[Amps] Fwd: Fwd: *** PGXL Questions on CW Operation

[Manfred Mornhinweg]

Jim,

interesting replies there. I would like to add some comments, for the 
possible benefit of readers:

> when using a Flex transceiver with the amp, there is no control 
> cable between it and the transceiver. Both plug into the internet and 
> all communication is via the internet.

I would think that the LAN (local area network) is meant, rather than 
the internet. Your LAN might or might not be attached to the internet. 
If any radio actually requires a functioning internet connection, that 
would be a serious reason against purchasing that radio!

 > (Also, Flex transceivers time
> shift CW as it's sent to allow any other brand of amp to come on-line 
> gracefully and to eliminate any potential latency. I don't believe other 
> transceivers have this capability.)

Flex transceivers are SDRs. Every SDR can easily time-shift signals. 
Most modern transceivers, even if they aren't true full SDRs, do use DSP 
for modulation, demodulation, filtering, etc, so they are SDRs in this 
limited sense, and can do time shifting. Of course it's up to the 
manufacturers to decide whether or not they include software to adjust 
the time shift.

What's more of a problem is that SDRs cannot work WITHOUT time shifting. 
  The digital processing of signals requires many samples of the signal, 
both from "before" and from "after" the current sample. So they must 
keep a certain number of samples in memory, which means that the signal 
going out is always somewhat time-delayed relative to the input signal.

While completely analog transceivers also have some signal delay, most 
of it in the crystal filters, that's pretty short. The delay caused by 
digital signal processing is typically longer, but is under control of 
the software, allowing trade-offs between the amount of time delay and 
the quality of filtering.

Depending on the implementation and configuration, an SDR's delay might 
be anything from a few milliseconds to several hundred milliseconds. 
Only the shortest of these delays allow QSK, while non-QSK operation is 
comfortable up to at least 100ms delay, and possible even with several 
hundred ms delay.

If a radio can intentionally add a delay to the signal, to cope with a 
slow amplifier, that's great and useful, but the combination definitely 
cannot do QSK.

Just for reference, some data on mu Kenwood TS-450SAT, which I measured 
in RX mode many years ago. With the stock 2.4kHz second IF filter, and 
no first IF filter, the delay is 1.22ms. With its widest filter, a 12kHz 
  wide one, it's just 0.35ms, while the combination of the two narrowest 
filters I have (1.8kHz in the first IF and 500Hz in the second IF) 
causes a delay of 3.50ms.

In TX the delay in SSB should be about the same as in RX with the 2.4kHz 
filter, and in CW it could be much shorter, because no IF filter is 
necessary.

The delays of my SDR, based on a Red Pitaya board and PowerSDR software, 
are much longer, but highly configurable.

> In addition, on CW the amplifier 
> achieves about 70 percent efficiency, which allows it to run cooler than 
> most solid state amps, thus allowing less fan noise.

This is something that really every power amplifier can do, but most 
radio manufacturers don't take advantage of this and run the amplifiers 
in fully linear mode even on CW. On the other hand, transmitting CW with 
a saturated amplifier results in a less than optimal keying waveform. 
Most operators wouldn't worry about this, but I know CW purists who do 
worry, and only use highly linear amplifiers even for CW!

> *The current recommended transmit delay is 20 ms but we will drop that 
> to 10-12 ms soon subject to further testing. **For FlexRadio only, this 
> added delay is about to go away and will be incorporated into the 
> handshake interchange, so any transmit delay is transparent to the 
> customer.? For Flex radios only, the radio delays the start of the CW 
> transmission, so no leading CW elements are lost. **For other brands of 
> radios, we will soon see a shortening to less than 15 ms. Other brands 
> of radios are not as sophisticated as to delay the transmit stream.*

A comment about this reply: This time delay implemented in SDRs will 
certainly cause the station's transmission to get out without any 
missing or shortened dits - but more time-delayed than the transmission 
of a non-SDR. When trying to break in, your transmission will always 
come late. When briefly switching to RX between dits, even if the radio 
hears anything it will get through to the speaker much delayed, making 
QSK less practical, or perhaps not practical at all. How is that, QSK 
operators? When you listen between dits, is it OK to hear what was sent 
by the other station a few dits ago, or does it have to be current?

The SDR might actually put the received signal on top of what you are 
transmitting, in your headphones.

In my opinion, QSK and SDR don't go together very well. Good QSK is 
easier to implement in conventional radios.

> *The relays are rated for a maximum contact bounce of 7 ms.? That plus 
> some time to control the relay we allow for 10 ms.? This is on the order 
> of the fastest transceiver key down to RF out.? As stated above, the 
> relays are not hot switched.? The relays are rated for 10 million 
> mechanical cycles.? You can see the Finder 40.52 specification sheet 
> attached. Contacts are AgNi.*

Flex has a point there. Modern relays, even quite cheap ones, are faster 
and more enduring than those of olden times.

Of course PIN diodes are even much faster...

> *We have significantly improved fan noise beginning with firmware 2.8.7 
> or later.? CW mode operates at 70% efficiency and a fairly low duty 
> cycle compared to RTTY and FT8 so fans typically will remain on the 
> lowest speed in most CW operation.*

The best solution to fan noise is adopting high efficiency 
amplification, to reduce heat generation so much that either fans become 
completely unnecessary, or can run at very low speed. I'm working on 
it... Maybe in 50 years I finish the project! Fat chance... ;-)

Manfred

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