EMRFD Message Archive 916
Message Date From Subject 916 2007-08-03 13:33:01 Ted Bruce KX4OM Fig 11.14 Transmit Chain questions
and 15m SSB/CW rig. I need some help in determining what the required
input level is to U3. I will be using a different low-level circuit to
drive that MMIC. The 90 degree hybrid and 2-way splitter are throwing
me off, as I don't know the losses associate with those circuits in
trying to calculate what the input level of the circuit as shown is.
Also, L6 and L7, the 51 ohm resistors and capacitors are described on
page 11.18 as a "low-frequency loading network". I assume those L and
C values would be good for 21 and 14 MHz.
Obviously, I will have band-switched switched filter circuits
following T4. The L8 and 9 filter/Z-transformation circuit I
understand, but the following Pi network/TR switching components look
to be too complex for me to translate for a 2-band rig. Since this is
an "EMRFD" design, I'd like to stick as closely as I can to modules
from the book, but I would probably have to follow the impedance
transformation network with other filter circuits and a simpler T/R setup.
tnx,
Ted KX4OM917 2007-08-03 19:57:07 Allison Parent Re: Fig 11.14 Transmit Chain questions 918 2007-08-04 09:06:01 Wes Hayward Re: Fig 11.14 Transmit Chain questions
This transceiver is Bob Larkin's design, but he is not available
right now. So let me take a stab at it and let Bob add details when
he is available.
First, it is not as much the drive level to U3 that is important as
are the levels through the balanced modulator, M1 for this case. If
you want good IMD, a good level for a single TUF-1 is an input max of
around -10 dBm, resulting in an output of -16 or so. Considering
that you have two ring mixers here, you can run those levels up by 3
dB. Some folks run these mixers a bit hotter than that, but I don't
like to push my luck.
It is important to measure levels with the balanced mixers. I think
that the most significant place where folks get in trouble when
building homebrew SSB rigs is when they fall prey to the urge
to "tune for max smoke." That worked in the old AM days when we
didn't much care about spectral purity or IMD. But we have moved
well beyond that point today.
Once you have the balanced mixer running at the right level, it is
just a matter of adding gain to get up to the desired output power.
I would guess that the chain should be pretty good as a wideband
amplifier and do a good job on both 20 and 15 meters. You might
want some sort of a gain control that could be changed as you switch
bands. I would start with the existing L and R values for L6/L7,
etc.
L8,C8,L9, and C9 just form a low pass filter along with perhaps some
impedance matching. You might want to analyze that one to see just
what it is doing. The T/R system is narrow band. That is, the one
shown is designed for the 17 meter band and won't really work well on
20 or 15 meters. The design is that shown on page 6.71, Fig 6.129
and related text.
The quadrature hybrid of T2 and related components is a narrow band
network and would probably not do a good job over the half octave
range you intend to tune. Switched networks might work well.
Alternatively, you might look at a wideband variation. This makes
more sense in a SDR like this, for you can then do adjustments at
baseband to take care of the problems related to the lack of
perfection in the quadrature hybrid. There is room here for
experimentation.
My personal take on a project like this is that I would take it in
steps. I would start with a single band design, perhaps 20
meters. After I had it going there and had done measurements, I
would then test the RF power chain to see what it did at other
frequencies. Work on it to make it functional where you need it.
Then add a band. Trade out the hybrid network for a wide band
variation. I might opt for a different T/R scheme considering the
desire for multiple bands.
You said that "Since this is an 'EMRFD' design, I'd like to stick as
closely as I can to modules from the book......" Well, I would
argue just the other way. That is, because it is a design from
EMRFD, feel free to do your own thing, but follow the underlying
philosophy of measuring and understanding what you have when you get
to various stages in the process.
One change I would make from the start today is to junk the IRF-511
and start with different transistors. The Mitsubishi RD16HHF1 is
inexpensive (RF Parts), but is actually an RF part specified for
linear service. The IRF511 is pretty poor for IMD when operated at
12 volts, as outlined in Ch 6. That would be my starting choice
today. A good friend built a SSB rig this last spring and had good
luck with that part. He has more measurements to do on it before he
publishes, which he is being urged to do. (hint hint)
One additional thought: W7PUA used a free running VFO for this
rig, which worked just fine in spite of the relatively high frequency
of 18 MHz. Temperature compensation was mandatory, but as Bob
pointed out, was not all that difficult. But it takes more than
just counting the drift of the room temperature oscillator. An
oscillator that works for 20 meters probably won't be stable at 15,
with the reverse also being true. Rather than doing the design
twice, I would be very tempted for your two band design to start with
a basic 20 meter oscillator. (Actually, run the VFO at 7 and use the
doubler that Bob did.) Once this system is stabilized with
appropriate compensation, run the 14 MHz into a X1.5 multiplier as is
used in several places through the book. The fractional drift
(including compensation) obtained at 14 is then there at 21 MHz. (I
love that circuit, but have not heard from others who have picked up
and used it.)
Anyway, I hope this helps get you started Ted. Good luck -- it
sounds like a fun rig.
73, Wes
w7zoi919 2007-08-04 09:06:26 Ted Bruce KX4OM Re: Fig 11.14 Transmit Chain questions
Thanks a lot for the information. The input and output of the mixers,
I knew, but other than one commercial splitter that I have, I wasn't
sure of the losses there. I've read the book from cover to cover, but
I haven't yet got everything imprinted in my mind so I can readily
access what I've seen and where.
