SSB Carrier Supression

[Space Varmint]

Like flat5 mentioned, "shield the heck out of everything!"
The RF could still be getting in through the MIC or somewhere else. If it was getting into the PLL you'd notice a lot of FM happening as well I would think. Do you have a frequency counter? RG-174U is good for between PCBs and will handle 80 Watts at 30Mhz with no problem. I wouldn't use it as a 100ft antenna feed line though.

LOL...I'm inclined to agree with you there I've thought about that mic being a problem. I did not have the standard connector so had to rig something up that doesn't look real rf tight.

 

[Space Varmint]

You could also decouple stages more generously
Throw a few chokes and .1 caps around the low level circuits.
Can you see with a scope RF on the power supply?
I don't know if that is meaningful when you have an eighty watt
amp next to the scope, what do you think?

That's very interesting. It just happens to be the technique I used to prevent any idle feedback. Mainly capacitors. Don't want to get too generous . The problem with using the scope is, it will actually becomes a source of feedback in operation. It's almost like opening a stage and sticking an antenna on an amplifier or something. I mean if I have the 80 watt fired up. Well it will do it at 8 watts too in most cases. You almost have to guess the hot spots, but it sounds like you know that. You might have to elaborate a little more on that one.

 

[Space Varmint]

Even if you are using dual shielded coax, high quality N connectors, dummy loads, shielded enclosures, etc, you'll still pick up the transmitter on a decent receiver if they are tuned to the same frequency. Unless you are running full duplex, what you need to do is mute the receiver with the PTT signal from the transmitter.

You mentioned something very interesting here kchriste. I know of VOX, but how would you operate full duplex unless you are using two separate frequencies and a diplexer?

 

[xanadunow]

This thread..

I know of VOX, but how would you operate full duplex unless you are using two separate frequencies and a diplexer?

will outlast all others..

xanadunow

 

[kchriste]

I know of VOX, but how would you operate full duplex unless you are using two separate frequencies and a diplexer?

That's exactly how you'd have to do it. I've seen duplexers that operate at 50Mhz where the cavities are as tall as the ceiling in the "shack". A duplexer made with cavities would get pretty big at 2Mhz! You'd need some pretty tight notch and bandpass filters to pull it off. Separating the TX and RX antennas would help somewhat.

 

[flat5]

Could you do it by very fast switching between transmit and receive? No I guess not. It would in effect be another modulation of the signal (say 20k switching) and the bandwidth would be too great at 14mhz and below. Is that right?

 

[Space Varmint]

That's exactly how you'd have to do it. I've seen duplexers that operate at 50Mhz where the cavities are as tall as the ceiling in the "shack". A duplexer made with cavities would get pretty big at 2Mhz! You'd need some pretty tight notch and bandpass filters to pull it off. Separating the TX and RX antennas would help somewhat.

I think you use a diplexer right? I'm just going from memory. But isn't that what they use in most repeaters now days?

 

[Nigel Goodwin]

I think you use a diplexer right? I'm just going from memory. But isn't that what they use in most repeaters now days?

No, a diplexer doesn't have the isolation required - feeding the transmitter directly into the receiver isn't a good idea.

 

[kchriste]

I think you use a diplexer right?

At first I thought you made a typo. No, a duplexer would be required. A diplexer is only a highpass and lowpass filter combo where as a duplexer has a sharper response using bandpass and notch filters. I've only seen duplexers used in repeater systems. As Nigel mentioned, the diplexer can't supply the needed isolation that a duplexer can.

 

[Space Varmint]

At first I thought you made a typo. No, a duplexer would be required. A diplexer is only a highpass and lowpass filter combo where as a duplexer has a sharper response using bandpass and notch filters. I've only seen duplexers used in repeater systems. As Nigel mentioned, the diplexer can't supply the needed isolation that a duplexer can.

Yeah, but what they do is use those "bucket brigade" chips so that the transmitter does not send what it is receiving at the same time but there is a delay. This prevents feedback.

 

[Nigel Goodwin]

Yeah, but what they do is use those "bucket brigade" chips so that the transmitter does not send what it is receiving at the same time but there is a delay. This prevents feedback.

That's just to help prevent audio feedback, it doesn't do anything to stop the receiver been destroyed as you feed 80W directly down it's aerial lead.

Even assuming the receiver is far enough away in frequency that a diplexer will prevent it been destroyed, it's still going to completely wipe out any incoming signals.

 

[Space Varmint]

That's just to help prevent audio feedback, it doesn't do anything to stop the receiver been destroyed as you feed 80W directly down it's aerial lead.

Even assuming the receiver is far enough away in frequency that a diplexer will prevent it been destroyed, it's still going to completely wipe out any incoming signals.

They use the same antenna? I don't know why they would do that. Most repeaters work at high enough freqs that they can be made very small. Usually a quarter wave whip antenna.

