Rob Sherwood NC0B is one of the real authorities on receiver performance. Many of us have relied on his ratings of commercial receivers for many years. His recent presentation to the Madison DX Club has a lot of really interesting information. There is also, I think, some stuff that homebrewers will find distressing.
Just some things that I noticed:
-- Rob mentioned a move back to 9 MHz IF filters and a move away from dual-conversion rigs with a high IF. He also mentioned the combination of a 9 MHz IF and a 5 MHz VFO as a way of easily getting on both 75 and 20 meters.
-- Rob discussed phase noise from synthesizers, a topic we discussed at length (some would say ad nauseum!) a year or so ago.
-- Rob really praised the "Pure Signal" system of one of the SDR manufacturers. He showed the completely rectangular waterfall display of a Pure Signal transmitter. I'm afraid that simple crystal rigs might never live up to this standard. An embrace of this high standard could discourage the construction of simpler, HDR rigs. We should not let the perfect be the enemy of the good!
-- We often hear SSB ops complaining that some other SSB op is "splattering all over the band." It often turns out that what is really happening is that a clean SSB signal is just overloading the receiver of an operator who does not know how to turn off his pre-amp or turn on an attenuator. Rob shows us how to really know if the problem is in fact at the other end: He looks at key clicks from two different CW signals on 160 meters. Both are at roughly the same level in his receiver But one is clicking all over the place while the other is not. With this kind of comparative info, we can be sure that the problem is the transmitting station's fault.
-- In discussing when to turn on the pre-amp (or the attenuator) Rob revives the old practice of just listening to the band noise. If you can hear the band noise when you switch from dummy load to receive antenna, you have enough RF gain. Adding more will only make things worse.
-- There was an interesting question about how to evaluate the performance of receivers when there are many signals inside the receiver's passband. This is the case with FT-8. Rob said this situation needs more research.
I don't mean to be critical here -- Rob is the guy who evaluated commercial rigs. And he is a contester. So his presentation is, of necessity, going to have a very "appliance operator" orientation. There seems to be an assumption that the only "rigs" that modern hams can use are commercial products. At one point Rob admits that most hams just can't repair these rigs. There is much for homebrewers to learn from experts like Rob, but presentations like this also remind us of what a tiny minority we really are, and how most hams have moved completely away from the old ham tradition of building our own rigs.
Thanks to Rob Sherwood and the Madison DX Club. And thanks to EI7GL for alerting us to this important presentation.
It kind of pains me to do this. These articles are from a long time ago, and the author is an esteemed Silent Key, but the myth about the origins of the USB/LSB convention is still out there, and as a homebrewer of SSB gear I feel obligated to point out these examples of the error that that myth is based on.
Last Friday, Pete WB9FLW and I were talking about homebrewing SSB rigs. I recommended a series of QST articles by Doug DeMaw. "Beginner's Bench: The Principles and Building of SSB Gear" started in QST in September 1985. There were at least five parts -- it continued until January 1986. (Links to the series appear below.) I hadn't looked at these articles in years, but when I did, a big mistake jumped right out at me: In the first installment, on page 19, Doug makes the same mistake that he made in his Design Notebook:
"Now comes the conversion section of our SSB generator. We must move (heterodyne) the 9-MHz SSB signal to 3.75-4.0 MHz. Our balanced mixer works just as it does in a receiver. That is, we inject the mixer with two frequencies (9 MHz and 5 MHz) to produce a sum or a difference output frequency (9 - 5 = 4 MHz, or 9 +5 = 14 MHz) If we are to generate 75 meter SSB energy, we must chose the difference frequency. We could build an 20-meter SSB transmitter by selecting the sum of the mixer frequencies. The RF amplifiers and filter (FL2) that follow would then have to be designed for 14-MHz operation. In fact, many early two-band homemade SSB transmitters were built for for 75 and 20 meters in order to use this convenient frequency arrangement. The use of upper sideband on 20 meters and lower sideband on 75 meters may be the result of this frequency arrangement (the sidebands become inverted when switching from the difference to the sum frequency.) "
Those last two sentences are incorrect. They repeat the "Myth," or the "Urban Legend" about the origins of the LSB/USB convention. Contrary to what many hams now believe, with 9 MHz filter and a 5.2 MHz BFO it takes more than just switching from sum frequency to difference frequency to invert one of the sidebands.
There are two conditions needed for sideband inversion to take place:
1) You have to be taking the difference product (DeMaw got that right)
2) The unmodulated (VFO or LO) signal must be larger than the modulated signal. (DeMaw and the ARRL obviously missed that part. Repeatedly.)
This is another way of stating the simple, accurate and useful Hallas Rule: Sideband inversion only occurs when you are subtracting the signal with modulation FROM the signal without modulation.
For DeMaw's claim to be correct, one of the SSB signals going into the balanced mixer would have to invert, and the other would have to not invert. Let's see if that happens: He has the sideband signal being generated at 9 MHz and the VFO running around 5 MHz.
9 - 5 = 4 But we are not subtracting the modulated signal FROM the unmodulated signal. SO NO INVERSION
9 + 5 = 14 We are not subtracting at all. SO NO INVERSION.
EI7GL reports some very interesting IARU activity that could possibly result in the 5 Meter band coming back to amateur radio use:
"The 60 MHz or 5 metre band has the potential to be a future allocation for the Amateur Radio service. The International Amateur Radio Union (IARU) are currently encouraging member societies to try and obtain small allocations at 40 MHz and 60 MHz."
https://ei7gl.blogspot.com/p/60-mhz.html
Regaining 5 meters was, of course, the objective of Frank Jones and the Five Meter Liberation Army. Wouldn't it be great of Michael Hopkins' fictional tale actually ended up coming true!
Posts
All Comments
Posts
All Comments