I am aware of the IRF510 frequency issue; in fact, I'm building the
WA2EBY amp, but I'm using a relay-switched (8 in all) set of filters
from the CDG2000 rig. I had to synthesize the values for 30m using
RFSim99, approximating between 40 and 20m, and then checking it with
the EMRFD programs. I should have recalled the information on the
termination.
tnx agn,
Ted KX4OM
--- In emrfd@yahoogroups.com, "Allis923 2007-08-04 18:06:41 Ted Bruce KX4OM Re: Fig 11.14 Transmit Chain questions
Thanks for the detailed comments. I guess need to explain some more
about this rig. On the transmitter side, I'm starting with the circuit
of Fig. 6.152 for the SSB Generator and CW oscillator. That's from the
"Monoband SSB/CW Transceiver", built for 20 and 15, instead of 6m.
The LO is a 2-band, HFO-switched 5 MHz version of the same rig's VFO
circuit in Fig. 6.148, using dual mixers. Next are HFO mixer
terminations and LO bandpass filters, using guidance from the circuits
in the book, and then on to the transmit mixer of Fig. 12.31 (the Dual
Band QRP CW Transceiver). Based on the -20dBm output from that stage,
I will need to add a 10dB amp to get it to the input that Allison
calculated of -1dBm to U3. I assume the feedback amp circuit should
work for that. I've built the Log Power meter, and it's been
calibrated to a secondary standard, so I can measure the stage outputs
easily.
The primary reason I used the RF driver chain in Fig. 11.14 is that I
haven't had the opportunity to experiment with MMICs. I have several
of the HP/Avago versions of the MAR-3 and MAR-6. I guess this sounds
like a patchwork rig so far, but I've modularized it924 2007-08-04 18:06:59 Ted Bruce KX4OM Re: Fig 11.14 Transmit Chain questions
Thanks for the detailed comments. I guess need to explain some more
about this rig. On the transmitter side, I'm starting with the circuit
of Fig. 6.152 for the SSB Generator and CW oscillator. That's from the
"Monoband SSB/CW Transceiver", built for 20 and 15, instead of 6m.
The LO is a 2-band, HFO-switched 5 MHz version of the same rig's VFO
circuit in Fig. 6.148, using dual mixers. Next are HFO mixer
terminations and LO bandpass filters, using guidance from the circuits
in the book, and then on to the transmit mixer of Fig. 12.31 (the Dual
Band QRP CW Transceiver). Based on the -20dBm output from that stage,
I will need to add a 10dB amp to get it to the input that Allison
calculated of -1dBm to U3. I assume the feedback amp circuit should
work for that. I've built the Log Power meter, and it's been
calibrated to a secondary standard, so I can measure the stage outputs
easily.
The primary reason I used the RF driver chain in Fig. 11.14 is that I
haven't had the opportunity to experiment with MMICs. I have several
of the HP/Avago versions of the MAR-3 and MAR-6. I guess this sounds
like a patchwork rig so far, but I've modularized it925 2007-08-04 20:55:08 Allison Parent Re: Fig 11.14 Transmit Chain questions 926 2007-08-04 20:55:49 mdgolfbum Re: Fig 11.14 Transmit Chain questions
I think a lot of folks would love to see that RD16HHF1 power amp. I
know I would.
73
jim ab3cv927 2007-08-05 09:30:16 Hari G Re: Fig 11.14 Transmit Chain questions
On 8/5/07, mdgolfbum <jim@jtmiller.com> wrote:Hi Wes
I think a lot of folks would love to see that RD16HHF1 power amp. I
know I would.
73
jim ab3cv
--
De Hari
VU2GHB928 2007-08-05 09:30:31 Allison Parent Re: Fig 11.14 Transmit Chain questions 929 2007-08-05 09:31:18 w4zcb77 Re: Fig 11.14 Transmit Chain questions 932 2007-08-05 14:10:16 Ted Bruce KX4OM Re: Fig 11.14 Transmit Chain questions
I had forgotten about it when I replied that I hadn't seen the
RD16HHF1, but I actually stumbled across it some months back. The
device is being advertised by online CB shops as a "drop-in"
replacement for the 2SC1969. I would imagine there's more to it than
"dropping it in".
I checked RFParts.com, and their price is very attractive, about the
same as the common 4 to 5W BJT CB PAs we often use. The price is much
higher on some sites I found by Googling.
Thanks for the pointer to the EMRFD supporting circuits, Allison.
73,
Ted KX4OM
--- In emrfd@yahoogroups.com, "Allis934 2007-08-05 19:58:00 Jim Miller Re: Fig 11.14 Transmit Chain questions
My design target is for a 5W CW-only with 10-12V supply for 160-10M.
My interest in a linear rather than the usual switch approach is that I want
to feed the final with a shaped keying envelop.
The usual saturated switch finals while more efficient would tend to wipe
out any such shaping.
73
jim ab3cv935 2007-08-05 20:04:18 Allison Parent Re: Fig 11.14 Transmit Chain questions 936 2007-08-05 21:22:48 Allison Parent Re: Fig 11.14 Transmit Chain questions