 

[Nigel Goodwin]

They use the same antenna? I don't know why they would do that.

Because it saves the cost of a second aerial, and it means performance is the same on both TX and RX. As they are very close in frequency you'd still need the cavity resonator to let it work, even with separate aerials.

We used to have mobile radios in the vans at work, they had a talk-through facility, whereby the vans could talk to each other via the base station - even though it used separate aerials, and had a 10MHz split (78.0625MHz and 88.0625MHz), reception was worse on talk-though as it de-sensitised the receiver.

Most repeaters work at high enough freqs that they can be made very small. Usually a quarter wave whip antenna.

I would have suggested repeaters probably use co-linears rather than just whips?, or possible dipoles - whips require a groundplane.

But really the whole discussion is pointless, you've no need (or use) for full duplex, and is it even allowed under your licence conditions?.

 

[Leftyretro]

Because it saves the cost of a second aerial, and it means performance is the same on both TX and RX. As they are very close in frequency you'd still need the cavity resonator to let it work, even with separate aerials.

We used to have mobile radios in the vans at work, they had a talk-through facility, whereby the vans could talk to each other via the base station - even though it used separate aerials, and had a 10MHz split (78.0625MHz and 88.0625MHz), reception was worse on talk-though as it de-sensitised the receiver.



I would have suggested repeaters probably use co-linears rather than just whips?, or possible dipoles - whips require a groundplane.

But really the whole discussion is pointless, you've no need (or use) for full duplex, and is it even allowed under your licence conditions?.

Well split band operation (say send on 80 meters and receive on 15 meters)would allow full duplex operation and I don't think it's unlawful for hams?

This has sure turned out to be a train wreak of a posting thread

Lefty

 

[Space Varmint]

Because it saves the cost of a second aerial, and it means performance is the same on both TX and RX. As they are very close in frequency you'd still need the cavity resonator to let it work, even with separate aerials.

We used to have mobile radios in the vans at work, they had a talk-through facility, whereby the vans could talk to each other via the base station - even though it used separate aerials, and had a 10MHz split (78.0625MHz and 88.0625MHz), reception was worse on talk-though as it de-sensitised the receiver.



I would have suggested repeaters probably use co-linears rather than just whips?, or possible dipoles - whips require a groundplane.



But really the whole discussion is pointless, you've no need (or use) for full duplex, and is it even allowed under your licence conditions?.

They use full duplex all the time on the high band repeaters like 2 meters and 70 centimeters etc. The transmit frequency is only 5KHz away from the receive frequency.

 

[Nigel Goodwin]

They use full duplex all the time on the high band repeaters like 2 meters and 70 centimeters etc. The transmit frequency is only 5KHz away from the receive frequency.

No they don't, it's just simplex operation, and the frequency split was 600KHz last time I looked. It certainly won't be 5KHz, as that's lower than the channel spacing

The repeater itself is 'duplex' (if you can call it that?), as it recieves on one frequency and retransmits on another - hence the cavity resonator to separate TX and RX.

 

[kchriste]

Yeah, but what they do is use those "bucket brigade" chips so that the transmitter does not send what it is receiving at the same time but there is a delay. This prevents feedback.

The only time I've seen this was in a "simplex repeater". In this case it was used for a remote ambulance station to extend the emergency pageout coverage. It would only activate when it received the paging tones for that station, record the message from dispatch, and then rebroadcast the message on the same frequency when the carrier from dispatch dropped.

They use the same antenna? I don't know why they would do that. Most repeaters work at high enough freqs that they can be made very small. Usually a quarter wave whip antenna.

Sometimes there is more than one repeater on a single antenna using multicouplers and combiners which is basically a bunch of duplexers strung together. Quite common on the mountain/building top sites around here. You can't get enough separation on a tower for multiple antennas so this is the preferred method.

 

[Space Varmint]

No they don't, it's just simplex operation, and the frequency split was 600KHz last time I looked. It certainly won't be 5KHz, as that's lower than the channel spacing

The repeater itself is 'duplex' (if you can call it that?), as it recieves on one frequency and retransmits on another - hence the cavity resonator to separate TX and RX.

OH yeah, your right 600KHz. LOL I forgot.

 

[Space Varmint]

Hey Mikey? I never got around to trying a different front end, but I did find that applying some negative feedback to it, goes a long way toward eliminating intermod.

 

[Mikebits]

Hey Mikey? I never got around to trying a different front end, but I did find that applying some negative feedback to it, goes a long way toward eliminating intermod.

Sounds about right. Feedback will improve IP3 but at the same time will degrade noise figure. It is a trade off. Best bet is to use an off the shelf monolithic amp like Mini-circuits ERA-3SM which is about 2.8 NF and IP3 of 25 dBm. I seen em for sale on e-bay for 5 bucks.