigiThin - Pre-production Info

Post by G4EWJ » Mon Aug 31, 2015 12:56 pm

Thanks for the comments Mike.

I've heard of a couple of people losing the MMIC in service, for no good reason. I've run mine without a load, into bits of wire, had my fingers on it and probably shorted it out and I'm still on the original. I've only ever connected it to an analyser. One theory is that floating power supplies may somehow be inducing high voltages like the tingle you can get from some unearthed equipment. I don't think the centre pin of an SMA plug can get to the socket before the shell connects though. I've considered putting a snubber resistor of 1k across the MMIC output, but I need to test the effects of this.

I think the not-MAR6 turned out to be an MSA-686. This has a P1 of only +2dBm, so the output does need to be kept well below this. The SGA-3586 has a P1 of +13dBm and the aim was to have an output power of 0dBm to +3dBm to keep it well below the P1 point.

The attenuator was a compromise to give some isolation between stages. The output of the modulator and the input of the MMIC are not going to be 50R, so I chose 110R as they were used elsewhere in the circuit.

It certainly needs a band filter for the harmonics. I can see some closer-in spurious signals, but I can see those on the LO directly, so I need to sort that out. I can see low levels of harmonics of the 1.333MHz DAC update frequency. I wonder if that correlates with yours? There's nothing getting to the MMIC via its 5v feed, as I've disconnected the output capacitor from the modulator and the analyser trace is flat.

I think it's well worth investigating the power supplies and trying external ones and using more filtering on the internal ones. It's a very noisy environment. I haven't had a lot of time for DigiThin investigations over the past few months, but things should quiten down after CAT15.

Brian

g0mjw
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Re: DigiThin - Pre-production Info

Post by g0mjw » Mon Aug 31, 2015 3:40 pm

G4EWJ wrote:Thanks for the comments Mike.

I've heard of a couple of people losing the MMIC in service, for no good reason. I've run mine without a load, into bits of wire, had my fingers on it and probably shorted it out and I'm still on the original. I've only ever connected it to an analyser. One theory is that floating power supplies may somehow be inducing high voltages like the tingle you can get from some unearthed equipment. I don't think the centre pin of an SMA plug can get to the socket before the shell connects though. I've considered putting a snubber resistor of 1k across the MMIC output, but I need to test the effects of this.

Brian
Interesting - thinking about it, I wonder if oscillation could be the culprit? As we are driving from 5V via a 51 ohm load its possible it could take enough current under a bad load to do some thermal damage but more likely its input power rating that has been exceeded. My failure was sudden. It had been showing low gain of around zero dB and then failed completely. The low gain may have been due to an oscillation at some other frequency - oscillators have zero gain while oscillating. These devices operate up to several GHz with high gain so unless a lot of care is taken with grounding and screening its surprising when they don't oscillate.

Mike

G4EWJ
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Re: DigiThin - Pre-production Info

Post by G4EWJ » Tue Sep 01, 2015 7:26 pm

Oscillation is a possibility. Maybe some capacitance across the input would help, or some absorbing foam stuck to the chip.

Brian

g8vpg
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Re: DigiThin - Pre-production Info

Post by g8vpg » Mon Sep 14, 2015 12:50 pm

I have just discovered that the SGA3586 on my Digithin has failed. Does anyone know a source for these or can suggest an alternative?
73 Shaun.

G4EWJ
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Joined: Wed Feb 17, 2010 10:11 am

Re: DigiThin - Pre-production Info

Post by G4EWJ » Tue Sep 15, 2015 10:06 am

I have a few spare. I'll send you a PM.

I don't know of a source of one-offs. RF-Microwave in Italy used to do them, but no longer.

It was chosen for its high gain and P1 figures. A rule of thumb for a replacement would be a gain no more than 12dB greater than the P1 figure. The supply resistor may need to be changed to set the standing current.

I would recommend a 1k resistor from output to ground, so that it always has some load.

Brian

MN-tech
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Joined: Thu Aug 30, 2012 6:01 pm

Re: DigiThin - Pre-production Info

Post by MN-tech » Tue Sep 15, 2015 11:02 pm

Cross Reference Match For: SGA3586

AG303-86G 1: $1.96

http://www.mouser.com/ProductDetail/Qor ... %252b9w%3d

Searching Mouser for SGA3586 returned this part number.

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