[TenTec] Receiver ratings question, IP3 and synthesizer noise

[rick@dj0ip.de]

Gary,

You just posted the most recent $64K question!

Even in the past, the DR3 value was not all-telling of a radio's overall performance, but now with SDR technology, it is just a number without a lot of meaning.

There is a group of at least 4 people I know of actively discussing how to best test and measure SDR radios.  
DR3 relies on just two strong signals to produce results.
Obviously our crowded bands, especially in contests, have many more signals.
Arguably Adam's NPR test (well it's not "his" test, but rather the test he runs and reports on) is a better indicator.
However the cost of equipment to run it is very high.

Besides testing with two signals, some have begun testing with a 3rd, out of band signal.
But how far out?  How strong?
And yet another suggestion was for a 3rd in-band signal.

BOTTOM LINE:  We cannot use the DR3 ranking anymore to rank receiver performance.  Actually we never should have and Rob has always said in every presentation that there is a lot more to it than just DR3, but at the end of the day we have to pick some spec or some suite of specs to rank them buy.

IMO, the time has come again for someone to produce a good quality pre-selector, such as the famous Braun pre-selectors. 
It seems to me this would solve the 7300's problem.

Once again we find a problem with unexperienced engineers working for the transceiver OEMs making decisions about what we need in our transceivers.  SDR technology has enabled us to monitor the entire band, or even several bands at one time with fairly good efficiency, but in order to do that, you must leave the ham band only band pass filter out.  Leaving that out creates other problems.

As I have said for the past 50 years, I want to have a general coverage receiver but it does not have to be inside of my ham radio transceiver and MUST NOT be there if it is detrimental to performance inside of the ham band.
THE TRANSCEIVER OEMs have failed to understand this basic request for the 55 years that I have been a ham.

One could argue that the FLEX 6500 and 6700 are exceptions because they have ham band BPFs but then I ask, where are the knobs.  > Optional for $1300.

My Dream:  A simple, ham band only transceiver with a good BPF, in a radio the size of the OM7 or slightly larger, real S-METER (not sparklers), LARGE readout, uncluttered from information I don't need to change, LARGE JUMBO VFO knob like the Hammerland HRO (!) and large knobs for the controls one adjusts often such as RF gain and Audio gain.  Maybe we even make it a bit larger and have a front facing speaker.  Then there can be a black-box optional 2nd RX with I/Q out for using as our General Coverage radio.  And when I am running SPLIT mode, I want it to light up bright so that I cannot oversee it.

And of course it must have a TX just as clean as the cleanest RX.

73 - Rick, DJ0IP
(Nr. Frankfurt, Germany)



-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Gary J FollettDukes HiFi
Sent: Friday, September 02, 2016 3:56 PM
To: Discussion of Ten-Tec Equipment
Subject: [TenTec] Receiver ratings question, IP3 and synthesizer noise

Having reread the definitions from Sherwood’s page on receiver test ratings, I have to ask, what is the significance of IP3 in a receiver that has no mixers (any direct digital SDR)?

 I also have to ask, what is the significance of synthesizer noise in a receiver that has no synthesizer (any direct digital SDR)?

These two existing ratings indicate that almost every DSP receiver is better than any heterodyning receiver, even though it is common knowledge that IC-7300’s are being dumped on the used market because they perform so poorly in tough RF environments and even Icom recommends use of a preselector to correct the problem.

Also, now that I pay closer attention to the full description of the Flex and other SDR’s, I see the “B” rating for the front end selectivity. This tells me that the various DSP radios must use band limiting input filters to help the DSP in overcoming the problems that would occur if a strong broadcast signal were being received at the same time as a 2 microvolt CW signal. Apparently some (Flex) do a better job of this than others (Icom).Either that or the front end selectivity ratings for these radios is boiler plate stuff with no real significance.

Perhaps it is time for a new select criterion for future DSP designs?

Gary
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